General Notes Flashcards
What recommended dose does NCRP 138 give for workers in the case of emergency interventions? If levels are expected to exceed, what should workers be made aware of?
500 mSv
If expected to exceed, workers should be made aware of the inreased risk to themselves, both acute and long term
During a radiation disaster, what is a good general workflow to run through?
Treat life threatening conditiosn first without regard for radiation or contamination
Isolate patients and restrict access to treatment and evaluation areas
Prevent and minimize internal contamination ASAP
Minimize contamination to medical personnel
Control contamination using a 1 way flow of rooms that includes contamination room and treatment room –> cleaning and disposal room –> survey room –> release
Seek assistance from professionals within organization with relevant radiation safety training, ideally older workers as long term effects are less likely to manifest for their life remainder
Follow patients overtime with significant radiation exposure
Counsel patients and family members on long term health risks and dangers
In the GM region, what is response proportional to?
Nothing
At this energy, townsend avalanching has produced so many secondary electrons, that there is no longer proportionality to voltage or energy deposited. All information is gone, only counts
What type of distribution does radiation detection follow? What about radiation counts?
Detection follows Poisson statistics
At high counts, this collapses to Gaussian
What is the workflow of PACS image transfer?
Modality –> Gateway (console computers that allow user to verify or input demographic info –> Archive –> reading stations
What does HIPAA stand for?
Health Insurance Portability and Accountability Act of 1996
As window width decreases, what happens with contrast?
Contrast increases
What causes aliasing?
When a high frequency signal is sampled at a low frequency
What has better contrast, a high speed film or a low speed film?
High speed film
What has better measurement exposure range, high speed film or low speed film?
Low speed film
What type of Gen CT is a typical CT Sim?
3rd gen
How is planar image magnification affected by source to image distance and source to object distance?
As SID increases, magnification increases
As SOD increases, magnification decreases
What is the main functional difference between a fan beam and a cone beam?
fan beams scan only a few slices at a time along a single axis
Cone beams scan in both the X and Y axis for all slices at the same time
How is speed of sound affected by density and bulk modulus? Is speed of sound faster in bone or water?
Proportional to sqrt of bulk modulus
Inversely proportional to sqrt of density
Speed of sound in bone > water
This is because, although bone has a higher density, it also has a much higher bulk modulus than water. Hence it over compensates
Bulk modulus is a value that defines the compressability of an object. Something that can’t compress easily and is very still has a high bulk modulus
Where is lateral resolution best for a ultrasound scan? What about axial resolution?
Lateral resolution is best at the focal point
Axial resolution is pretty consistent across all depths
Per TG-128, what is the tolerance for the needle alignment test? What is the inherent uncertainty in having the setup be in water bucket? How can you reduce this uncertainty?
3 mm
Speed of sound in water is slightly slower than tissue
To minimize this effect, either raise water temperature or raise salinity
What phantom did we use for TG-128 annual?
CIRS Model 45 Phantom
Conceptually, what is CTDI?
Dose from a single slice at a particular depth in a phantom
Conceptuallym what is CTDI100?
Cumulative dose at the center of a 100 mm axial scan
Conceptually, what is CTDIw?
Weighted average of CTDI100 measured at the center of the phantom and the edges
Conceptually, what is CTDIvol?
Normalized dose froma helical scan with an arbitrary pitch, to a pitch of 1
Conceptually, what is SSDE?
Size specific dose equivalent
Attempts to make CT dose estimate more applicable to individual patients by applying a conversion factor which takes patient dimensions into account
If you’re setting up a new clinic, where can you look to find scan protocols for a CT sim in your clinic?
AAPM has a list of recommended scan protocols on a site-by-site, treatment-by-treatment, and machine-by-machine basis
What is the purpose of a bow-tie filter in a CT
Equalize image noise for varying thickness body parts
What is pitch? What does a pitch < 1 imply? What about > 1?
Table increment per rotation / beam collimation
> 1 implies gaps between slices (less dose, decreased image quality)
< 1 implies overlapping slices (higher dose, increased image quality)
What is the difference between EMR and EHR?
EMR capture information from a single care provider, which is only available to that one provider and their clinic
EHR is designed to be used by multiple care providers and healthcare organiations
How can you doa craniospinal irradiation without kicking couch?
HBB and rotate collimator to limit divergence
How would you treat a internal mammary chain breast? What field arrangements?
They’re hard to cover with traditional field arrangements, so some sites utilize VMAT to cover all nodes
What are most CT detectors made of?
Solid state scintillators
Cadmium tungstate, gadolium based or ceramic scintillator
True or False
Ring artifacts are only present in 3rd generation and 4th generation CTs
False
Only present in 3rd generation
How is workload determined for CT sim shielding?
Either with an isodose map given by the vendor
Or with CTDI1000 or DLP applied to an equation. But these are less common
Why is TAR useful for Co-60 beams but not MV beams?
Because at higher MV energies, the buildup cap size required for CPE is so large, that it effectively acts like a mini phantom and the readings are no longer in air
What two physical parts make up the collimator scatter factor?
Scatter from jaw backscattering to chamber, reducing dose/MU
Scatter from jaws reaching patient, increasing dose/MU as jaws get larger (more surface area to scatter)
The 2nd effect is the more dominant
True or False
EDWs do not harden the field, thus PDD/TMR are unaffected for EDWs
True
What is the relationship between electron output factor, cutout factor and applicator factor?
Output factor = AF x CF
Applicator factor corrects output between 10 x 10 cm2 to open cone size you’re using
Cutout factor corrects output between open clinical cone size to cone size with cutout you’re using
Output factor is the full conversion relative to a 10 x 10 cm2 open cone to your clinical setup
How is attenuation corrected for in PET imaging?
Using the CT component of the PET/CT
Algorithm takes into account that peripheral annihilations travel less through tissue, thus attenuate less, meaning that the counts detected will be higher than central, even if true counts are the same
What organs are naturally bright on a PET scan?
Bladder, kidneys, heart, brain
Anything that either clears agents from the body or circulates them
The brain has a high glucose intake, so that’ll also be naturally bright
What are some things that you would do during commissioning of a TPS
- Check for accurate image interpretations (scanned catphan)
- Check that patient positioning and orientation is accurately defined
- Check that immobilization devices, if available, are modeled in TPS
- Commission beam model (TG-106, 119, MPPG 5a)
- Calculation and easurement of sample test cases for EBRT or HDR
- Check of entire treatment plan normalization and MU calcs on a series of plans
- Accurate DVH calculation and construction
- Limit testing of algorithms for when they begin to fail in dose prediction
- End-to-end testing
Per TG-53, what is the recommended monthly and annual QA for TPS?
Monthly - CT data input verification (Accurate SSDs, spatial representation, CT number vs electron density)
Annual - dose calculation accuracy and consistency of all clinically utilized algorithms
The purpose to TPS QA in essence is to
1. Make sure there was no unknown changes to the beam model (checked with calc, recalcs and checksums)
2. Make sure there was no unknown changes to electron density curve and image quality
Why on a HU curve is part of the curve linear and the other part non-linear?
In the linear part, Z is low, so dominant interaction is compton. The compton cross section increases with electron density, however, most soft tissue has effectively the same electron density. The key difference then is physical density, which will increase the linear attenuation coefficient proportionally, hence linear
In higher Z region, photoelectric starts to become more prominent, which increases linear attenuation coefficient at a non-linear rate
What are the most common forms of imaging for thyroid cancer?
Radioactive iodine SPECT imaging which assesses thyroid function and potential disease spread
Ultrasound which assesses nodule size and visualizes microcalcifications
What two guidance documents are used for IMRT QA?
TG-218 is the main one for tolerance limits and methodologies
TG-230 focuses more on dosimetry tools and techniques
Explain the difference between tolerance limits and action limits as used in TG-218 for IMRT QA. What are the recommended tolerance and action gamma criteria in TG-218?
Tolerance limits are meant to be a warning that something might be out of the ordinary in the specific plan, treatment system, or TPS. Plans failing to meet tolerance limit should be investigated, but may still be clinically acceptable if they are able to pass the action limit
Action limit defines the amount a measurement can deviate from calculation without risking harm to the patient. Plans failing to meet the action limit should be replanned to improve deliverability.
3%, 2mm, 10% minimum threshold
Tolerance limit is pass rate >= 95%
Action limit is pass rate >= 90%
**Note: ** exact same tolerances as what were used at CTCA
What is the Roentgen-to-rad conversion factor and how is it found?
It is a factor that converts exposure to absorbed dose to air.
fair = 0.876 rad/R which is found by noting that 1 R = 2.58 E-4 C/kg and 33.97 J are required to ionizie 1 coulom of charge
Given a dose to air for a survey meter, how do you relate to dose to water for a person?
Dmed = Dair * ratio of mass energy absorprtion coefficients
Note: Why is it not ratio of limited mass stopping powers? This is because bragg gray cavity theory and spencer-attix cavity theory is only valid for small cavities, in which only electron fluence contribute absorbed dose to the cavity and photon interactions from in the cavity can be neglected
In survey meters, exposure is measured and photons are the ones contributing the dose, thus we talk about the energy absorption coefficient ratios
In regards to CT imaging, what is a ramp filter?
A digital filter applied to k-space (frequency domain) data that acts to suppress low spatial frequencies
Side effect is an increased image noise which is largely contained in the unsuppressed high frequency portion of k space
If a 4D CT appears choppy, what are some things you can do to reduce the choppiness?
Decrease pitch, allowing for sufficient data to be collected at all breathing phases
Coach patient to try to turn an irregular breathing cycle into a regular one
Reposition camera or surrogate device to make sure it’s reading an amplitude appropriate to the breathing cycle
Manaully re-bin or assign peaks on the respiratory cycle if automatic peak selection fails, which it may for irregular breathing cycles
What is a laminated barrier? Why might it be useful?
A barrier consisting of different layers of different shielding material
Commonly may be used is lead or steel sandwiched by concrete
It is useful in reducing the overall thickness of the barrier, which allows for better space saving
What does medical professionalism mean to you? (there is no need to memorize information, just speak)
“Medical professionalism is a belief system about how best to organize and deliver health care, which calls on group members to jointly declare what the public and individual patients can expect regarding shared competency standards and ethical values and to implement trustworthy means to ensure that all medical professionals live up to these promises.”
In essence, it’s a belief system that sets a standard for what the public should expect competency and ethically wise in a medical professional
What are some tenets of professionalism? (no need to memorize all of them, they’re mostly common sense)
- Professional Competence
- Honesty with patients
- Patient Confidentiality
- Maintaining appropriate relationships with patients
- Improving quality of care
- Improving access to care
- Just distribution of finite resources
- Scientific knowledge
- Maintaining trust by managing conflicts of interest
- Processional responsibilities including self-regulation
What are some codes of professional behavior/ethics that you are aware of? (there should be three that you really should know exist)
- Code of ethics for the AAPM
- TG-109
- Hippocratic Oath
- Declaration of Geneva
- Good medical practice - UK
- AMA Code of Medical Ethics
What are the 4 general steps of commissioning a TPS dose calc algorithm?
- Data acquisition and processing
- Model creation
- Validation of model
- Establishment of ongoing QA program
Per NCRP 151, what is a typical workload for a high energy Linac and what should be considered when determining the workload?
Typical workload is 500 Gy/wk
But you want to consider your own clinic practices, primarily
* How many patients are treated per week
* What are the typical prescriptions given?
