2nd Year: SRS/SRT Flashcards
For small fields, why do you NOT measure PDD? Only TMR?
It’s very difficult to measure PDD due to chamber movement from CAX as the chamber changes depth. Your alignment is critical and has to be centered perfectly the entire range of depths
For TMR, since the chamber only stays in one spot, it only needs to be aligned once
For cone, why do we always use the same jaw setting for every cone attachment and every cone plan you do?
Changing jaw size affects output factor due to differences in scattering surfaces. You need to stay consistent since your output factors are defined purely for one jaw setting
What does isocentricity measure? What tests do we do that check it?
Size of isocenter
Spoke shot and WL both check it
What does isocenter coincidence check? What test do we do that checks it
Coincidence of radiation and mechanical isocenter
WL checks it
You can argue, if lining WL up using imaging, that imaging isocenter coincidence with the others is also checked, since you define imaging isocenter as the center pixels, so if center sphere aligns to that, then you say imaging isocenter is checked against radiation and mechanical
What test done daily for TB3 checks imaging and radiation isocentricity?
Isocal
(WL also does check mechanical and radiation isocentricity)
For WL, do we collimate with MLC or Jaws?
Always MLC
This is because…
A) We always define the last layer of field collimation using the MLCs
B) MLC positioning is more accurate than Jaw
C) You always want to test the mechanical aspect which is defining a field. For all of IMRT, that is MLCs
What is the isocenter of an image, by definition?
The literal location of the center pixel in your imaging array, rotated about all angles
That is, if something is aligned to imaging isocenter, Ex. a WL cube, then it should be at the center pixel across all projections
What is the benefit of frame delivery vs frameless? What is the drawback
Frame delivery is more accurate and does not require IGRT or any external tracking. It forces the skull to a certain orientation
However, it’s often much more uncomfortable and painful for the patient and requires more setup/time commitment to attach and to do everything same day
How are frames attached? How is patient aligned for treatment?
Screwed to the skull with local anesthesia
There’s a special device that shows torque on skull so you don’t crack it
The frame attaches to the couch top in a known and indexed manner. This makes alignment as close to exact as possible, and does not allow for wiggle
Frame has a known coordinate system for alignments
What are the benefits to using noncoplanar beams over coplanar beams?
Noncoplanar beams can achieve higher conformality and steeper dose gradients, in addition to lower surface dose
In general, what is mechanical tolerances for mecahnical devices?
< 1 mm
What are the main defining characteristics of SRS?
Steep dose gradients
High targeting accuracy (<1 mm)
High dose per fraction ( >= 5 Gy)
High conformity
1 to 5 Fx
Target diameter < 4 cm
How do cones achieve steeper dose fall-off?
Multiple noncoplanar arcs
Typically use lower energies
Smaller collimator-to-tumor distances
How does GammaKnife manage to get steeper dose gradients than Linac based SRS?
Large number of noncoplanar isocentric beams
Small collimator-to-target distances
Lower energy photons, that means lower energy and lower range secondary electrons
How often will a GammaKnife unit require a source exchange?
Every 5 to 6 years
(about 1 half-life)
What is the typical initial dose rate at isocenter of a GammaKnife unit when the sources are first received?
3 Gy/min
What is typical prescription (1 Fx) for the following metastases sizes?
<= 2 cm
2.1 - 3 cm
3.1 - 4 cm
<= 2 cm - 20-24 Gy
2.1 - 3 cm - 18 Gy
3.1 - 4 cm - 15 Gy
What is typical prescription dose for a trigeminal neuralgia?
80 - 90 Gy x 1 Fx to the 100% isodose line
What are the advantages to using GammaKnife over Linac based SRS?
Higher accuracy and pecision
Less moving parts (easier to get everything within QA tolrances)
Quicker QA
Used longer in the field (more research/experience)
Very rare downtimes
What are the disadvantages to using GammaKnife over Linac based SRS?
Can only treat brain
Usually only treats single fractions
Longer treatment times
Not many units come with IGRT capabilities
Requires source exchange after 5 - 6 years
Radiation safety precautions
When would multi-fraction SRT be preferred over single fraction SRS?
Retreatments
Tumors > 4 cm max dimension
Lesion located near critical structures
For a 1.5 T MRI scanner, what the is approximate geometric distortion due to fluctuations of gradient field near the center of the stereotactic space?
What about near the edge (near headframe base for example)
< 0.5 mm near center
about 2 mm near headframe base
How does the strength of magnetic field influece the geometric distortion thatis seen in MRI?
Distortion magnitudes are approximately proportional to field strength
Ex. For a 1.5 T field, the distortion at center is < 0.5 mm. For a 3T field it is < 1 mm
Ex. For a 1.5 T field, distortion near edge is 2 mm. For a 3T field it is 4 mm
What MRI protocol is the go-to for drawing GTV for SRS/SRT?
T1 with contrast
What is one dose concern for multi-lesion plans that is not present for single lesions? How do you avoid it?
Dose bridging
That is, a certain isodose line bridges between two nearby lesions, giving high dose to the healthy brain between lesions
To minimie, draw control regions around this dose bridging, where you can optimize down the dose
What are some special contours that we draw for SRS/SRT optimization? How are they used?
What are some special tricks we also employ?
Outer brain - 2 cm cropped from PTV. max dose 30% in optimizer
0.5 cm ring - 0.5 cm cropped from PTV. max dose 90% in optimizer
1 cm ring - 1 cm cropped from PTV. max dose 50% in optimizer
2 cm ring - 2 cm cropped back from PTV. max dose 30% in optimizer
Brain - GTV - cropped 0 cm from GTV. You want V12 Gy < 10 cc for SRS
Bridging control - locate bridging dose region after first run. Draw structure around region and minimize dose to this area
NTO = 120 (as opposed to the usual 100)
No entry through eyes
Back-to-back method - that is, give the optimizer the impossible task of having an immediate dose fall-off after giving prescription dose to target, and also conformal dose in target. This will make your coverage worse, but fall-off and conformality better. When you normalize, it will be goood
What couch kicks do we use for our SRS/SRT plans?