* How much QA is performed in a given week?
* Are there any special procedures (especially TBI)?
Why is the highest electron energy used for TG-51 parallel plate cross calibration?
To minimize gradient effects across the reference cylindrical ion chamber
Order the following steps into their proper order in a TG-100 FMEA process.
Fault tree analysis
Process tree mapping
Development of mitigation strategies
FMEA
Process tree mapping –> FMEA –> Fault tree analysis –> development of mitigation strategies
What is the purpose of fault tree analysis in TG-100 FMEA process?
Evaluates how failure propagate through a process, and helps identify mitigating strategies for a given failure
Are machine performance checks a form of QA or QC?
Kind of both…
On one hand, you’re evaluating the current status of treatmen tparameters, compared to tolerances, and making adjustments if they fail. Thus you can argue it’s QC
On the other hand, most reports refer to them as QA, and you are demonstrating whether a safe and successful treatment or a correctly functioning machine is being output
Who decides on acceptance testing procedures and is this negotiable?
The vendor outlines acceptance testing procedures but these can be negotiable prior to purchase and specified in the purchase agreement
What order should you do the following general Linac acceptance test categories?
Mechanical system operations
Radiation tests
Safety tests
Imaging tests
Safety –> mechanical –> radiation –> imaging
What is electrical arcing and why is SF6 used in microwave waveguides to prevent it?
The waveguide carries RF power to the accelerator waveguide by propogating the energy based on the conductive walls of the waveguide tube, which acts as distributed inductors. The space between the walls acts as capacitors
Arcing is an occurence in which electricity from one conductor jumps to another, causing a flash of electricity
SF6 is a dielectric, which acts as an insulator preventing the arcing from discharging in the waveguide
Why must the accelerating waveguide be kept in a vacuum?
Fill gas would cause energy degradation and unbunch the electrons
What does BEIR stand for?
National Academy of Science Biological Effects of Ionizing Radiation
Where does data for the BEIR four risk assessment vs radiation models come from?
Most of it is from atomic bomb survivors
But the BEIR study includes data from people exposed for medical reasons and nuclear workers
Why do radiaiton risk assessment models disagree in the low dose region?
Because at low doses, the probability increase for radiation induces cancer is so low, that it is very difficult to distinguish against all other environmental and genetic factors
What are some cellular defense mechanisms to radiation?
Activation of enzymatic DNA repair
Apoptosis (programmed cell death)
Cellular signaling of radiation damage to initiate nearby cells to activate defenses and attack damaged cells
Why does the kq curve in TG-51 decrease for increasing %DD(10)x?
%DD(10)x increases with increasing beam energy
As beam energy increases, the water to air stopping power ratio decreases, which reduces kQ
What is point of measurement for a cylindrical ion chamber?
Simply the central axis of the chamber
Why is EPOM used to determine kQ?
Minimizes impact of gradient effects within the sensitive volume which impacts depth-ionization curves
Why is POM used for the dose output portion of TG-51 and not EPOM?
Because POM is used to align the chamber at the ADCL, not EPOM
You and a small team have completed work and prepared an initial draft of a paper to publish. One of your team members suggests that you seek out a freelance editor to help edit the paper. What do you do?
Is the freelance editor allowed to be listed as an author?
Hiring a freelance editor is appropriate, however, you must take appropriate cre to ensure that no confidential information is disclosed to the editor
Assuming the editor’s only role was to edit the draft, they should not be listed as an author. However, they should be acknowledged as a contributor to the manuscript
After publishing research, your research nurse who was responsible for recruiting human research subjects is angry you did not include them as an author or acknowledge in the work. How do you respond?
If all the nurse contributed was recruitment, that is not grounds for authorship. However, they should absolutely have been acknowledged in the final paper.
In this scenario, you should express your apologies for excluding them in the acknowledgements and emphasize their importance in the study, while also being firm and respectful in your decision that they should not be listed as an author
When publishing a paper, what are some frequirements for someone to be listed as an author?
- Contribute substantially to the concept or design of the study. Including data acquisition, analysis, OR interpretation of analysis
- Draftng the work or revising it critically for important intellectual content
- Approval of final version to be published
- Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of the work are appropriately investigated and resolved
What is an honorary authorship and when is it appropriate?
An authorship conferred without fulfilment of the authorship criteria
Honorary authorship is never appropriate
Per 10 CFR 20, what are the requirements for a worker to have to have their radiation dose monitored?
If personnel is likely to receive more than 10% of their annual exposure limits
Or if they declare pregnant
Or any individual entering a high or very high radiation area
Ring badges are different, and are usually only worn by individuals who handle radiation sources or work in fluoroscopy
What information is needed to demonstrate that a group of workers does not require radiation monitoring?
Demonstration that none of the workers is likely to receive 10% of dose limit
This is demonstrated by use of area surveys, or previous experience monitoring workers in similar environments
A calculation may also be performed
What are the ALARA levels and the follow-up process?
ALARA 1: 10% of quarterly limit exceeded. Should be brought up at next RSC meeting, and discussions should be had if that level of exposure is appropriate for that individual. If so, no further action
ALARA 2: 30% of quarterly limit exceeded. Investigation by RSO to determine cause of exposure, in addition to actions that may be taken to mitigate future exposure. Results should be reported to RSC
ALARA 3: 40% of quarterly limit exceeded. RSO will supply RSC with written documentation about how to reduce dose
ALARA 3b: 70% of quarterly limit exceeded. RSO required to take action to change person’s work environment until end of the year
Will increasing the threshold dose in a monte carlo calculation increase or decrease computation time?
It should decrease computation time.
Threshold dose is used to assume the point at which a particle is so low energy, it cannot escape a given voxel and thus deposits its remaining energy into said voxel. Increasing threshold dose will decrease accuracy, but also decrease treatment time since you’re ending particle histories a bit earlier
Up until what point in a particle path does the phase space file have pre-calculated monte-carlo code for and why?
Up until the ion chamber
Anything above the ion chamber is assumed to be constant for every patient
True or False
During commissioning, the phase space file is tweaked to closer match your individual machine
True
In what scenario can you skip the need for daisy chaining in small field dosimetry?
If you have a detector that is calibrated to give dose in both the reference condition and the non-reference small field condition
This usually doesn’t exist, since calibrated detectors are typically ion chambers which are not good for small field dosimetry
What is the rationale for the size requirement for a reference class dosimeter in TG-51 addendum?
Chambers below 0.05cc are expected to have higher noise, and unuaul polarity and recombination behavior and are also typically impacted by irradiation history
In MCO, what does “Pareto-Optimal” mean?
A plan is considered “Pareto-Optimal” when it is not possible to improve the plan with respect to one objective without worsening it with respect to another objective. That is, when the plan is as good as it can be without making a trade-off
In MCO, what is a “Pareto-Surface” and how are “Pareto-Optimal” plans generated?
A “Pareto-Surface” is the st of all Pareto-optimal plans which are generated by optimizing, usually by direct machine parameter optimzation, to produce a set of Pareto-Optimal plans
These plans are then interpolated providing the set of plans along the surface
What are some responsibilities of a peer reviewer of a manuscript prior to publication?
- Must assure that the quality of published work meets journal’s standards
- Provide an unbiased, independent, critical assessment of the manuscript
- Must be technically proficient in the area they are reviewing
- Should provide feedback that is constructive and polite, while also being honest and forthcoming
- Should not be influenced by conflicts of interest
In peer review of a manuscript, is the system single or double blind peer review?
Double blind
The author doesn’t know who the reviewer is
The reviewer doesn’t know who the author is
This allows for honest feedback without fear of retaliation, while also limits the personal bias a reviewer may have of the author
What is “duplicate publication” of a manuscript?
Submission of substantially similar manuscripts and publication of said manuscripts multiple times or the simultanous publication of a single manuscript in multiple journals
Does a publication based substanially on a abstract presented at a conference constitute duplicate publication?
No
This is generally acceptable “secondary publication.” But it is recommended that full disclosure of the original abstract be in the final publication
Who must be present in a RSC meeting?
RSO
Member of nursing staff
Representative from management who is neither RSO nor AU
And an AU for each byproduct material listed on the license
What typical occurs in a RSC meeting and how often do they meet?
RSC typically meet quarterly
Most meetings include…
* Review of minutes from previosu meeting
* Review exposure or incidents
* Review recent and upcoming radiation safety program events, surveys, etc
* Update roster of committee members
* Plan for institutional and equipment changes
Per 10 CFR 25, what sites are required to have a RSC?
Any site that is authorized for two or more different types of byproduct materials
So I site only involved in I-131 treatments, does not need an RSC, BUT still needs an RSO
What are requirements for an RSO?
- Be certified by an NRC approved specialty board
- Hold a masters or doctorate degree in physics, medical physics, physical science, engineering or applied mathematics
What affect does electron beam obliquity have on…
Depth of max dose
Therapeutic depth
Dmax
Surface dose
Practical electron range
Increased obliquity will…
Decrease depth of max dose
Decrease therapeutic depth
Increase Dmax
Increase surface dose
Increase practical electron range
Label this diagram
True or False
The Radiological Physics Center has compared the use of plane-parallel and cylindrical ion chambers for reference dosimetry in 5 MeV and 20 MeV and found no measurable difference
True
Can plane parallel chambers be used to calibrate photon beams?
In principle yes, but in practice no since kq values are not listed in TG-51
The addendum also recommends that only cylindrical chambers be used as there is insufficient information about wall correction factors in photon beams other than Co-60
Can plane parallel chambers be used to measure beam quality in TG-51?
Yes
In fact they may even be BETTER than cylindrical chambers
Why not send a plane parallel chamber to ADCL?
Because plane-parallel chambers can be very sensitive to small changes in their constructions, including those can can occur due to rough handling during transportation
Why should the highest energy electron field be used to cross calibrate plane parallel chambers?
To minimize gradient effect across cylindrical chamber
Your rad onc department has made a deal with a nearby hotel to provide rooms to patients undergoing RT at a reduced rate. The deal requires the hotel to be listed as the “preferred hotel” and as a radiation oncology sponsor in any patient handouts.
Is this a conflict of interest. Are there any ethical issues with this arrangement?
This does qualify as a conflict as interest as there is an ongoing financial relationship with the hotel
This may or may not pose an ethical issue. Context matters. If the arrangement provides a real benefit to the patients by lowering the cost of stay and still allows for hospital to provide information about other hotels, than this is acceptable. But if anything is done to place the patient at potential financial risk, it becomes unethical
What is stark law?
A US Federal law preventing physicians from self-referral. That is, referral to a service in which themselves or a family member has a financial relationship with
How does mutual information image registration work?
Seeks to pair voxels of a given intensity in image A, with voxels of a corresponding but uniform, but not necessarily similar, intensity in image B.
That is, it is able to consider matching all light voxels in image A to dark voxels in image B, provided that the shape and size of the voxel areas is similar
Why should you not use collimator 0 for VMAT?
Because the interleaf leakage is in the same plane as the rotation of the arc. This will yield larger leakage radiation in plane
In short, what is the belmont report?
A report that defines ethical principles and guidelines for research involving human subjects
What are the three principles of the Belmont Report, and what do they mean?
- Respect for personons
Protecting autonomy of all people and treating them with courtesy and allowing for informed consent - Beneficence
Do no harm. Maximize the benefits of the research while minimizing the risks - Justice
* Ensure reasonable, non-exploitive, and well-considered research procedures
* Ensure fair distribution of costs and benefits to potential research participants
* Ensure research participants are treated equally
What are IRBs?
Institutional Review Boards
Committee that oversees research, especially that involving human subjects. Are responsible for assuring research on human subjects is conducted in a manner that is scientific, ethical, and comply with all relevant regulations
A patient who received Pd-103 permanent seed prostate implants three days ago calls in and tells you that he has urinated a seed into the toilet at home. What do you do?
What about if instead of calling, they show up at your clinic carrying the urinated seed?
In scenario A, there’s really nothing to do. Pd-103 is very low energy, so the toilet water will absorb pretty much all the dose. At that point, flush the seed in the toilet. It may be beneficial to get an idea of the scenario from the patient as well, how many seeds?
In scenario B, you should place the seed in a shielded container and allow it to decay for 10 HLs prior to disposal. You should also ask the patient how they carried the seed, and inform them of any side effects and increased risk of exposure they may expect
How common is prostate seed loss post-implantation?
Approximately 9% of LDR prostate patients experience seed migration.
Of these 9%, 75% are lost through the urethra, 25% become lodged in the lungs after migrating through prostativ veins
For an electron beam, is mean energy the same as nominal energy? If not, how do you find mean energy?
No, mean energy is not the same as nominal energy. Nominal energy is the electron beam mean energy as it exits the bending magnet. However, the electron beam will degrade through the scattering foil, air, treatment head, and cone. Along this path, the mean energy will drop
Mean energy is defined at patient/phantom surface, and is found by 2.33*R50
For an electron beam, what happens to PDD when the field size drops below the size where lateral charged particle equilibrium becomes lost?
Depth of max dose is reduced
Relative skin dose increases
Decrease the slope of dose falloff
What are these three types of bending magnets and which manufacturers use which ones?
- 90 deg bending magnet, not commonly used
- 270 deg bending magnet, used by Varian
- Slalom type bending magnet, used by Elekta
What is this a schematic of? Label the components
Well chamber
What TG report gives recommendations for well chamber QA? What are some general tests you would perform on a well chamber?
TG-40
Most recommended tests are done initially when the well chamber is acquired, or after repairs of the well chamber. These include (do not memorize these)…
* ADCL calibration
* Precision
* Linearity
* Collection efficiency
* Geometrical/length dependence
* Energy dependence
* Source wall dependence
* Venting
Others include a redundancy check, which is done with another well chamber whenever you would use them (source exchange), tolerance is 2%. Another is a leakage check which is done whenever you would use the chamber (source exchange). These you should probably know
Which well chamber do you have, what is the collection volume, what bias do you apply, how long does it take the chamber to stabilize after connected to well chamber? What is the outside body made out of? Is it vented or sealed?
HDR 1000 Plus (standard imaging)
245 cc
300 V
10 mins to stabilize after being plugged in
Outside body is aluminum
It’s vented (requires TP correction)
What are some advantages to MOSFET detectors?
Extremely small volume
Minimal perturbance of underlying dose distribution
Allowfor instantaneous readout
Some MOSFETs provide permanent dose record
What are some disadvantages to MOSFETs?
Finite lifespan of about 100 Gy
Strong temperature dependence
Energy dependence
Sensitivity changes with integral dose as trap centers fill
What does MOSFET stand for? How does it work?
Metal Oxide Semiconductor Field Effect Transistor
Consists of three leads, a source, the drain , and the gate
When voltage applied to the gate is too low, the region between the drain and source contrains excess electric charge, peventing current flow. When voltage applied exceeds threshold voltage of the gate, current is able to flow. When a MOSFET is irradiated, holes are created in a SiO2 layer underneath the gate, separated out, and increase the threshold voltage of the drain
Per 10 CFR 35, what criteria classify a medical event?
An event not caused by patient intervention in which dose differs from prescribed dose by more than 50 mSv effective dose equivalent, 500 mSv organ or tissue dose equivalent, or 500 mSv shallow dose to the skin
AND one of the following
* Total dose delivered difference of 20% from prescription
* Single fraction dose delivered difference of 50% from prescription
* Wrong radioactive drug was administered
* Wrong route of administration
* Wrong individual received the intended treatment
* A leaking sealed source
* Wrong mode of treatment
Following a misadministration, how should you interact with a patient?
Physician determines when/if to inform the patient
Physicist may be brought in the consult with patient regarding…
* Nature of error
* Expected impact of event
Physicist must be careful to speak only within their scope of expertise, and provide contact information should the patient have questions later
It is important to speak honestly but with empathy for the emotions of the patient
Following discover of a medical event, what must you do per NRC regulation?
- Noticy NRC operation center no later than the next calendar day
- Inform physician within 24 hours of discovery
- Physician then informs the patient at their discretion
- Generate a written report and submit to NRC within 15 days
What do pacemakers and defibrillators do?
Pacemakers send small electrical signals to the heart muscles that regulate heartbeat for treatment of bradycardia (low heart rate) or irregular hearbeat
Defibrillators use a capacitor to deliver a high voltage shock to the heart when the device detects ventricular tachycardia (abnormally fast heartbeat)
What would placing a magnet ontop of a defibrillator do?
Deactivates tachycardia sensing mode of the the defibrillator
This is important because during RT, the dose rate may increase reading of the defibrillator, and trick the defibrillator into thinking tachycardia is occuring, thus false shocking the patient
A patient is planned to receive RT but has a CIED, what are some steps you should follow to ensure safe delivery?
Identify type and model of device, and contact vendor for guidance/recommendations
Contact cardiologist or electrophysiologist to get status of dependence of patient, if a magnet can be placed, and when most recent interrogation occured
Do not treat beams entering or exiting device
Use energies 10 MV or lower
Keep cumulative dose below vendor and TG 203 recommendations
Follow TG 203 guidance to estimate dose to pacemaker, and to classify risk level for patient
Given risk level, follow TG 203 and clinic specific policy for practices during treatment course, including device interrogation scheduling with cardiology team
What are some advantages to EPID imaging as opposed to kV planar OBI imaging? What about disadvantages?
Advantages:
* Uses actual treatment geometry
* Displays block/MLC positions relative to treatment anatomy
* If used as during imaging, that is, delivered with the beam, will contribute no extra dose to the patient
Disadvantages:
* Worse contrast
* If not delivered with the treatment field, it contributes more dose than kV planar
How does EPID device work?
Most common design consists of a metal copper plate (1 mm thick) which attenuates the photon beam, emitting electrons
These electrons are incident on a phosphor screen which emits scintillation photons
The scintillation photons are detected by an amorphous silicon panel which converts photons to electric signal
Per TG-142, what QA tests should be performed on OBI?
Daily
* Collision interlock functionality
* Positioning/repositioning accuracy
* Imaging and treatment coordinate coincidence at single angle
Monthly
* Imaging and treatment coordinate coincidence at 4 cardinal angles
* Image scaling
* Spatial resolution
* Contrast
* Uniformity and dose
Annual
* Full range of travel SSD
* Imaging dose
What does flatness and symmetry look like when there is an angular error in beam steering? What about positional?
What does flatness and symmetry look like when there is an angular error in beam steering? What about positional? Draw it out
In computer programming, what are FLOPS and what is clock speed? What is a floating point operation?
FLOPS are floating point operations per second, which is the measure of the computation speed of a computer
Clock speed refers to the rate at which logic gates are able to change state
Floating point operations refer to arithmetic operations using formulaic expressionf of real numbers. These are similar to scientific notation. It requires less memory for very small and very large real numbers, resulting in less memory requirement and quicker operations. However, it also introduces rounding error as they are approximations of real numbers
How do traveling waveguides and standing waveguides operate? Which venodrs use which waveguide? What are pros and cons to waveguides?
Traveling Waveguide
EM wave progresses longitudinally through cavity, any remaining energy of the target end of the waveguide must be absorbed. Electrons are accelerated down waveguide in manner similar to surfer.
Pros: less expensive
Cons: Generally longer and less stable
Vendors: Elekta
Standing Waveguide
Microwave electric field changes its amplitude vs time, rather than its position. Energy remaining at the end of the waveguide is allowed to reflect backward. Forward traveling and backward traveling waves produce a net stationary waveform which oscillates in sign vs time. Nodes (which are cavities with zero electric field at all times), are fixed and never provide any acceleration and can be side coupled which allows for a shorter waveguide. The microwave still passes through these side couples on it’s way to the rest of the cavities, however the electron doesn’t have to, meaning waveguide can be shorter.
Pros: Shorter, more energy efficient, more stable beam
Cons: More expensive
Vendors: Varian and Siemens
How does a traveling wave vary accelerating energy?
By increasing or decreasing the number of electrons in a bunch
Electrons must share the microwave energy present. So more elctrons means less energy per electron
For H&N CT sim, how far sup/inf should scan extend to?
Superior border is top of skull
Inferior border is at least 5 cm beyond PTV (typically T7ish is more than enough)
For any treatment, what are some considerations you may want to think of when deciding if a replan is necessary?
- When do changes occur? How early in the course are we?
- Is surface changes greater than 1 cm? How much of an effect will that make on raw TMR calcs?
- Are anatomical changes causing target and/or OARs to move relative to PTV and/or OAR contours?
- Are we losing ability to reproducibly immobilize/position patient?
What requirements does HIPAA place on a PACS system?
HIPAA requires that a PACS system has procedures to protect and secure patient data
That means that the system is secured against inappropriate access (Ex. password gating) and that date should be encrypted.
It also requires that data must be secured against unintentional or intentional destruction, usually via off site backup and/or offsite DICOM server mirroring
What is the difference between data mirroring and data backup?
Both are ways to store copy of data, main difference is time frame
Backup is a scheduled task in which data is copied at regular intervals
Mirroring is a near real time, as data is stored, moved or deleted from the primary server, the mirrored data is a near exact copy of the main server data at any given time
What is the difference between direct machine parameter optimization and fluence map optimization?
Fluence map optimization creates a fluence map which produces a dose distribution that minimizes cost function to true global minima. The planning sytsem then attempts to create setup points to produce the fluence map. The cons of this are that the optimized fluence map may not match the actual planned fluence map due to machine limitations. The pro is that it is a quicker optimization calculation
Direct machine parameter optimization considers machine parameters during the optimization process. It’s slower, and will lead to local optimal solutions, requiring simulated annealing. However, the resulting fluence map and control points from the optimization plan will better resemble the deliverable plan
What are the two main optimization types? Which do we use in Eclipse?
Analytical Method and Iterative/Stochastic method
Iterative/Stochastic method is used in Eclipse
Are there any exceptions to the adult non-pregnant occupational dose limits per 10 CFR 20?
Yes, two types of exceptions
- Planned special exposures, the limit being 50 mSv added to the annual limit in which those 50 mSv come from the planned special exposure itself. Worker must be informed about the dangers and scope of their work
- Emergency action exposure. These are NOT regulated by the NRC, provided that they are necessary to protect health and safety. EPA gives a recommendation of < 100 mSv to protect infrastructure, and < 250 mSv for life saving activities, but these are guidelines not regulation
Linac vaults are designated as high radiation areas, however, the expected dose rate at 1 meter from isocenter exceeds the requirement for a Very High Radiation Area sign. Why then do we use high radiation area instead?