0, 45, 315
What is the brainstem max dose constraint in 1 Fx SRS?
15 Gy
What is the chiasm max dose constraint in 1 Fx SRS?
12 Gy
What is the Brain-GTV constraitn in 1 Fx SRS? Where does it come from?
V12 Gy < 10 cc
Quantec and TG-101
What is the cord max dose constraint in 1 Fx SRS?
14 Gy
What is the cochlea max dose constraint in 1 Fx SRS?
9 Gy
What is the optic nerve max dose constraint in 1 Fx SRS?
10 Gy
What global hotspot do we keep our SRS/SRTs to?
125%
What is the coverage goals to GTV and PTV that we normally use in our clinic?
**GTV: ** Dmin = 20 Gy
**PTV: ** V100% > 95% (18*0.95)
What is the chiasm max dose constraint in 5 Fx SRT?
25 Gy
What is the lens max dose constraint in 5 Fx SRT?
7 Gy
What is the chiasm max dose constraint in 5 Fx SRT?
25 Gy
What is the brainstem max dose constraint in 5 Fx SRT?
31 Gy
What is the optic nerve max dose constraint in 5 Fx SRT?
25 Gy
What is the Brain-GTV constraint for 5 Fx SRT?
V28.8 Gy < 7 cc
What is the global max hotspot we use for 5 Fx SRT?
125%
30*(1.25) = 37.5 Gy
What is sphere packing? What is it used for?
The technique of filling non-spherical target volumes with a composition of multiple spherical dose distributions of varying sizes
It is used to create a conformal plan for a non-spherical target when you can only deliver spherical distributions (such as with Cone or GammaKnife)
What is the difference between MLC conformal arc and VMAT?
MLC conformal arc moves the MLCs to conform to the target at all angles. This is forward planning
VMAT is inverse planning and moves the MLCs to fit a required fluence mapping patter
What are the pros and cons to using MLC based conformal arc therapy versus cones?
Pros: can treat multiple targets with a single isocenter. Requires shorter treatment times. Can achieve lower hot spots for irregular targets
**Cons: **Larger penumbra than cone due to final collimation being farther from target
When is it better to use cones or MLCs?
Cones: single, spherical targets or targets where you want fastest possible dose fall-off (Ex. Trigems). Lesions far from inhomogenous material in body and atleast 1.5 cm deep
MLC: larger or irregular shaped targets. Multiple targets
What are some new features added to HyperArc that don’t exist in RapidArc?
- Optimization tools specifically for SRS (automatic lower dose objective and SRS NTO tool)
- SRS specific quality metrics (conformity and dose bridging miniization)
- Virtual dry runs
- 1 Click automated delivery with imaging done between kicks
List off as many clinical uses of SRS as you can
AVMs
Meningiomas
Acoustic Neuromas
Trigeminal Neuralgia
Movement disorders
Malignant brain tumors
Metastatic lesions
What are cones made out? What is their approximate length?
Cerrobend lead encased in stainless steel
Approximately 15 cm long
Give a general outline for an “ideal” end to end test
An ideal end to end test is one that tests every step of the SRS workflow and involves everyone who would be responsible for individual steps
This would mean going very deep into detail with an anthropromorphic head phantom, with a mask placed on it during sim by sim therapist, planned by the dosimetrist, approved by physician and physicist, QA performed, and treated by a therapist
Where do we get the majority of our SRS/SRT dose constraints?
TG-101, Quantec and Timmerman
Why in some clinics is it preferred to get a very high hotspot at the center of a lesion?
Center of tumors are typically hypoxic
So a high hotspot at the GTV center can actually partially compensate for the loss in effectiveness due to OER
For what dose per fraction is the LQM generally supported?
1 - 5 Gy/Fx
Beyond this range, the validity of the model is debated
What three non-LQM mechanisms form the basis of SRS radiobiology?
Abscopal Effect
Vascular Damage
Immune-Mediated Response
Describe briefly the abscopal effect
Phenomenon in which local radiotherapy is associated with regression of metastatic cancer in unirradiated lesions
For what dose per fractions does the abscopal affect contribute more? Low or high?
High
Briefly describe the idea of vascular damage in relation to SRS radiobiology
At large dose per fractions (> 10 Gy), death of vascularture leads to lack of blood supply to the tumor
Cell killing continues up to 3 days after irradiation due to tumor starvation
Briefly describe the mechanism of “Immune-Mediated responses” as it pertains to SRS
For high dose/fraction it has been shown that induced local release of tumor specific antigens combined with pro-inflammatory and pro-axidant cytokines improves priming of tumor T cells, contributing to increased immune response to combat cancer
**Note: **For standard fraction the opposite actually occurs. Lower fractionation induces immunosuppression
What does the “Geometric Window of Opportunity” refer to?
Ability of highly conformal dose distributions to avoid critical structures all together
How does one improve the “Geometric Window of Opportunity” (two ways)
Improved localization techniques
Lower intrafraction time
How is TCP affected by inter-fraction time?
You need to give some time between fractions for re-oxygenation and re-distribution to occur, increasing TCP
But at the same time it allows for proliferation and repopulation of tumor, reducing TCP
It’s a balancing act
Describe the procedure of a WL test
A tungsten ball is centered at isocenter as defined by either lasers or imaging
At a combination of different couch, gantry, and collimator angles, a small radiation field will be shot at the ball
During analysis, the center of the radiation field, as defined by the FWHM, is compared to the center of the tungsten ball shadow on the field profile
What is the purpose of the WL test?
It’s both an isocentricity test and a coincidence of radiation and mechanical isocenter test
It measures the size of your isocenter but also how the radiation isocenter agrees with mechanical isocenter
What does a spoke shot test measure?
The size of your isocenter (isocentricity)
What is “Stereotaxis”
Use of a 3D coordinate system to localize a target
In general, how does the size of the tumor affect the prescribed dose? Why?
As size of tumor increases, prescribed dose decreases
This is due to the higher amount of normal tissue dose due to larger beams
What are the positional and dosimetric accuracy requirements for a SRS delivery?
Dose: 3%
Positional: 1 mm
What imaging modality is used for AVMs?