10 CFR 20 provides an exception to this requirement if the room is within a hospital or clinic, access to the room is controlled, and prsonnel take necessary safety precautions
What material are flattening filters typically made out of?
Aluminum
What is typical width of a 10 MV beam penumbra? What about Co-60 teletherapy unit?
10 MV: 6 mm
Co-60 teletherapy unit: 15 mm (due to larger source size)
Linac source size is 1 mm. Co-60 teletherapy unit source size is 1-2 cm2. Which greatly increases geometric penumbra, even despite shorter SAD
Does penumbra increase or decrease with increasing electron beam energy?
Decreases with increasing energy
What is a bridge made of fetus shielding made out of? What are pros and cons to using one?
5 - 7 cm of lead, which is about 4 - 5 HVLs
Pros: Reduces fetal dose by > 50%
Cons: difficult to construct and increases treatment time
You’ve treated an unknowingly pregnant patient with a tangent breast plan for 3 Fx after discovering they are pregnant. What do you do next?
- Estimate dose to fetus and have an open and honest conversation with the patient regarding potential risks to the fetus
- Consult with physician regarding if remainder of treatment can be pushed until after baby is delivered. Consider how far along the patient is and how aggressive the disease is
- If plan will be continued, consider planning strategies to reduce dose as highlighted by TG 36. Including shielding bridge and dose monitoring
What is annual limit on intake (ALI), what is it based on, where would you find this limit?
ALI is limit of amount of radioactive mateirla taken into body by inhalation or ingestion per year
It’s determined on a nuclide-by-nuclide basis such that the “reference man” would receive a committed whole body dose equivalent of 50 mSv/yr or a organ/tissue of 500 mSv/yr
Found in 10 CFR 20 appendix
What is the “reference man” used by the NRC?
A concept used in dosimetric calculations by the NRC and ICRP
It’s a standardized, caucasian male between ages of 20 - 30 with an average weight and height
How is TERMA calculated for dose calc algorithms using voxel kernels?
A model of the primary photon fluence incident on the patient is generated for each beam/control point
Ray tracing projects the fluence distribution through the patient
TERMA is then computed as the product of fluence and mass attenuation coefficient through a given voxel. That product is inherently the energy released in that voxel
In simplest terms, what is a Kernel?
A kernel represents the energy spread resulting from radiation interactions at a given point
Do convolution calculations account for beam hardening? If so, how?
Yes
Multiple kernels make up a single kernel. These kernels correspond to different photon energies, and have relative weighting depending on the depth and expected energy spectrum at a given depth
What two techniques does AAA utilize to improve accuracy of convolution dose calculations?
Superposition
Kernel tilting
What does CCC do differently than standard convolution algorithms in order to speed up calcs?
It models the kernel as a finite number of rays emitted from the point of interaction, rather than as a continuous 3D object
Results in a loss of accuracy far from point of interaction
In a klystron, how is microwave produced at the catcher cavity?
As electrons reach the catcher cavity, they induce a retarding electric field, which causes additional electrons to decelerate quickly
This deceleartion releases energy at the microwave frequency at a much higher amplitude than that input into the buncher cavity
What are the advantages of Magnetrons vs Klystrons? What about vice versa?
Magnetrons > Klystrons
* Smaller
* Simpler
* Cheaper
* Produce their own microwaves
Klystrons > Magnetrons
* Have longer life expectancy (approx 5x longer)
* Can produce a higher power output
What are the four stages of acute radiation syndrome?
- Initial onset of symptoms
- Temporary reduction in symptoms
- Increase in severity of symptoms (most severe stage)
- Death or recovery
Is GI acute radiation syndrome fatal?
Most likely yes. Especially without immediate medical intervention
Above 8 Gy whole body exposure, the probability of survival is very very low
Cause of death is most often infection due to leukopenia (drastic drop in disease-fighting leukocytes). Bone marrow transplants can improve survival chances
How do TLDs work? What are they made of?
TLDs are crystals doped with lithium fluoride
Irradiating TLD creates electron-hole pairs
Electrons are carried into the conduction energy band to electron traps, caused by impurities in the crystalline structure
Holes migrate to a hole trap via the valence energy band
The pairs are stored in their respective traps until readout
Readout is accomplished by heating the TLD which imparts enough energy to the electrons that they are able to escape their trap centers
Once free, the electrons enter recombination/luminescent center swhere theye mit luminescent photons
The photons are measured by a PMT or camera
Is TLD response (calibration curve) impacted by reuse (annealing) process? Why or why not?
Yes
Competitive centers (trap centers that do not contribute to luminescence) fill up through TLD exposure, but are not emptied by annealing
This means that with each TLD use, there are less competitive centers, which means the sensitivity of the TLD increases (supralinearity)
What are the pros and cons to OSLDs vs TLDs?
OSLD Pros
* Can be read out multiple times
* Fade much less than TLDs
* Readout and annealing is quicker
OSLD cons
* Made of an aluminum oxide, which is less tissue equivalent than TLDs, meaning more energy dependence
What is SQL?
A domain-specific language used to manage actions within a database
What is TD5/5 and how is it used in RT?
TD5/5 is the dose delivered to normal tissues that causes a 5% adverse reaction rate over 5 years
This dose level is often chosen as the dose level to evalute therapeutic ration in RT studies
Why does LDR have a better therapeutic ratio than HDR?
because normal tissue, which has a lower alpha/beta, benefits more from sublethal repair than tumor cells
What is a parallel organ
An organ in which many or all of the sub-units must be disabled to cause organ failure
What is EUD?
Equivalent Uniform Dose
It’s defined as the absorbed dose that, if homogeneously delivered to an organ, causes the same expected number of cells to survive as the actual non-homogenous absorbed dose
For EUD calclation, what alpha value is used for a serial organ? What about a parallel organ? What about your target?
Serial organ: alpha around 10 (approximates max dose constraint)
Parallel organ: alpha of 1 (reduces to mean dose)
Target: alpha of negative value
Which report is the reason why we measure output for 60 deg EDW daily? What is the tolerance?
MPPG 8
This test is not mentioned in TG-142. Only to test functionality
Tolerance is +-3% from baseline
What is the increase in out of field dose for physical wedges and EDW?
Physical wedges can increase out of field dose by a factor of 2 or more depending on angle and mounting position
EDW barely increases out of field dose
What phantom is this? Which module is used for spatial integrity testing? How many different plugs are in image B?
- This is the ACR 464 CT Phantom
- Module D is used for spatial integrity (in addition to uniformity). It’s really hard to see, but there’s two small objects that are diagonally 10 cm away from each other
- Module B has 5 different plugs. There’s one for water (HU 0), so you can’t see it
What is modular transfer function? How is it used? What’s a typical threshold value?
MTF is a measure of a detector system’s ability to render contrast as a function of spatial resolution
MTF is used as a metric in spatial resolution in QA and manufacturer spec
MTF = 0.1 is generally taken to be limit of detectability of humans although manufacturers often quote an MTF of 0.05 or less in their spec
What CT/CBCT beam or viewing parameters affect contrast resolution?
kVp (decreases kVp increases contrast, but also noise)
mAs (increasing mAs decreases noise)
Slice thickness (increase slice thickness decreases noise)
Window and leveling during analysis
What criteria determine how small a detector should be for adequate small field dosimetry?
Per TRS-483, needs t be small enough such that volume averaging correction for measured field sizes < 5%
Per TG-10, needs to be small enough such that field output is uniform within 1% over detector sensitive volume (much more strict than the TRS-483 definition)
This condition is met when Detector width <= FWHM - 2 x rlcpe
What is “fault tolerance” in the context of data transfer?
Ability of a system to prevent a data transfer error from reaching the patient
What are some tools and literature recommendations you can use to ensure your data transfer program is “fault tolerant?”
First identify potential points of failure and quantify their potential impact using TG 100 workflow
Then, weaknesses in the system should be addressed either by eliminating the potential source of error, or where that is not possible, via a robust QA and QC program
TG-201 recommendations
What is the difference between QA and QC in the context of IT systems?
QA is a program that ensures that the IT system, and each of its subsystems, is performing according to spec and intent of physicist
QC is a process designed to ensure that an individual patient’s treatment meets intentions of the physician prescription
What are some components of a good IT QA program?
- Acceptance testing of each component of the system
- Commissioning of each component of the system to assure proper configuration and function within clinical environment
- End-to-end testing of data communication along clinical workflow
- Annual subsystem QA, often similar to commissioning, to assure that the subsystem was not unintentionally changed since initial commissioning
How do you perform QC of data transfer?
Two types of QC
- Automated checksums to ensure data is the same before and after a transfer occurs
- Human visual checks of data transfer (time outs, weekly chart checks, etc)
Under what conditions can a dosimeter be used to measure outut of the reference field, and also a 2 x 2 cm2 field? Are there any dosimeters that work for that?
What about for 0.5 x 0.5 cm2 field?
Conditions you require are that…
1. Chamber is a reference class chamber as delineated by TG-51 (so must be an ion chamber)
2. Chamber has minimal small field energy spectrum dependence
3. Chamber is small enough to limit volume averaging to less than 5% per TRS 483 or 1% per TG 106
It’s very difficult for a chamber to meet all 3 of these conditions. The smallest reference chamber that does meet this condition is the Exradin A26 with a sensitive volume of 4.3 mm3 which may be small enough for 2 x 2 cm2, but not for 0.5 x 0.5 cm2
What is the difference between daisy chaining and intermediate field size method for SFD?
Daisy chaining does not utilize correction factors and is therefore not recommended by TRS 483, it’s simply a multiplication of measurement ratios.
The intermediate field method does use correction factors to effectively do…
Output factor of small field to intermediate field using SFD x output factor of intermediate field to reference field using reference ion chamber
Why are two dosimeters recommended for SFD instead of just one?
Because correction factors can be large and somewhat uncertain across the range of field sizes in which reference chambers can be suitable
What do each of these factors mean?
Sk is the air kerma strength of the source
Delta is the dose rate constant expressed per air kerma strength hour
GL is the geometry function which accounts for the inverse square law only
gL is the radial dose function which accounts for dose fall off due to scattering and attenuation along the transverse axis
F is the anisotropy function which accounts for variation in dose with angle relative to transverse plane
Does your TPS use the 1D or 2D TG-43 formalism? Why is there no 3D formalism?
Most TPS use the 2D formalism
There is no 3D formalism because sources are cylindrically symmetric, meaning rotations along the long axis do not impact dose
How does NIST measure Air Kerma Strength?
Using a WAFAC chamber with seed placed 30 cm from source. The beam passes into the chamber via a8 cm circular aperture. Aluminum filter is placed in aperture to remove low energy photons from source encapsulation. Source is rotated to average out radial asymmetry. Source placed on plastic holder to minimize scatter
End result is a seed with a known air kerma strength
How many detectors would be best to use for small field measurements?
Ideally two if you have two available
SFD is prone to error, so making two sets of measurements can allow you to cross check your values
Most people don’t do this though
What is the 2019 intersociety conference statement of professionalism? What are some key points? Why is it important to us?
It’s a statement that the ABR requires that all diplomats should abide by to maintain their certification
Key points include…
* All members should feel valued and respected
* Foster a collaborative and inclusive culture
* Non-discrimination or harassment
* Freedom from harassment
* Qualified freedom of speech
* Advocate for those who cannot advocate for themselves
What ethical principle is generally regarded as the most important for a medical professional?
To always hold paramount the interests of patients in all circumstances
What is TG 109? What does it talk about?
Code of ethics for AAPM
Talks about professionalism, ethics, and authorships/research publication practices
What department governs NJ state law on radiation safety?
Department of Environmental Protection Radiation Protection Element
When comparing TG-43 calcs to a model based algorithm, what scenarios should you calc side by side?
First, start with a liquid water phantom. Both calcs should be very close. Tolerance per TG 186 is 2%
Then move onto simple heterogeneities
Then side by side with actual patients
Per TG 186, when should you switch to model based for dose calculations alone, as opposed to TG-43?
NOT YET
There is not yet enough data relating prescriptions with TG 43 vs model based, and dose to water vs dose to medium calcs
TG 186 recommends for now not making the switch until more data is available
What NJ department is responsible for radiation safety regulations?
New Jersey Department of Environmental Protection
Bureau of Environmental radiation
What does SBAR stand for?
Situation (what happened)
Background (context for how the situation may have happened)
Assessment (what the potential impact was)
Recommendation (what steps can be taken to reduce likelihood of future occurence)
What does ROILS stand for?
Radiation Oncology Incident Learning System
Sponsored by both AAPM and ASTRO
What kind of information should be documented in an incident learning system?
- Incidents (in which an error reaches a patient)
- Near misses
- Unsafe conditions
- Unexpected failures of equipment
What are some benefits of enrolling in an incident learning system?
- Contribute to a national database and collectively improve the field of radiation oncology.
- Track and review internal incidents, near misses and unsafe conditions.
- Receive legal protections from discovery afforded by the Patient Safety Act for information that meets necessary requirements.
- Receive regular education based on events reported throughout the country, including suggestions on how to prevent errors.
- Receive practice-specific reports.
- Gain access to analysis tools within the RO-ILS Portal.
- Receive regular program education and announcements about features and tools, best practices and general patient safety initiatives.
- Learn from RO-ILS user meetings and webinars.
- Meet the requirements for up to two Improvement Activities in Medicare’s Merit-based Incentive Payment System (MIPS).
- Meet the requirement to participate in a PSO within the radiation oncology alternative payment model (i.e., the “RO Model”).
- Meet components of accreditation programs, including ASTRO’s APEx® program.
- Meet criteria for a practice quality improvement (PQI) toward the purpose of fulfilling requirements in the ABR Maintenance of Certification (MOC) Program.
But the big one is really, provides a tool to help work towards safer and more effective treatments and workflows for your patients and clinic
How is dref related to R50?
dref = 0.6R50 - 0.1
Why do we measure at dref for electron beams?
Minimizes dose gradient across sensitive volume of ion chamber
What is this? How is it used?
This is a source positioning ruler
- Device is attached to aferloader transfer tube
- Dummy cable is extended to a known position, and this position is verified using the in-room camera system
- The brachytherapy source is then extended to a known position and verified using the in room camera system
What is the NRC requirement for source positioning accuracy of a HDR afterloader (frequency and tolerance)
+- 1 mm tolerance
Done morning before treatments
After source replacement/significant repair
And ATLEAST quarterly
What guidance documents would you use in settling up an HDR brachytherapy QA program?
10 CFR 35
TG 59 (HDR Brachytherapy treatment delivery)
TG 56 (code of practice for brachytherapy physics)
TG 40 (comprehensive QA for rad onc)
TG 41 (remote afterloading technology)
How do you measure output factor for an electron beam?
A / B
Where A is the dose per MU measured at dmax, with the clinical condition, that is with the cutout in, at the SSD that will be treated, and with the cone that will be used
B is the dose per MU at dmax, with a 10 x 10 cm2 open cone at 100 SSD (calibration condition)
Note also, same dmax is used. In reality cutout dmax might be slightly different than reference dmax at CAX, but this effect is small
An alternative is that you can measure all applicator factors at 100 SSD, and apply a effective SSD inverse square correct in addition to the applicator factor at 100 SSD. But in practice its easier and more accurate to simply measure the applicator factor at the clinical SSD
Remember: OF = AF x CF
Is it possible for a cutout factor to be greater than one for electrons?
Yes
Due to in scattering from the walls of the cutout reaching central axis
If your machine fails a PF test, what is the appropriate course of action?
Step 1: re-initialize or recalibrate the MLCs in service mode
Step 2: retake PF and see if it passes. If not move ontop step 3
Step 3: Call for engineer to inspect and potential replace MLC drive motors
What does the Varian tounge and groove look like? Why is tounge and groove design used?
Interlocks MLC leaf edges, allowing leaves to slide relative to neighboring leaves while preventing gaps between the leaves. Used to reduce interleaf leakage
When must a Geiger counter be calibrated?
Annually and after repair
Whether or not you use it for surveying, it doesn’t matter. U still get it calibrated annually
Why is it not ideal to use Geiger Counters for Linac vault shielding surveys?
- High deadtimes lead to significant understimates of leakage
- GM counters are not able to measure dose
- Most GM designs are not able to detect neutrons
How do you commission a CT simulator?
Essentially it’s all the tests you would perform during a monthly
+ Radiation profile
+ X-ray generator function
+ HU conversion tables
+ shielding survey
+ creation of imaging protocols
+ end-to-end testing
What is arguably the single most important test that you should be checking for a CT simulator that will perform SRS/SBRT planning scans?
Spatial Integrity
How would you perform an End-to-End test on your CT simulator?
Pretty much the same way you would do it for the entire planning/delivery workflow
Take extra care to look for laterality, setup orientation, spatial integrity, and data transfer integrity
Per 10 CFR 35, how often does a source inventory need to be performed? Who is responsible for monitoring radiation source inventory? How long must the source inventory record be maintained?
Every 6 months (NRC)
RSO (NUREG)
3 years (NUREG)
What are some data communication protocols commonly used in healthcare? What are they used for?
DICOM/DICOM-RT - Standardization of digital imaging information and RT plan related information that can be transmitted, stored, retrieved, printed, processed and dislayed across different vendors
Health Level 7 (HL7) - set of international standards for transfer of clinical and administrative data between software applications used by various healthcare providers. Ex. demographic information transfer of Epic to ARIA or Mosaiq
TCP/IP - Transmission control protocol/internet protocol, which is a suite of communication protocols used to interconnect network devices
For Co-60 what is the relationship between TVL1 and TVLe in concrete, steel and lead? What about for a 4 - 30 MV beam?
For Co-60, TVL1 = TVLe for concrete, steel and lead
For polyenergetic beams, TVL1 > TVLe for concrete, and TVL1 = TVLe for steel and lead
For polyenergetic beams, TVL1 in concrete is around 2-4 cm thicker than TVLe
For concrete, the increase in compton scattered component dominates primary beam hardening. For Steel and Lead, beam hardening and scatter are approximately equal contributors to increasing TVL1 and TVLe
What field settings are typically used for TBI…
Dose rate
Energy
SSD
Gantry angle
Collimator angle
- Dose rate at dmax of 10ish cGy/min
- Energies of 10 - 18 MV for uniformity, however reduced skin dose may be undesirable so 6 MV can also be used
- Extended SSD around 4 - 5 meters (also improves uniformity by reducing inverse square law effect)
- Gantry at 90 or 270 deg (for pediatrics treated AP, gantry is 0)
- Collimator rotation of 45 deg
In TBI, what is the desired level of uniformity and what are some things you can do to improve uniformity?
Desired level is +- 10% of midline dose across body
- Some things to help achieve this,
- Extended SSDs
- If lateral setup, add compensating filters to account for thinner areas (head and feet). Compensating filters should be placed several inches from patient to facilitate some level of skin sparing
- AP setup has more uniform body thickness and can be considered if body size geometry allows
Briefly, how does cyberknife work?
Single energy linear accelerator mounted on a robotic arm. Capable of produced a 6FFF photon field for all treatments. System includes an orthogonal kV imaging system with ceiling mounted sources and floor detectors, as well as optical respiratory tracking system which allow for real-time tracking of target position during delivery. X-ray tracking occurs at user defined time increments. Synchrony tracking is realtime using the optical tracking system correlated with the x-ray system to produce model of expected target positioning
Treatment delivered using 30 - 300 non-coplanar, non-isocentric beams at various angles of incidence and couch angles
What collimator options are available in Cyberknife and how does it change collimators?
Fixed collimators ranging from 5 -60 mm. these require manual changing during treatment.
Also a variable collimator called Iris which can produce 12 discrete field sizes ranging from 5 - 60 mm. These can be switched out automatically during treatment
There is also a small MLC called InCise which can produce field sizes up to 115 mm x 100 mm
How is output calibrated for a Cyberknife? What conditions? What relevant TG?
1 cGy/MU using the 60 mm cone at a depth of 1.5 cm, 80 SAD
Vendor recommends using TG-51 protocol, but also provides a physics guide explaining additional recommended steps
To find kQ, measure PDD10 at 100 SSD, 60 mm cone. Correct for the PDD for field size and shape (find equivalent square field) to a 10 x 10 cm2 field at PDD(10)x. Correction factor found in literature
What are some advantages and disadvantages to MOSFETs?
Advantages
* Extremely small collecting volume, making them well suited for SFD and high dose gradient measurements
* Minimall perturb underlying dose distribution, making them advantageous for in-vivo dosimetry
* Allow for instantangeous readout
* Some MOSFETS also provide a permanent dose record
Disadvantages
* Finite lifespan of about 100 Gy
* Strong temperature dependence
* Energy dependence (SiO2 layer is where electron-hole pairs are created during irradiation)
* Sensitivity changes with integral dose as trap centers fill (irrevesable)
What does MOSFET mean?
Metal Oxide Semiconductor Field Effect Transistor
How does threshold voltage of a MOSFET change with increased absorbed dose?
Proportionally
As absorbed dose increases, the number of trapped holes increases, and thus threshold voltage also increases
What is the general construction of a triaxial cable? What makes triaxial cables ideal for measurement of charges?
Three leads, the collector, guard and ground separated by an insulating layer. Colelctor and guard are held at the same voltage, meaning there is no leakage from the collector, making triaxial cables ideal for measurement of small charges
Wher eis posterior fossa located for CSI treatment?
Part of the cranial cavity located between the foramen magnum and the tentorium cerebelli
What are these (top 3)? Why are the ends plastic instead of steel or another metal?
These are applicators for a kV x-ray therapy unit
The ends are made of plastic to reduce the amount of electron contamination, while also providing adequate collimation. It also allows easier setup as target area can be directly seen through plastic collimator
What are these (bottom two)? What are they made of? Why? What is their purpose?
- These are filters for a kV x-ray therapy unit
- Made out of Aluminum for lower energies and copper for higher energies
- They take advantage of the PE to filter out the low energy photons in the spectrum while letting high energy photons pass through. This is useful because the low energy photons increase superficial dose and reduce beam calibration accuracy
- The filter material should have a PE absorprtion edge higher than the tube potential, otherwise you get disproprotionately filtering
You would not want to use a high Z filter (such as lead or tungsten), because they would be so efficient, that they would have to be built incredibly thin, making them mechanically unstable
Per MPPG 9a, what is the minimum staffing to run an SRS/SBRT program? And who is ultimately responsible for the success of the program?