Digital subtraction angiogram (DSA)
What is “General Supervision”
Procedure and staff are under control of rad onc and QMP, but their presence is not required during procedure
What is “direct supervision”
Procedure and staff are under control of rad onc and QMP, and they must be immediately available in the department during procedure
What is “personal supvervision”
Procedure and staff are under control of rad onc and QMP and they must be present during procedure
What level of supervision do we have at our site?
Hybrid,
QMP is personal supervision (they are at the treatment the entire time)
Rad Onc is direct supervision and is only present during setup and time out
Which two reports recommend that QMP should provide personal supervision for atleast the first fraction of SRS/SRT?
TG-100 and MPPG 9
Per MPPG 9, what are some QMP responsibilities in an SRS program? List as many as you can, don’t have to know all
- Be available for consult at CT
- Be available for personal supervision during treatment
- Oversee acceptance testing and commissioning of IGRT/localization, hetereogeneity calcs, TPS, SFD
- Develop E2E tests and perform yearly
- Develop an ongoing QA program
- Develop SOPs and SWIs for everyone involved in the SRS workflow
- Develop safety checklist
- Incorporate an incident learning system
- Supervise or perform treatment planning
- Initial and final plan review
- Perform appropriate plan-specific QA
What is the definition of the RTOG and ICRU defined conformity index?
Ratio of volume of prescription isodose surface and PTV volume
What is the ideal value for the conformity index?
1.0
What does a conformity index > 1 represent? What about < 1?
> 1 means over-coverage
<1 means under-coverage
What is the major limitation of the conformity index?
It is only a ratio of volumes. It gives no indicate whatsoever about the overlapping of the volumes
For all you know the volumes of the target and the prescription line can be exactly equal, so you get a CI = 1. But in reality the isodose line may be completely offset from the target volume
What is the Paddick Conformity Index? What is the equation for it?
It’s a modified version of the conformity index used to account for the overlap of the prescription isodose line and the target
CI = (TVPIV)^2 / (TV*PIV)
- TVPIV is the target volume covered by the prescription isodose volume
- PIV is the prescription isodose volume
- TV is the target volume
What is the ideal value of the paddick conformity index? What is an expected value?
Ideal is 1.0
Expected is > 0.8
True or False
The conformity index is not applied to OARs
True
What is the one limitation of the Paddick Conformity Index?
It does not distinguish between overcovering and undercovering
If a value is not = 1, you don’t know if you’ve over or undercovered the target
Pairing it with the Conformity index may help
What is the definition of the gradient index? What is the ideal value for SRS? What is the expected value for SRS?
Ratio of 50% isodose volume and prescription isodose volume. It’s an indicator of dose fall-off
Ideally 1.0, but this is physically impossible
For SRS, expectation is 3 - 4
Will a smaller target yield a larger or smaller gradient index?
Smaller
What is the homogeneity index?
Max dose in target divided by prescription dose
It’s an indication of the uniformity across the target volume
What is the expected homogeneity index for a Linac based SRS? What about GammaKnife?
Linac SRS: around 1.2 - 1.3
GammaKnife: Around 2.0 or more
Eclipse defines the gradient index a bit differently than most reports. How does eclipse calculate it?
Difference of equivalanet sphere radii of prescription and 50% isodose lines
(unit is cm)
Describe the general function of a CyberKnife unit. How does it work?
Compact x-band LINAC mounted a robotic arm weighing 160 kg
6 MV x-rays are generated froma tungsten alloy target with primary and removable secondary collimators
The arm can move in a hemisphere around the patient
Radiation dose delivery is non-isocentric
Couch is also automated and robotic for repositioning
Arm and couch movements are determined by the computer system
Synchrony respiratory motion management available for lung tumors
How does the CyberKnife Synchrony respiratory motion management tool work?
Can correlate external infrared monitoring system and fiducial movements to internal tumor movement and adjust the targeting of the beam to focus on the tumor in real time while beam is still on
What kind of secondary collimation is available for a CyberKnife?
Circular apertures with dimensions 5 to 60 mm
Why do you need to warmup the chambers of a CyberKnife daily?
They are open chambers and require about 6000 MU to warm them up to the average temperature that they’ll be operating at during a typical treatment day
**Note: ** Closed chambers wouldn’t require this warmup
Newer models have closed chambers. Older models have open chambers
What is the term used to define a combination of location of couch and arm that is beam is delivered from for CK?
Node
What two types of dose calc algorithms are employed for CK?
ray-tracing or monte carlo
How is geometric isocenter defined for CyberKnife?
There is a isocrystal mounted on an isopost, and using the orthogonal x-ray system it serves as the origin for the CyberKnife coordinate system
For CyberKnife, what is the recommended frequency for E2E testing?
Monthly for both intracranial and extracranial
For small field dosimetry of a CyberKnife system, which secondary collimation attachment do you use as your msr field?
60 mm
For all msr fields you want to use the largest possible field size if your machine is incapable of producing a 10 x 10 cm2
Which TG report covers CyberKnife QA?
TG-135
Which protocol is used for CyberKnife reference dosimetry and output calibration? How?
TG-51
But since you can’t find kQ in the traditional way, there are publications that compare PDD10 of a 60 mm collimation to a 10 x 10 cm2 field
You still measure PDD for SSD = 100 cm
What additional safety tests should you consider for Cyberknife?
Since both the couch and arm move often for CyberKnife treatments, there is a lot of collisional safety checks needed for CyberKnife
Additionally, since the arm as it moves can cover camera, you need to make sure you have multiple cameras so not all can be blocked at the same time
There’s also a patient safety zone programmed in which, unless overriden by difficult treatment plans, will not allow the robotic arm to move into
Which TG report covers QA for GammaKnife?
TG 178
What are the three currently available models of GammaKnife in order of oldest to newest?
Model C –> Perfexion –> Icon
How many sources are available in each model of the GammaKnife?
Model C - 201
Perfexion and Icon - 192
What is the Half-Life and Average Energy of Co-60?
5.3 years
1.25 MeV
What is the “unit center point” for GammaKnife?
It’s the location in which all sources/beams are directed towards
In the coordinate system, it is defined as 100,100,100
How does the older (Model C) unit collimate its sources?
Primary collimator for source holdings, with secondary collimation in the form of one of four special collimation helmets worn by patient (4,8,14,18 mm) helmets
Helmets need to be changed manually
How do the newer GammaKnife (Perfexion and Icon) collimate their beams?
Single Internal collimation with the sources residing on one of 8 moveable plates (sectors) which slide to different collimators
4, 8, 16 mm
Each sector contains 24 sources, so 24 x 8 = 192
Collimating units are then 192 x 3 = 576 channels
Channels that shouldn’t be treating at a given shot are plugged
What is the typical GammaKnife prescription isodose line?
What CTV to PTV margin is given for GammaKnife?
50% isodose line
0 mm margin
For GammaKnife, what is a “shot”
Dose delivered at a specific point in the patient with specified collimation and beam on time
Which sources are shown, which are covered, how long are they like that?
In between shots, the sources are blocked or defocused while patient gets into next position or the next set of collimators open
How is TMR measured for GammaKnife?
It isn’t, you cannot measure TMR for GammaKnife
Instead everything is dervied from Monte Carlo
How long is typical treatment time for GammaKnife?
Newer models: 40 - 60 minutes
Older models: 2 - 4 hours
True or False
You can treat multiple lesions with GammaKnife?
True
It would just require more shots and longer treatment times
Which GammaKnife units REQUIRE frames? Which are Frame Optional?
Perfexion and Model C require frames since they do not have patient motion monitoring built in
Icon does not require frame and can treat with mask
How does the Icon utilize patient motion monitoring?
Four reference marks and one patient mark are built into setup
Motion of patient marker relative to the four other markers is tracked during treatment
What is the point of TG-178?
Give recommendations on calibration and dosimetry of GK
Background info on use of GK
Recommendatiosn on QA
Examples and instructions on how QA’s should be performed (appendix)
What detectors can be used for GammaKnife calibration?
Only ion chambers are recommended
Do NOT use microionization chambers due to the instability of their ADCL correction factor over 2 years. But they may be used for spot checks
What is the IAEA TRS 483 (ALF) formalism for GammaKnife msr calibration?
Dose rate = Chamber and environment corrected raw reading * ADCL calibration factor * msr to 10 x 10 correction factor
**Note: **Do not need to correct the ADCL calibration by beam quality because GammaKnife is Co-60. kQ = 1
How is the 10x10 to msr correction factor calculated for GammaKnife? What does it take into account?
Monte Carlo
Takes into account field size differences, geometry, phantom material and beam quality between conventional reference field (10x10) and msr field (maximum collimation, 16 mm for Perfexion and Ion, 18 mm for Model C)
NOTE: Elekta calculates this specifically for both solid water phantoms and the ABS phantom. Meaning you must use one of those two
What are the only two phantoms you are allowed to use for calibration of GammaKnife?
ABS plastic phantom or solid water phantom
These are the only phantoms where Elekta did MC to get the msr correction factors. No other phantom is valid
How is dosimetry spot check for GammaKnife compared to baseline?
Compare dose rate at UCP corrected for decay to the commissioning dose rate in TPS
Every morning, how should the GammaKnife dose rate be checked?
Hand calc compared with TPS calculation
Exactly like HDR
What is the required tolerance of coincidence of UCP and radiation focal point for GammaKnife? How is it performed?
0.2 mm
Elekta has a QA tool and specialized phantom to measure it
What types of machines does MPPG 9a give QA recommendations for?
LINACs that we see
CyberKnife
Helical Tomo
**Note: **No Gamma Knife
True or False
NRC has regulatory enforceability over GammaKnife usage?
True
Co-60 is byproduct material
What two dose calculation algorithms are available for GammaKnife? Which is more commonly used? Which is more accurate?
Ray-tracing (TMR10) (more common)
Convolution (more accurate)
What is the difference in treatment times due to dose calculation between TMR10 and Convolution algorithm for GammaKnife?
Convolution leads to increased total treatment time by 4-10%
How is the reference coordinate system related to real life setup and alignment for GammaKnife treatment planning/delivery?
Patient has frame
Fiducial localizer box is attached to frame
Frame is CTed and MRIed with patient wearing it
On TPS, you mark the identifying marks on the frames and a least square fit maps the frame space to the image set
The images are then defined with the coordinate system (stereotactic images)
What are the commonly used secondary MU check softwares for GammaKnife? What are recommended tolerances per TG report?
RadCalc and MUCheck
3% tolerance, 5% action level
Neither are as accurate as TPS calc
Where does the 2 mm margin come from for GTV –> PTV for LINAC SRS?
Slice thickness 1 mm, uncertainty of isocenter and setup 1 mm
Add them together and you get 2 mm uncertainty in setup and localization
What are some advantages to using W1 or W2 scintillation detectors? (8 listed)
- No correction factor for small fields
- Water equivalent
- Extremely small
- Energy independent
- No angular dependence
- Minimal temperature dependence
- Minimal dose rate dependence
- 1% measurement uncertainty
What are some cons to the W1 and W2 detectors? (2 listed)
Radiation damage over time causes plastic yellowing and a 2%/kGy decrease in response
Cerenkov stem effects
How often does the vendor recommend calibrating the W1 and W2 detectors?
Every 1,000 Gy
What are the dimensions of W1 and W2 detectors?
W1: 1 mm diameter x 3 mm length
W2: 1 mm diameter x 3 mm length or 1 mm length
What is the main capability difference between the W1 and W2 detectors?
W1 can only measure point dose measurements
W2 can scan
How does a scintillation detector generate signal?
Incident radiation excites atoms or molecules in the scintillating medium
When the excited states decay, they produce visible light
Visible light potons are channeled to photodetector and converted into electrical signal which is displayed by electrometer
What is the W1 made of?
Plastic scintillator and scintilaltor fibers. PMMA cladding
All material have a electron density close to water
What three methods for Cerenkov subtraction can scintillators utilize to minimize effect?