Physicist (responsible for technical aspects)
Radiation Oncologist (responsible for clinical aspects)
Therapists
Dosimetrists
Explain general, direct and personal supervision
General - professional is responsible for training and maintaining competence of the personnel performing a task, and for establishing the procedures and responsibilities. Ex. overseeing daily QA tasks completed by therapists
Direct - in addition to general supervision, professional must also be present in the facility and immediately available to provide assistance and direction during performance of task
Personal - In addition to direct supervision, professional must be physicall present in room during performance of a task.
what equipment is needed to establish an SRS/SBRT program?
Instruments
* Multiple SFD detectors
* End-to-end phantom
* Electrometer
* QC device to measure radiation isocentricity
Planning and Treatment delivery
* Imaging modalities
* Immobilization devices
* Respiratory and motion management devices
* Image fusion and contouring software
* TPS that is commissioned for small field calculation and heterogeneity
* Treatment machine capable of precise delivery
What are some important components of SOPs?
- Responsibilities
- Outlined process for task and parameters (Ex. for simming, slice thicknesses and FOV, for planning, margin selections and calc grid sizes, etc)
- Tolerances involved in task
- Outlining of expected time intervals
- Devices/equipment to be used in process
- Imaging to be used along step of process
- Checklists at every step of the process
Why does Borated Polyethylene (BPE) have both boron and polythylene? What percentage of BPE is boron?
5% of BPE is boron
The polythylene has a high hydrogen content, so it’s great at thermalizing neutrons
Boron has a high absorption cross section for thermal neutrons, so it absorbs them
If you had Boron only, it would be much less efficient to slow down then neutrons
What are the functions of the three layers of a vault door? Which lead layer is thicker, the 1st or 2nd layer?
1st layer of lead attenuates incident photons and also to some extent reduces energy of fast neutrons, amking the BPE layer more effective
BPE thermalizes neutrons and also absorbs them. Emitting neutron capture gamma rays
2nd layer of lead attenuates the neutron capture gamma rays
The 1st layer of lead is thicker than the 2nd layer of lead
What is MLC leaf position offset in the context of modeling? How does Varian account for it?
MLC leaf position offset refers to the difference in position between the MLC setting defined by light field or sensor calibrated to light field, and the radiation field edge (defined by the 50% isodose line). This offset is approximately 0.7 mm for Varian designs
DLG models this offset, but is 2x the offset. So typical DLG value is around 1.4 mm. Eclipse assumes leaf tips are square, and the leaf tip itself attenuates the beam down to transmission factor in a single step, instead of a smooth profile (so 100% –> 1% for open field –> MLC leaf tip).
DLG is the FWHM of the field with the two leaf tips touching each other (gap of 0). During beam calculation, the MLC positions are cropped away from field by DLG/2 to account for the radiation field being a bit past the physical leaf tip
But physical DLG measurement itself is sometimes used as a starting point, and the planning DLG can be adjusted to achieve best agreement with a set of IMRT/VMAT QA measurements
How is DLG measured during commissioning?
- Measure dose of a reference field (10 x 25 cm2)
- Measure dose of the field with bank A blocking, bank B blocking, and average to get a average transmisison factor
- Center MLCs over the reference field, and take repeated measurements using different MLC gaps
- Subtract MLC transmission from each measurement
- Plot corrected measurements (x) and gap size (y) to create a linear fit line
- DLG is the x intercept of the extrapolated fit line
That is, a theoretical corrected charge of 0 should give the DLG
In what scenario might using local normalization in gamma analysis be more favorable than global normalization?
Validation measurements in commissioning
Some errors, particularly in leaf modeling, are more pronounced in modulated lower dose areas. Therefore local normalization would be better at finding these errors
In Gamma Criteria, are min threshold and dose diff normalized by global normalization based off calculated maximum dose, or prescription dose?
Calculated maximum dose
What is the one major limitation of gamma index?
It’s generally less stringent than DTA in high dose gradient regions and dose difference in homogeneous regions
It’s also less striingent in high noise measurements
In theory, assuming perfect dose delivery with no uncertainty, which volume has to receive full tumorcidial dose to achieve local control?
CTV, because this includes all gross and microscopic tumor cells
The ITV and PTV are usually prescribed the tumorcidal doses however as you need to account for the imperfect dose delivery
What is the approximate equation to determine PTV margin creation?
Margin >= 2.5S + 0.7R
S is the systematic component of error
R is the random component of error, may also include internal margin (error accounting for internal movement)
What does ICRU stand for?
International Commission on Radiation Units and Measurements
What are the classes of OARs?
Class 1 - 3
Class 1 refers to when radiation lesions are fatal or result in severe morbidity
Class 3 refers to when radiation lesiosn are mild, transient, and reversible, or result in no significant morbidity
Class 2 is mild
What is PSO (patient safety organization)
A federally recognized entity that collects information about medical errors and safety risks in a confidential and legally protected environment. The gola being to reduce errors, improve quality and promote safety
Ex. includes the PSO used by the ROILS program
What types of legal protections are gained by working with a PSO, and if your institution particpates in a PSO, are you legally shielded from liability related to adverse effects on patients?
Legal protections
* Data provided to a PSO is privileged and not subject to subpoena, discovery, or freedom of information act
* Data provided to a PSO is also confidential
PSO’s do NOT provide a legal shield against malpractice or medical errors directly
Background:
The 2005 Patient Safety and Quality improvement act was created because healthcare providers feared legal liability in the event that medical errors were reported and discvoered. This act allows for aggregation of patient safety data on a national scale, by granting confidential and privileged investigations and reportin gof patient safety events legally to organizations involved in a PSO
What are flattening filters typically made of?
Low atomic number materials. Often aluminum
Besides flattening, what effect does a flattening filter have on beam properties?
- Increased beam hardening
- Increased scatter photon dose and electron contamination (superficial dose)
- Decreased dose rate
How does SSD impact the charge/unit dose measurement of a diode?
Increased SSD, decreases the dose per pulse
For diodes, this causes a decrease in the charge collected per unit dose. This is because charge liberated in the diode may find itself in a recombination center, resulting in electron-hole recombination prior to collection. When dose per pulse is higher, these centers fill quicker, meaning that there are less centers available for recombination and thus less recombination.
So a increased SSD, means more available recombination centers relative to pairs produced, which means more recombination, which means reduction in measured charge per unit dose
**Note: **this is opposite behavior to ion chambers, where recombination in an ion chamber increases as dose per pulse increases
What impacts affect diode response?
- Accumulated lifetime dose: accumulated dose produces additional recombination centers, which reduces diode response over time
- Temperature: increase in response as temperature increases (0.1 to 0.5%/celsius increase)
- Directional dependence: diode design is assymetric which means different amounts of attenuation and buildup vs direction
- SSD (dose per pulse)
- Energy
- Electrometer offset voltage
- Field size (changes in scattering conditions)
What is electrometer offset voltage? How does this impact diodes?
Electrometer offset voltage refers to the fact that when you set a 0V, there is actually some small voltage being applied still
In diodes this can be significant due to their low electrical impendance. Meaning a small offset can cause significant leakage current
How do you determine the required width of a primary barrier?
Project maximum field size to distance of primary barrier, then add 30 cm on each side
Why might it be a good idea to produce (and use) multiple treatment plans for a course of proton therapy?
Proton therapy is greatly impacted by changes in patient geometry
Multiple plans allow the oncology team to select the most robust plan for the patient’s anatomy at time of treatment
Ex. a prostate patient may have significant change in rectum filling, which would greatly impact the radiologic path length of a PA field (but not lateral)
Multifield uniform dose (MFUD) proton therapy plans are sometimes split to deliver only 1 or 2 fields per day. Why is this done in proton therapy by not photon?
Delivering less fields per day reduces treatment time, which is a huge advantage in a proton center where beam availability is limited
In proton beams, a single field can produce a uniform target dose (SOBP). In photon therapy, you need multiple fields to produce a uniform target dose. The therapeutic ratio will be greatly impacted in a photon beam if you treated only 1 field per day.
Describe the design of a kV treatment unit source tube. Why is the anode the way it is?
kV targets are angled to reduce self-absorprtion of Bremsstrahlung photons
The angle also produces a smaller focal spot, while spreading electron impact area and resultant heat across a wider surface area
The anode is also spinning to further increase surface area of heating
Drawback: angled target produces a heel effect in which output at the cathode side of the dose distribution is higher than the anode side (approximately 5% higher, but energy and geometry dependent)
Why might superficial photon beams be preferrential to superficial electron beams?
- Higher surface dose
- Cheaper and require less shielding and staffing than a Linac
- More uniform dose distributions for small fields and oblique/irregular surfaces
What is TG-61? What is this equation? Why is air kerma calibration used instead of dose?
TG-61 is a formalism for calculating dose to water at surface for a low or medium-energy photon beam. The below equation is the in-air method?
- Dose is dose tow ater at a depth of 0
- M is the fully corrected ion chamber reading
- Nk is air kerma calibration factor
- Bw is backscatter facot which accounts for effects of phantom scatter of a kV x-ray beam when the in-air method is used
- Pstem-air is a correction factor accounting for change in photon absorption and scattering between calibration (air) and measurement (air) due to presence of stem
At kV energies, kerma is approimately equal to collision kerma (very low Bremsstrahlung production, 0.1%), and thus also dose
Are ion chambers well approximated by Bragg-Gray theory in the 50 - 300 kV photon energy range? Why or why not?
NO!
- CPE does exist (this is compliant)
- Secondary electrons created inside cavity are deposited locally (this is compliant)
- All electrons causing ionzations in the cavity arise from the phantom mateiral is NOT true. Most arise from the cavity wall
- Secondary electron spectrum is unchanged by presence of cavity is NOT true
What is the purpose of Bragg-Gray cavity theory? What is the equation?
Purpose is to relate dose to medium to dose to cavity filly gas
Dmed/Dgas = ratio of mass collision stopping powers of medium to gas
What are the assumptions of Bragg Gray cavity theory? Which contradict? What assumption does Spencer-Attix and Burlin theory account for?
- CPE or TCPE exists
- All electrons causing ionizations in the cavity arise from phantom material
- Secondary electron spectrum is unchanged by presence of cavity
- Energy of secondary electrons created inside cavity are deposited locally
Assumptions 2 and 3 imply a need for a small cavity, while assumption 4 requires a larger cavity. These contradict
Spencer attix solves condition 4 for small cavities by using restricted mass collision stopping power, which only accounts for secondary electron with energies greater than a cutoff energy. That is, electrons below the cutoff deposit their energy locally, and electrons above are considered in the equation
Burlin forumation expands upon spencer attix and generalized for both large and small cavities by eliminating assumptions 2,3, and 4
Typical cutoff energy for restricted mass stopping power ratio is 10 - 20 keV
What is geiger discharge? What is another name for it? How long does it last?
In a GM counter, townsend avalanches produced a massive amount of ion pairs. The electrons in these ion pairs are collected much quicker than the positive ions, due to the positive ions having a larger mass. Because of this, there is an accumulation of positive charge in the cavity gas which neutralizes the electric field and stops further ionizations from occuring. This continues until the positive ions drift to the cathode and collect an electron, returning to their ground state
This process lasts 10 - 100 ms
This time is effectively a dead time, since no further counts can be registered
What is the difference between geiger discharge and townsend avalanching?
Townsend avalanche refers only tot he cascading ionizations caused in electron induced ionizations
Geiger discharge includes many townsend avalanches and also ionizations caused by UV radiation from excited atoms, which then further cause avalanching
What two types of gases are GM counters filled with and what is their function?