Dual fiber/channel method (one fiber measures signal, one measures background)
Chromatic method (two wavelength subtraction method) (Cerenkov has different spectrum than collection spectrum)
Time Delay (Cerenkov signal decays quicker than actual signal)
How does the W1 and W2 detectors subtract out Cerenkov radiation effects?
Two-wavelength subtraction process
Use calibration technique and phantom specified by vendor
10x10 and 40x40 field delivered at min and max position
Cerenkov light ratio is derived from equation and calculatedby electrometer for all future readings
This process should be repeated every 1000 Gy
What is the sensitive volume of the W1 detector?
< 2.3 mm3
What are the uses of W1 and W2 in SFD?
Excellent for Relative measurements (profiles, PDD, TMR, output factors, etc) since they do not require a correction factor
Can be used in absolute dosimetry only if temperature and energy corrections are applied to keep uncertainty below 2%
How does the uncorrected W1 and W2 SFD output factor measurements compare with corrected output factor measurements of other detectors?
Agrees within 0.5%
Suggesting that W1 and W2 don’t need a small field correction factor
What are the two available algorithms for cone? Which do we have at our site?
CDC and ECDC
We have ECDC
Give a very brief explanation of CDC/ECDC
Essentially a fancy MU calc
Interpolates and extrapolates reference commissioning tables of TMR, OAR and output factors to calculate dose. Assumes everything in body contour is water.
**Note: ** CDC/ECDC never use PDD
Two matrices, coarse matrix (5 mm separation of calc points), and fine matrix (1 mm by default)
Calculation is measured up to distances of edge 2*cone diameter at isocenter + 5 mm margin around
List the limitations of CDC and ECDC
No inhomogeneity corrections
Assumes everything in body contour is water
Does not account for any backscattering
Available for only 6 MV and 10 MV
In what scenarios would you not use cones due to the limitation of the algorithm, not the limitation of the physical cones themselves?
When lesions are within 1.5 cm of surface (poorly models buildup)
When lesions are near inhomogeneity (more beam angels pass through to get to target, and no backscatter is accounted for)
When sum of SSD and dept of any calc point is > 130 cm (data doesn’t allow for calc that far)
SSD must be between 70.1 - 100 cm
Roughly what percent error occurs due to CDC/ECDC assuming skull is water?
What about wen lesion is near bone or air?
Roughly 1-2% if only skull is issue
If near bone or air, 5-10% error
What data must be measured during commissioning for CDC/ECDC?
10 x 10 cm2 open field absolute dosimetry
Output factor for all cones (95 SSD, 5 cm depth)
TMR for at least three cones
OAR for at least three cones measured at ATLEAST one SSD and one depth
What data did we collect for commissioning of CDC and ECDC?
Profiles at 80, 90, 100 SSD for every cone at d = 5 cm
TMR at STD = 100 for all cones
Output factor for each cone at 95 SSD, 5 cm depth for all cones, 5x5 cm2 jaws
Which detectors did we use for cone commissioning?
Edge and W1
They were compared to one another, and W1 was compared to MC
In addition to data collection for TPS, what else is done for Cone Commissioning?
- E2E test
- Comparison of data vs golden beam provided by varian for 5x5 cm2 jaw setting
- Winston-Lutz isocentricity test
- Center check output factor
- Cone Jaw setting check
- Cone concentricity
Describe the cone concentricity test. What does it check?
Place each cone in one by one. Irradiate either film or EPID and find center of profile
Centers of each cone attachment shoudl agree with one another and be at center of image
This is a mechanical check and it gives us confidence that when we check WL for one cone attachment, the results can also be applied to all others if they do truly align
What is the cone jaw setting check for cone commissioning? How do you do it and why?
- Attach cone setup
- Set jaw to clinical setting (5x5 cm2 for us), Varian recommends max of 5.4 x 5.4
- Irradiate film and EPID with cone attached and ensure no leakage through edges yielding high dose outside of the cone
- Checks to make sure your jaw setting is large enough to not cover any part of cone opening, but small enough to be fully encapsulated in the attenuating material of the cone
Why is using SunNuclear dose calc especially important for cone plan secondary MU check?
SDC is model based and does include inhomogeneity corrections
Which detector was used for output factor determination for cones, and was it daisy chained with anything?
W1
Yes, it was daisy chained with an ion chamber at 4x4 cm2
True or False
For all SRS/SRT, you MUST insert the encompass couch while treatment planning
FALSE
If using AAA, yes definitely you must take it into account
If using Cone, you can insert it if you want but you don’t have to. Just make sure you don’t include it in the body. Same with the mask
Per TRS 483, what is the Exradin W1 field output correction factor for any field setting for CK, Tomo, Cone or MLC?
1.000
Which two reports are referenced for Truebeam SRS/SBRT QA?
TG-142 and MPPG9a
What is MPPG 8a?
Report that gives QA recommendations similar to TG-142
Recommends almost te exact same tests, but with slightly different tolerances, and also explains how to do tests
What does MPPG 9a include?
Report that includes minimum practices to be implemented for a stereotactic program for Tomo, C Arm LINAC and CyberKnife
Per MPPG 9a and TG-142, what daily QA tests should be performed for a stereotactic program with Truebeam? What are tolerances? (7)
- Laser localization (1 mm)
- Collimator size indicator (1 mm)
- Radiation Isocentricity (cone and MLC) (1 mm)
- IGRT positioning (1 mm)
- Safety interlocks (functionality)
- Output constancy (+- 3%)
- ODI @iso (2 mm)
**Keep in mind: **These are our tolerances, which are the most strict of either TG-142 or MPPG 9a. The individual reports differ in values
Per MPPG 9a and TG-142, what monthly QA tests should be performed for a stereotactic program with Truebeam? What are tolerances? (7)
- Radiation isocentricity (1 mm)
- Couch positioning indicators (1mm or 0.5 deg)
- Output constancy (+- 2%)
- Laser localization (1 mm)
- MLC travel speed (0.5 cm/s below max)
- Leaf position accuracy (0.5 mm)
- IQ metrics (baseline or < 1 mm)
Per only MPPG9a what are the minimum recommended annual QA tests for a stereotactic program, and their tolerances at our site? (7)
- SRS Arc Rotation mode (MU units set vs delivered: 1 MU, gantry arc set vs delivered: 1 deg)
- MU Linearity (+-5% (2-4 MU), +- 2% (>= 5 MU)
- Accelerator output (+-1%)
- Coincidence of rad and mech isocenter (+- 1mm)
- Verification of small-field beam data from baseline (1%)
- E2E localization hidden target (1 mm)
- E2E dosimetric test (+- 5% measured vs calc)
What is the MLC transmission test tolerance from baseline? When is it performed?
+- 0.5% from baseline
Annual
What is the MLC spoke shot tolerance? When is it performed? Which report recommends it?
< 1 mm radius
Annual
TG-142
What are the three possible conditions that, if one is met, makes a small field?
- Loss of LCPE
- Partial occlusion of the primary photon source
- Detector size being too large
Of the three conditions of a small field, which is the condition that is typically not met?
Partial occlusion of the primary photon source
For LINACs and most other machines, the source is very small
To achieve LCPE at central axis, what is the minimum field size (defined by FWHM) that a field must meet?
2r_LCPE
Where r_LCPE is derived from either %DD(10,10)x or TPR_20,10(10)
(both of which require field size 10 x 10 cm2)
For what field radius is LCPE at central axis lost for a Co-60 beam? What about 24 MV?
Co-60: 3 mm field radius
24 MV: 2.5 cm field radius
When the primary photon source is partially occluded by collimation, what happens to the penumbra?
Penumbra overlap with one another and the CAX is contained within the penumbra
Dose is reduced drastically
Given a detector diameter ‘d’, what is the minimum FWHM of the side of a field for it to NOT be considered small field?
FWHM >= 2r_LCPE + d
That is to say, if there is not lateral charged particle equilibrium in the area of the field with the detector, then it’s a small field
What effect occurs in detectors when LCPE is not met in the area of the field where the detector sensitive volume is?
Drastic volume averaging
For intermediate and broad fields, how is field size defined? What about small fields?
Non-small fields: 50% isodose OR jaw settings since they agree closely with one another
Small fields: FWHM
When partial occlusion occurs, does FWHM become larger or smaller than the field size setting?
Larger
This effect is called “penumbra broadening” or “apparent field widening”
What is the machine-specific reference field?
It’s a field size (large possible field size), that is used as reference in place of a traditional 10 x 10cm2 for machines that cannot generated that field setting
(Ex. GK, CK, Tomo)
What is the output factor in SFD?
Ratio of absorbed dose to water at a point in a non-reference field to absorbed dose to water at that point in a reference field
True or False
The machine-specific reference field (msr) is NOT allowed to be a small field?
True
the msr is used for reference dosimetry. If it, itself is a small field, you’re in trouble
If your machine is unable to generate a 10x10 cm2 field, how do you obtain the beam quality factor?
There’s an equation
Relates %dd(10,S) to %dd(10,10) where S is the non-standard reference square field
What is the purpose of the correction factor shown below? How do you measure this?
Corrects for field size differences, geometry, phantom, material, and beam quality between the conventional reference field and the machine specific reference field
You don’t measure it yourself, there is MC and TRS 483 tables that give the values
NOTE: It’s dose per response/dose per response essentially. In MC, they figure out what the charge reading would be for a given point dose of both the reference field and the clinical field and take the ratio
Why is a water calorimeter not the ideal detector for SFD?
Due to heat flow
For a broad field, heat flow inwards is roughly equal to heat flow outwards in the field
For small fields, the heat flow inwards is NOT equivalent to heat flow outwards, so heat measured at center is incorrect
What does the following correction factor account for?
Difference in detector response for a clinical (non-reference) field, and the msr or reference field
Conceptually, what is the equivalent square MSR field size (S)?
Size of square field in which the same amount of phantom scatter is generated as in the non-square field
A reference chamber used for an MSR field must be calibrated for said field. What are the two methods of doing so?
- Direct calibration for MSR field at ADCL in Co-60
- Calibration at 10x10, then correction factors applied to convert beam quality from 10x10 in Co-60 to msr for whatever beam quality
What is the chamber calibration coefficient stability requirement for a suitable reference class dosimeter?
<0.3% change in calibration coefficient over a 2 year period
What is the leakage requirement for a suitable reference class dosimeter?
<0.1% of reading
What four dosimeters are most popular for relative dosimetry for small fields? Which is the most common?
Scanning diodes (most common)
Diamond detectors
Plastic scintillators (W1 and W2)
Micro-ion chambers woth low-Z electrodes
What is the maximum magnitude that the chamber specific correction factor is allowed to exhibit for a given field size?
5%
How do you find an equivalent square field for non-square small fields?
Find field size of small field that gives the **same area **as the non-square field
What happens to field energy spectrum as fields get smaller?
Less scatter yields loss of low energy contribution
So the average energy of the field increases as field size decreases
If you have a detector with correction factors over the full range of field sizes you are measuring at, how do you measure output factor?
Simply take the ratio of corrected measurements at clinical setup and msr, and multiply by the beam quality factor to go from msr to clinical
When would you have to daisy chain?
When you don’t have a detetcor that can measure over the range of small and reference fields you want to measure
How do you measure output factor by daisy chaining?
Use the small field detector and its correction factor to measure dose ratios from small field to intermediate field, then multiply by ion chamber dose measurement corrected from intermediate field to reference field
The intermediate field is commonly 4 x 4 cm2 and should be the smallest field that an ion chamber can accurately measure
**Note: **the correction factors are from small to int and int to msr. Meaning, all correction factors get normalized to the intermediate field factor
How do you measure small field correction factors?
Trick question, you don’t
They have been derived from MC and either tabulated inTRS-483 or some other literature
What happens if you are daisy chaining, but you do not have a correction factor for the intermediate field to get to msr?
You can assume that if the intermediate field is not a small field, then the correction factor from intermediate to MSR = 1
This is pretty close to true for most detectors, including the ion chamber you’d likely use
What is kerma?
Kinetic energy released per unit mass
That is, energy of primary beam transferred to secondary radiation when the interaction occurs IN THE MASS. It does not need to be absorbed in the mass, that is dose
What is absorbed dose?