Noble gas - provides chemical stability during repeated ionizations
Quench Gas - reduces false signals generated as the ionized fill gas returns to ground state after a discharge
An alternative to quench gas is external quenching, where voltage is reduced for a fixed perod following a reading
How does a false reading occur?
Positive ions after discharge combine with electrons. The difference in energy in this reaction can ionize in the cavity, and if not quenched, can cause avalanching which gives a signal that didn’t come from radiation
What is this?
This is a standard probe geiger counter. There are also pancake probe and modern pocket geiger counters
Difference designs have different uses based on wall design. For high energy radiation, the wall is primarily responsible for creation of electrons into the cavity. Low energy radiation can interact directly with cavity. It is VITAL to select your GM counter specific to it’s intended use. Different wall materials are better at detecting certain types of radiation
Note also: thickness of wall has no significant impact on response for high energy radiation. This is because, as thickness increases, production also increases, but the probability of electrons reaching the cavity also decreases. These two effects cancel each other out
What is this? Does it produce a continuous or pulsed beam? Is it capable of mono or polyenergetic production? What is typical diameter?
- Cyclotron
- Continuous beam
- Monoenergetic (range has to be modulated by energy degraders)
- 3.5 - 5 m diameter for protons
- Synchrotron
- Pulsed beam (due to time required to cycle magnets)
- Energy modulation is possible
- 8 - 10 m diameter for protons (25 m for carbon)
What are energy degraders made of in proton beam therapy?
What two types of nozzles exist for proton beams?
Energy degraders made of carbon
Nozzle can either container lucite then lead scatterers (double scattering) (scattered beam) or steering magnets (scanning beam)
What types of image guidance are commonly used in proton?
CT on rails
CBCT
Out of room CT using a movable but fixed couch
Room mounted orthogonal kV x-ray imaging
Portal imaging via kV x-ray source mounted along beam path
You notice oil leaking from your LINAC kV source. What do you do?
What QA do you perform after the source is repaired?
What to do?
* Immediately call service and local engineer for recommendations and for them to come in for repair
* Avoid treatment of patients for as long as practically achievable until issue is resolved
* Clean up any spill if oil is not hot
QA to perform after
* Verification of kV and radiation source isocenter coincidence
* Verify image quality of CBCT and kV planar imaging
* Any other QA engineer may recommend
What are the two major proton system vendors and which proton beam generator do they use?
Varian and IBA
They both use cyclotrons
Infact almost all proton beam treatments use cyclotrons. Synchrotron is typically a niche
What is the modular transfer function? What are typical values? What does a high MTF and low MTF indicate? Use this image below as an aid
MTF is a quantitative measure of a detector system’s ability to render contrast as a function of spatial resolution. The value of MTF is in units of % contrast lost. That is, an MTF of 0.2 means you’ve lost 80% of contrast. An MTF of 1 means you’ve lost no contrast
MTF is at a maximum 1, and at a minimum used for imaging of 0.1 (10%), but this minimum can be
A high MTF indicates that the system is able to resolve a given spatial frequency without too much loss of contrast to distinguish high contrast objects
A low MTF indicates that the system loses too much contrast for a given spatial frequency to be able to distinguish high contrast objects
How are bonner spheres constructured? What do they measure?
Lithium iodide scintillation detector or boron-trifluoride proportional counter surrounded by a polyethylene sphere
The sphere thermalizes incident neutrons
These thermal neutrons are then captured in the scintillator, which then emits a scintillation photon. This photon is detected by a proportional counter
Using bonner spheres of varying diameters allows you to measure the energy spectrum of the neutron field
What is the difference between a rem ball and a bonner sphere?
Rem ball is a specific type of bonner sphere with a diameter of 10 - 12 inches
Rem ball has a response that allows for direct measurement of equivalent dose
What are some alternatives to bonner spheres?
For neutron spectrometry, moderated activation foils
For general monitoring, bubble detectors, activation foils and other proportional counter designs
Per NCRP 151, is neutron surveyingrequired for a high energy linac vault constructured of concrete?
Required at vault door
Not required elsewhere. It’s assumed that concrete shielding that can adequately attenuate a photon beam, will also provide sufficient neutron shielding
Explain the two source rule and how it works
The twosource rule treats patient scatter and leakage components of secondary radiation as distinct sources
- If patient scatter and leakage transmission factors are approximately equal, shielding thickness may be taken as the larger of the two barrier thicknesses + 1 HVL
- If the thickness of each source differs by 1 TVL or more, the larger barrier thickness may be used
Per TG-235, is it recommended to use multi-channel or single-channel approach for scanning film?
Multichannel method
Multihannel method allows for correction of a variety of disturbances in the digitized images including…
* Nonuniformities in the active coating on the radiochromic film
* Minimizes Scanner related artifacts (such as lateral position dependence)
* Separate out nondose-dependent abnormalities from the film images
It also reduces random noise in the images
You are creating an interstitial breast HDR brachytherapy plan and you notice that the interstitial needle connectors are placed medially (i.e., requiring transfer tube hookup at patient midline).
Why is this an issue?
What steps would you recommend to correct the issue?
the TGTs will likely rest on the patient’s skin with this setup, and thus increase skin and midline dose to the patient
Some corrective steps would be…
1. Notify doctor of error
2. Place pillow or other spacer between TGT and patient skin during treatment
3. Identify root cause of error and develop procedure to prevent in future
What are some common types of errors associated with HDR?
- TGT errors (failure to correct, breaks in tube, unecessary running of tube over patient anatomy, TGT connected into incorrect applicator or channel
- Inccorrect source acvitity used to adjust dwell times (wrong date at console)
- Damaged applicators
- Afterloader retraction failures
- Errors in planning
For portal image IGRT in the lung, what structure is reliable for use in alignment? Why?
Carina
It’s cartilage, so it’s rigid
It shows up well on portal imaging
It moves very little with respiration
And it can be contoured very accurately during planning
It won’t show well on a DRR, but it shows very well on a portal image
How does a Varian Linac modulate dose rate?
Varian linacs have a fixed pulse rate (energized accelerating cavity)
That is… there are x amount of pulses per second, always
What is being modulated is how many of those pulses are electron-loaded by the electron gun
The Electron gun is a triode design, in which an electrified “grid” at the end of the gun is used to deflect electrons away from accelerating cavity whenever electron-loaded pulse is not desired
Why for a flattened beam, at depths > 10 cm, do you start to see forward peaking profile a bit more? (two reasons)
- Beam at center is harder so is getting attenuated at a lower rate than peripheral
- More scattering towards central axis than there is away from central axis
True or False
For electron beams, there is no clear equivalent square field relationship
True
It’s best practice to directly measure a cutout’s PDD and output
However, if you have rectangular field you can approximate PDD in the following way…
Identify the bony structures shown in these structures. Which MRI sequence is shown?
T2 is shown
Identify these bony structures of the male pelvis
What is this? What is it for? What are key structures of the image? How does it work? What is typical dimension for the length of the cavity?
This is a cutaway view of a standing wave accelerating waveguide used for a 6X Linac
Key features include accelerating cavities, coupled side cavities, the electron gun on the left, and the target on the right
The length of each cavity is approximately 5 cm (half the wavelength)
What anatomical points do you use to measure fetus dose per TG-36 in-vivo?
Fundus, Umbilicus, Pubic Symphysis
What is the difference in image divergence of a DRR and a scout image?
Scout image only has divergence in the transverse direction
DRR has divergence in both transverse and longitudinal direction
Take note in the image on the prior page, you see anatoy in the head on different plans overlapping, this is due to the longitudinal divergence. Scout doesn’t have that
Also DRR utilizes SAD = 100 cm
Scout is SAD = approximately 60
For MV photon beams, is compton scattering primarily foward, side, or backscattering? What angle are most of the electrons scattered to?
Forward for photons
Most electrons are scattered at large angles
Which of the following gives the highest increase in relative surface dose?
Lower physical Wedge
Upper physical Wedge
Dynamic Wedge
Upper wedge < Dynamic Wedge < Lower wedge
This is a tricky question actually…
- Upper wedges (those we typically use), actually decrease relative surface dose
- Dynamic wedges result in very minimal change in relative surface dose until an increase at 60 Wedge angle
- Lower mounted wedges (we never use), will increase surface dose
What are these nodes?
What was this patient most likely being treated for? describe the blocking
This is a child most likely treated for Hodgkin’s Lymphoma
Blocking is arranged to permit treatment of neck, chest, and underarm lymph nodes, while sparing portion of the lungs to retain respiratory function, and humeral head to minimize impact on bone development
What routine QA is to be performed on your EPID system?
Both of these images were made by x-rays, why do they look so different?
Image on the left was kV xrays, imaging on the right was MV xrays
What is the device in this image causing the artifacts you see?
Breast tissue expander
What is the exposure rate corresponding to each packaging label type of RAM? Where are these values defined?
49 CFR 172
What is the difference between film HD curve for diagnostic and therapy uses?
Radiation emitted from what portion of the Linac head is calculated in a phase space file?
The patient independent part. See red region shown below. This is the head setup that is identical for all patients, so phase space file can be pre-calculated up until then
What is a conventional simulator and simulator CT? How does this differ from CT simulators?
Conventional simulation implies the use of a machine capable of the same mechanical movements as treatment units, and same couchtop inserts and immobilization devices. Images can be taken with this simulator with CBCT acquisition, to create a 3D CT scan. It’s essentially imaging with something similar to a Linac delivery system
CT simulators are CT scanners that are accompanied with special software that allows one to do virtual simulation (in TPS)
What does the plot show? What are those 3 plots?
This plot shows the variation of compton cross section per electron/cm2 with energy
The top curve is the total cross section of interaction
The second curve is the partial cross section relating to transfer of energy to compton scattered photons
The bottom curve is the partial cross section relating to transfer of energy to compton recoil electrons
So key takeaway, at lower energies, the photon retains most of its energy during the scatter, at higher energies, the electron gets most of the energy from the photon
How does CCC work?
What happens to PDD if you insert a cork slab infront of a 10x10 or a 2x2 field size?
For an 18 MV beam, what is the dominant photon interaction in soft tissue, bone, and prosthesis?
How does cyberknife TPS calculate dose?
It’s effectively just an MU calc. The only heterogeneity correction portion is tha tthe depth used for TPR is the effective radiological depth.
What are the TG 43 parameters/functions? Just say the names. Which factor dissapears in the 1D formulism?
The 2D anistropy function is replaced by a 1D anisotropy function which averages out the anisotropy factors at distance r from the source at all theta locations
What is this?
This, surprisingly enough, is a schematic of a doorless maze
That extra corner + having some thicker concrete in between would actually be enough to lower neutron and photon fluence down enough to the point where you wouldn’t need a door
How do photon attenuation coefficient curves look for the 4 photon attenuations in water, iron and lead?
Notice how compton goes up then down
What two effects compete with one another in TVLe?
Beam hardening, which increases the average energy of the photon beam, thus decreasing attenuation coefficient and increasing TVL needed
BUT, also increase in compton scatter, which decreases the average energy of the beam, thus decreasing the TVL needed. This is true mainly for broad beam conditions however
Turns out the 2nd effect actually acts more strongly than the first, so TVLe decreases with depth
In pb, which is larger, TVL1 or TVLe?