Energy imparted per unit mass in the volume and also absorbed in the volume
As charped particle equilibrium is lost, what happens to the ratio of dose and collisional kerma?
D/kcol decreases
This is because the energy released in the volume that escaped the volume is not compensated by energy outside of the volume coming back into the volume
True or False
In a small field, dose will always be less than collisional kerma
True
What is charged particle equilibrium and does it ever truly occur?
It is a condition in which the number of charged perticle energy entering from outside of a volume is the same as that exiting
For photons this is never truly possible due to attenuation with depth, but transient CPE can occur where kerma and dose are proportional
With transient charged particle equilibrium, which is greater, dose or kerma?
Dose
This is because electrons coming in from upstream are greater than what is lost in the volume because of attenuation
Will volume averaging correction factor be greater than or less than 1?
Always greater than 1
This is because you sample lower doses, so your average is lower than the CAX dose. So correction needs to be > 1
What are the two major components of the field output factor for detectors?
- Difference in perturbation due to material of detector
- Volume averaging
For SFD, what is the main limitation of diodes. What affect does it have on diode reading ratio with IC as field size changes?
Diodes are made of silicon, so for high energies mass energy absorption coefficient varies very little. For low energies, mass energy absorprtion increases dramatically and diodes over-respond
As field size gets lower, the scatter contribution decreases, so the field energy increases
So as field size dereases, diodes will under respond relative to ion chambers, and for higher field sizes they will over respond
For shielded diodes, how is the field size dependence minimized?
Shielded diodes tend to filter out the low energy electrons anywasy, so their energy dependence is not as drastic as unshielded diodes
It does still exist to some extent of course, but less drastic
Blue shows shielded diode, green is unshielded
Why for shielded diodes is the output correction factor more drastic than unshielded?
Because the shielding material of shielded diodes causes more density effects than unshielded diodes
Why for ion chambers does the outut correction factor increase as field size decreases, but diodes decrease?
At small fields, ion chambers lose signal due to volume averaging, so the output correction factor hsa to compensate the loss
For diodes, they over respond due to density effects
What are the two most common devices for reference and absolute dose determination?
Ion Chambers (reference)
Calorimeter (absolute)
TRS 483 gives reommcendations on detector scanning orientations, What two factors determine their recommendations?
- Physical reason (such as perturbation angular dependence or stem irradiation, etc)
- Lack of research/data showing affect of different orientations
What are the pros to using silicon diodes for SFD?
Small
Sensitive (good SNR)
Minimal temperature dependence
What are the cons to using silicon diode detectors for SFD?
- Unshielded diodes over respond in large fields
- Shielded diodes have large perturbations in small fields due to present of shielding
- Angular dependence
- Change in response over time due to radiation damage
What is the minimum field size you can use a diode for SFD?
Unshielded: can use to smallest sizes
Shielded: 1.2 cm (at this point correction factor exceeds +-5%)
This suggests, with scanning with Edge for Cones, maybe you would want to consider a depth or location specific correction factor curve
What is the correction factor for ion chambers from 2.4 cm field size and above?
What happens below 2.4 cm
Close to unity
Below 2.4 cm the correctionf actors increase drastically due to volume averaging
What are the cons to using air filled ion chambers for SFD?
- Low sensitivity (size limit constraint and volume averaging)
- Polarity effects
- Difference in stem irradiation (calibrated with entire stem in field, but portion of stem in field changes significantly with small fields)
For the following detectors, at what field size does correction factor magnitude exceed 5%?
Mini Ion chambers
Micro ion chambers
Liquid ion chambers
Mini Ion chambers - 1.2 cm
Micro ion chambers - 0.7 cm
Liquid ion chambers - 0.3 cm
What are the pros (2) and cons (2) of liquid filled ion chambers?
Pros
1. High sensitivity (small volume)
2. Near water density
Cons
1. Less stability (leakage and liquid deterioration)
2. Dose rate dependent
What are the pros (3) and cons (4) of radiochromic film for SFD?
Pros
1. High resolution
2. 2D distirbutions
3. Self developing
Cons
1. Laborious readout procedure
2. Noisy if low dpi
3. Energy dependent
4. requires time for development
What are the pros to diamond detectors for small field docimstry?
- Small
- Sensitive
- Minimal angular dependence
- Minimal temperature dependence
- stopping power and mass energy absoprtion coefficient ratios of diamond/water are independent of energy
What are the cons to diamond detectors for SFD? (3)
- Recombination leads to dose rate dependence
- Expensive
- Construct of detectors and influence of components on output factors is not entirely known/researched
What does the shape of the correction factor curve look like for a unshielded diode? What about a semiflex ion chamber?
For daisy chaining, when using an ion chamber for the int to reference region and a diode from the small to int fied, what does the normalized curve of correction factor look like?
What machines are correction factor and beam quality for reference field correction factors given for in TRS 483?
LINAC Cone and Jaw fields (6 MV and 10 MV)
CK
Tomo
GK
We never daisy chain during annual. Why is this?
During annual, all we do is compare output ratios of small and reference field to baseline ratios from commissioning
We assume that the correction factors for the detectors will not have changed between annual and commissioning, so as long as we use the same detectors, all we have to do is compare to baseline
What is the general spread of calculated uncertainty for correction factors in TRS 483?
For small field sizes uncertainties can reach 2-3%
As field size increases, uncertainties decrease to about 0.5%
What phantom must you use for all Linac small field relative dose measurements at commissioning? What must it be commssioned for before use?
Full scatter 3D tank
Commissioning for alignment, orthogonality, distance accuracy and hysteresis effects
For relative dosietry scanning, why is it recommended to use a slower scan speed?
- Less water ripple effects
- Improve signal (less noise)
What is hysteresis? What happens if you detect it in your water tank?
The idea that scanning or measuring something one way, and scanning or measuring the exact same thing but in a different way, can lead to two different results
Ex. if you scan a cross profile left to right vs right to left, they may not perfectly overlap
If detected in water tank, send to manufacturer for repair
What is typical collecction volume of a diode?
0.2 mm3
Which is preferred for SFD, shielded or unshielded diodes?
Unshielded
Shielded has larger correction factors due to the metallic shielding material perturbation effects
For PDI -> PDD curves of broad fields, the conversion is fairly simple for photons
Why is it not as simple, and requires a per depth correction for small fields?
Tkaing this into account, what is the most ideal detector for SF PDD?
For small fields, the perturbation factor can vary by up to 10% as a function of depth (for broad fields, it’s at most 3%)
Most idea is W1 or Radiochromic film, since both are water equivalent
**Remember: **PDD is a measure of doses, not a ratio of readings
How does energy spectrum change vs depth for PDD scans for small and very small fields??
Small Fields
As depth increases, influence of low energy photons increases (more spread fields, more scatter)
So some detectors, like diodes, would overrespond with deeper depths
smaller Fields
As depth increases, beam is hardened, so opposite effect occurs, and since field is so small, the scatter contribution is still small and can’t compensate hardening
Main takeaway
Spreading of field with depth gives more low energy contribution, but beam hardening counteracts this effect. How small the field is determines which effect is stronger
For OAR do you require a location specific correction factor for small fields? Why or why not?
Yes
Energy spetrum changes depending on location off axis, so the output correction factor will also change. So you need a location specific equation to correct
OAR is a ratio of doses, so you do need to correct readings before you make the ratio
BUT, in actual practice, such as commissioning, nobody really apply any correction factor for relative measurements because it’s assumed the change is small
For annual, why do we not care about location specific output correction factor correction for OAR measurements and similarly, depth dependent for PDD of small fields?
Because we compare to uncorrected baseline ratios from commissioning just to make sure that the distribution and profiles haven’t changed
What does ECDC stand for?
Eclipse Cone Dose Calculation
What is the downside to using W1 and W2 for anything other than output factors for small fields? Primarily, scanning?
Scanning requires ALOT of MU’s. THis will result in yellowing of the palstic (recall, sensitivity drops 2%/1000 Gy)
Thus for small field scanning, you’re best off using a diode (either edge or microdiaond)
For small field dosimetry scans during commissioning, down to what field are scans taken, and what is the preferred scanning detector?
1x1 cm2
Diode detectors (either edge or microdiamond)
For Eclipse, down to what field sizes do the models take for in and cross plane profiles? What about depth dose curves?
In and cross plane: 3x3 cm2
depth dose: 1x1 cm2
During the ohio commissionign trip you visited, how was daisy chaining performed for small field output factors?
Farmer chamber measured dose for open 6x6
Use microdiamond to measure response at 6x6, this gives you dose/response
You then, for 1x1 -> 3x3, multiply the reading by dose/response and by the correction factor to get to 3x3 field
Note: this suggests the intermediate to broad field daisy chain portion is simply just a dose/response ratio with your reference ion chamber, which is true!
For small field scans you need to acquire TMR scans (Ex. Cones), how do you do it if your 3D tank is incapable of measuring directly?
Measure PDD, then a british journal of radiology study gives a way to convert your PDD curve to TMR
Studies have shown which detector to be the most suitable for cone scanning?
Edge
True or False
To convert from PDI to PDD for small fields you do not HAVE TO apply depth specific correction factors?
True
Although this would be the most accurate approach, it is deemed non-essential by commissioning standards, as the actual energy spectrum, and thus perturbation correction factor barely changes vs depth. Most commissioning don’t apply any factors for scans, only point measurements
This is due to field spreading increasing low energy scatter, while beam hardening increases energy, thus resulting in a somewhat balance. The factors will change, but not by a significant amount
How would you QA MRI distortion? (two methods, one for frame and one for no frame)
Frame: you know the exact dimensions of frame, so you can measure distances on MRI and compare to dimensions of real frame, tolerance is 1mm
No frame: visual assessment of MRI registration with CT, especially at peripheral of scan
True or False
Edge detector is unshielded diode
False
It is shielded with copper
How does W1 convert light signal to electrical?
Using a shielded photodiode that converts to charge
True or False
It is generally known that varian gold beam data is not completely trustworthy for small field output factors and scans
True and False
You shouldn’t throw all your faith into it. BUT, if you do your own measurements and they agree with varian data then it’s still a good double check
Why do you orient the microdiamond and W1 detectors vertically (parallel to beam axis)?
So that their smallest collection dimension is in the scanning direction
What is the photon source size for the linac?
It is not one singular value, source size is defined by the algorithm, NOT physical size, and depends on if you collimate MLC or Jaws
For AAA: x source size is 1 mm and y is 0 for MLC. Both are 0 for jaw
For acuros: x source size is 1.5 mm and y is 0. Both are 0 for jaw
Both models just need to use those values for their calcs, taking into account MLC or jaw transmissions in either direction
What is a good expected eclipse gradient index for MLC SRS? What about cone SRS?
MLC SRS: around 0.5 cm (makes sense, 10%/mm is expected fall-off)
Cone SRS: around 0.2 - 0.25 cm (quicker fall-off)
Why for PDD to TMR small field conversion can you not use the Khan method?
That method relies on phantom scatter factors
These are very difficult to measure and yield large uncertainties
Also you would need the factors for field sizes even SMALLER than your reference field, so imagine how hard it would be for cones
Where do the 5 Fx SRT dose constraints come from?
Timmerman 5 Fx
TG-101 for the cord and Brain-GTV
Grimm 5 Fx for eye and lens
What is the difference between DLG and transmission factor? How are both measured?
**DLG: **takes into account field size widening due to transmission at rounded leaf end. Measured by having series of MLC defined fields, and a sliding window slit of MLCs. This process models/optimizes one single DLG value to fit the measured data
**Transmission factor: **transmission through leaf banks. Simply block field with A bank and B bank, and compare to open field.
True or False
GammaKnife daily QA is very similar to HDR daily QA?
True
For GK, you share a lot of tetss as brachy such as…
- light indicators
- interlocks
- timing accuracy
- audio visual
- dose rate verified with decay calc
- Survey meter functionality
In addition to some GammaKnife specific tests such as…
- UCP and radiation focal spot coincidence
- Remote couch movement functionality
Does AlignRT utilize deformable or rigid registration?
Rigid