They’re very close and depend on energy of the beam
For higher energy beams, TVL2 > TVL1, this is because beam hardening is greater in this instance. Since PP becomes more prevalent, and PP does not give lower energy compton photons, it doesn’t lower the average energy as much as compton would. Additionally, compton scatter is much less dominant in lead in general. As such, beam hardening is slightly more significant than compton reduction of average energy
Which manufacturer does this belong to?
Varian
How does electron cutout factor vary with increasing cutout field size?
Increases then eventually goes above 1.0 briefly then back down to 1.0
What is the dose limit for regions 5 and 8?
Trick question, there is no limit
How do you calculate the workload at 1 meter for a Ir-192 source for shielding calculations?
What kind of diodes do we use in our clinic?
Fluke VeriDose Model 37-705 Dual Diodes
What qualifications are needed to be an RSO? Which chapter gives these details?
10 CFR 35
How does a MOSFET work?
- Consists of three leads, a source, a drain and a gate
- Source and drain are connected to a p-type semiconductor
- This forms a PNP junction that has a one way flow that can only be overcome by applied voltage
- Under the gate is an oxide (insulator) that blocks current flow from source to drain
- To run a current, you have to apply a negative voltage that overcomes the threshold voltage
- The thresold voltage increases as holes from electron-hole pairs collect at the interface between the oxide and the N substrate and get trapped there
- These wholes create positive charge, making it harder for current to flow, thus increasing threshold voltage
For proton double scatter method, do you use high or low Z for the first and second scatterers? Why?
For the first scatter, you use a high atomic scattering foil of uniform thickness in order to spread out the beam
For the second scatterer, you use lucite, a low atomic number to help achieve a more uniform beam intensity profile
How do OSLD/TLDs work?
- A crystal lattice (LiF for TLDs and Al2O3 for OSLDs) is doped with impurities to create trap centers
- There are two bands, valence band and conduction band
- When the atoms are irradiated, the electrons will ionize into the conduction band
- When they drop back down in energy state, some electrons get trapped inside of trap centers from the impurities in the crystal (depth of trap center relates to energy needed to escape the center)
- The energy level of the trap center is somewhere between the conduction band and the valence band
- When heat or light is applied to the TLD or OSLD, these trapped electrons get enough energy to leave the trap center, then relax back down to a recombination center/hole trap (also from the impurities), in the process emitting visible light
There are also competitive centers, which trap charge carriers but do not contribute to luminescence.
What is the difference between CCC, AAA and pencil beam?
Pencil beam convolution separates the beam into many very small beamlets (pencil beams), and convolves TERMA distribution vs the pencil beam kernel for these small beamlets. Then sums the dose from each beamlet. Neglects modifying dose deposition based on medium density.
AAA does pretty much the same thing, the only difference being that it also stretches and shrinks the pencil beam kernel based on local heterogeneities. AAA can stretch the Kernel in 16 different directions
Point kernel based algorithms, such as CCC, do nearly the same thing as AAA, the only difference being they look at TERMA at the voxel, how much energy is released within that voxel and not along a beamlet, and are able to stretch and shrink that in more directions than AAA with a finer resolution
In the image shown below, AAA would be somewhere between monte carlo and pencil beam
How would a pencil beam electron beam profile look compared to a monte carlo beam? Why does it look that way?
Electron beams are essentially all scatter
Pencil beam ignores side scatters considerably, thereby grossly underestimating dose near central axis (ignores the scatters going towards CAX)
What definitions of flatness and symmetry does ICProfiler use?
ICP uses the TG 45 definitions
For acceptance, what formula does Varian give for flatness and symmetry, and what is the tolerance?
Flatness: +- 3.0% for a 30 x 30 cm2 field
Symmetry: <= 2%
Where along the electron path are the steering coils located?
Relating TCP and NTCP, what is often a function that is used to determine the ideal tradeoff between the two for optimization calculations?
TCP x (1-NTCP)
Plot out the relative output vs energy Bremsstrahlung spectrum curve for a filtered and unfiltered kV imaging machine. What are common filtered materials added to the machine? What are some inherent filtration materials?
Inherent filtration is the construction component of the tube window (usually Silicon (Glass) or Aluminum)
Added filtration is typically fabricated from Aluminum or Copper
“Unfiltered” part of the spectrum actually includes the inherent filtration
What kind of filters are used in the kV OBI of your Linac? What is their purpose? What are they made of?
Full fan bowtie and half fan bowtie filters
Bowtie filters are used to acquire a CBCT while reducing photon fluence at the periphery where paths are shorter and fewer photons are required for adequate signal, thus reducing unecessary dose to the patient
Both are made of Aluminum
Additional info: full fan bowties can measure around a 27 cm FOV. Half fan bowties can measure 41 cm FOV. So half-fans are used for larger anatomies
How do CT simulators measure intensity (what is the detector, what does it measure?)
The detector consists of a scintillator (commonly NaI(Ti)) to convert keV x-rays into visible light, which is then detected by an array of photon diodes and converted to electrical signal, which is further converted to digital signal
NaI(Ti) has the highest light output efficiency. Bismuth Germanate (BGO) is another possible scintillator material type, which produces 5-10x less visible light signal than NaI(Ti), BUT, it has a higher density and stopping power so you can pack it much smaller, allowing for better spatial resolution
For a direct shielded door, where are the weak points?
For a direct shielded door, how can you minimize it’s weaknesses?
To reduce weakness, you can either…
1. Increase overlap of the secondary barrier and the door or…
2. Line the secondary barrier with lead and BPE and add a concrete door stop on one side
In our clinic we use method 1 + the concrete stop from method 2
How does gEUD change as ‘a’ goes from negative infinity to infinity?
At negative infinity, gEUD approaches minimum dose to volume
At a = 1, gEUD is the mean dose
At infinity, gEUD approaches max dose to volume
Is this a triode or a diode electron gun?
Triode
What is this model called. What is “cell survival”? What are the four parameters in this curve and what do they signify?
This model is called the “multi-target model.” It’s an older model and has been largely replaced by the linear-quadratic model. Cell survival means that a cell retains its reproductive ability, or in the case of non-proliferating cells (such as nerves, muscles or secretory cells), cell survival is retaining their specific function.
- ‘n’ is the extrapolation number and is in theory the number of “targets” that must be hit in each cell to cause multi-hit death. It is also the width of the shoulder. (note: it refers to the shoulder region)
- ‘Dq’ is the quasi-threshold dose which defines the width of the shoulder reason and is a measure of the ability for cells to repair damage
- ‘D0’ is the final/terminal slope and is the dose at which 63% of cells will die (0.37 cell survival). It is an expression of radiosensitivity of a population, and is the slope of the multi-hit kill region
- ‘1D0’ describes the small initial exponential region slope falling right before the shoulder which is the result of random single hit killing.
What is the electron streaming effect (ESE)?
In MR linacs, it’s an effect that can produce large out of field doses that must be considered in the TPS
Highly dependent on angle of treatment surface with respect to magnetic field
Electrons backscattered from surface or present in air will scatter away from the field due to magnetic field and deposit significant out of field doses
How does magnetic field impact the PDD of a photon beam?
Shifts dmax more shallow
Decreases %dd(10)x very slightly
How would you perform dose output calibration on a MR Linac?
Similar formulism, except there’s a magnetic field correction factor, kB, which accounts for difference in chamber response due to presence of magnetic field. It’s very difficult to measure, so values are Monte Carlo derived
Additionally the magnetic field correction factor will be more significantly below 1.000 when chamber is oriented perpendicular to the magnetic field, and will be closer to 1.000 when oriented parallel.
Elekta recommends dose should be calibrated with gantry at 90 deg or 270 deg (due to annulus at 0 deg having a helium fill level which can affect the dose output by up to 0.9%). This means you’re actually irradiating the side walls of the water phantom, which needs to be accounted for
Calibration point is either at dmax (1.3 cm), 5 cm depth (recommended), or 10 cm depth
In TBI, what is compensator made of and where is it placed?
Made of aluminum
Placed on a shadow tray mounted on the Linac. The shadow tray allowed for drawing of outline of light field projected onto patient at treatment position to highlight borders of low thickness areas. This outline is then given aluminum compensator cutout to compensator for thickness
What is the order of components in the gantry head that your treatment beam passes through??
Target (or not) –> Primary collimator –> flattening filter (or not) / scattering foil –> ion chamber –> secondary collimator (jaws) –> MLCs
In proton beam output calibration, how is beam quality measured/specified?
- Measured PDD of SOBP
- Find practical range (where PDD drops to 10%)
- Find center of SOBP (Zref)
- Residual range = Rp - Zref
- The beam quality is then the effective energy, defined as the energy of amonoenergetic proton beam that has the same residual range measured as that of the given clinical proton beam
Note: this means that the beam quality is typically very close to the maximum energy in the proton energy spectrum at the reference depth
What is the construction of the Varian kVD?
Scattering grid to minimize scatter
Scintillation layer to convert incoming kV photons to visible light
Amorphous silicon thin film transistor to convert scintillation photons to charge for collection and eventual readout
Are thin film transistors indirect or direct imaging devices?
They can be used as both
If paired with a scintillation element (such as with EPID or kVD, it’s indirect)
If paired with a semiconductor capable of directly converting incoming radiation to charge (such as amorphous selenium), it is a direct detector
Describe the difference between amorphous silicon and amorphous selenium
Amorphous silicon is a photodiode that converts visible light to electrical signal. It’s used in indirect detection and is paired with an scintillator prior and a TFT post (TFT for charge collection and readout)
Amorphous selenium is used for direct collection since it’s capable of converting incoming x-rays into electron and hole pairs, and by aplying a electric field the charge is collected in the TFT. Even though amorphous selenium has lower z (so low collection efficiency), it can still be built thick to increase signal since the electric field stops the charge collection from laterally spreading
What are most CT detector arrays made of?
- Solid state scintillators (typically gadolinium based, but can also be cadmium tungstate)
- photodiodes
- Substrate
Scintillator converts x-rays to visible light, photodiode converts visible light to electrical signal
How does the contribution of out of field dose from photon fields change for the three source types as you move further and further from field edge?
Which of these curves is the raw measurement? Which is PDI? Which is PDD?
The long dashed lines are the raw measurements
The solid line is PDI (shifted by 0.5rcav)
The short dashes lines is PDD
Which TG report gives recommendations on the calibration of TLDs and OSLDs, and what is the general formalism?
TG 191
General formalism is similat to TG-51, but with additional LD dependencies
First you have the cross calibration factor which is found by the solid water + bolus setup method
Then you multiply your corrected reading by taking into account background signal, depletion correction factor, and individual sensitivity correction factors (individual LD response vs average batch response for a given batch) (will require bleaching/annealing afterwards. if you order from landauer, they pre-measure the sensitivites for you).
You then multiply by the linearity correction factor (which accounts for change in sensitivity vs irradiation history), the fading correction factor (which accounts for loss in signal due to spontaneous stimulation), beam quality correction factor (that is, the change in response due to beam quality), and an angular correction factor
This is the higher accuracy way of doing things, which is what labs like IIROC or IAEA will do. But at most clinics, we do a much more simplified approach.
On the SRS frame used in GammaKnife, what are the markers on the frame made of?
Copper Sulfate
Which MRI Sequence is this (the white line is created artifically in post processing, ignore it)
T2
Which MRI sequences are these images?
They’re all T2, just taken at different times
