2nd Year: SBRT Flashcards
What is a typical lung SBRT prescription?
5 x 10 Gy = 50 Gy
or
4 x 12 Gy = 48 Gy
or
3 x 20 Gy = 60 Gy
What is a typical spine SBRT prescription?
17 Gy in 1 Fx
or
24 Gy in 3 Fx
or
30 Gy in 5 Fx
What is a typical liver SBRT prescription?
40 Gy in 5 Fx (8 Gy/fx)
What is a typical prostate SBRT prescription?
8 Gy x 5 Fx = 40 Gy
We will do…
7.25 Gy x 5 = 36.25 Gy
(sometimes with a sequential boost to 8 Gy x 5 to part of the target)
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 supervision”
Procedure and staff are under control of rad onc and QMP and they must be present during procedure
Which two reports recommend that QMP should provide personal supervision for atleast the first fraction of SRS/SRT?
TG-101 and MPPG 9
What is the definition of the RTOG and ICRU defined conformity index?
Ratio of volume of prescription isodose surface and PTV volume
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 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 (non-eclipse)?
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
What is the intermediate dose coverage metric? What is the max value for a lung SBRT?
Ratio of volume receiving 50% of prescription dose to volume of the PTV
Should be less that 5-6
What is the homogeneity index?
Max dose in target divided by prescription dose
It’s an indication of the uniformity across the target volume
Eclipse defines the gradient index a bit differently than most reports. How does eclipse calculate it?
Difference of equivalent sphere radii of prescription and 50% isodose lines
(unit is cm)
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
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
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
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)
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
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
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 a correction factor in SFD? How are correction factors measured?
Accounts for difference in detector response for a clinical (non-reference) field, and the msr or reference field. This difference is due to two major components, detector composition and effect on field perturbation, and volume averaging
Correction factors are typically derived from MC
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
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
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 has to compensate the loss
For diodes, they over respond due to density effects
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 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)
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? (5)
- 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 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%
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
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. It’s essentially a modifier to help the model fit to your measured penumbra (as source size relates to geometric penumbra)
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
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
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.
What are the QA tests we perform daily that align with TG-142 and MPPG 9a for required for a stereotactic program? (9 listed)
- Laser localization
- Collimator size indicator
- Radiation isocentricity
- IGRT positioning
- Imaging Subsystem interlocks
- Stereotactic interlocks????
- Output constancy
- ODI @Iso
- OSMS Daily QA
What are the QA tests we perform monthly that align with TG-142 and MPPG 9a for required for a stereotactic program? (7 listed)
- Radiation isocentricity
- Couch position indicators
- Output constancy
- Laser Localization
- MLC travel speed
- Leaf position accuracy
- Image quality tests
What are some other uses of calypso besides prostate SBRT you may see at other sites?
Pancreas - Gating with long exhale and tracking of verticle motion of abdomen
Breast DIBH - two beacons palced on sternum, not implanted, tracks verticle motion of abdomen
Most clinics now don’t use calypso anymore
What is a major inherent limitation to calypso for prostate SBRT?
X,Y,Z and center of beacons relative to isocenter tends to be accurately measured, but angles (pitch, roll, and rotation) are highly variable and fluctuate day to day depending on rectal and bladder filling
How many beacons are implanted for a typical calypso? What is the minimum number of beacons needed?
Typically 3 implanted
Minimum needed is 2
Give a quick general rundown of how calypso works
Board knows where the beacons are relative to the board. It does not know where they are relative to isocenter.
Camera in ceiling knows exact array board position relative to isocenter. Therefore, the system can correlate beacons relative to isocenter.
Beacons are small copper coils with resonate due to RF signal emitted from the board. as they resonate, they emit an infrared signal and are located by the receiving arrays in the board
Why does calypso have a difficult time dealing with pitch, roll and rotation when a couch shift is needed?
Calypso will send LNG, LAT and VRT shifts.
When there’s a pitch, roll or rotation, a shift in the 3 cartesian coordinates are no longer independent of one another, they influence the other.
What are the uses of OSMS for SBRT at our site?
Trick question, there are none
What are the uses of OSMS for SBRT in outside sites?
Possibly breath holds in theory if you can get a nice surface (this is especially challenging in the abdomen as there’s no distinguishable features)
What registration does OSMS use to track surface changes?
DIR
Hence it has inherent large uncertainty, especially when there’s no distinguishable surface features
What TG report gives recommendations on plan and chart checks?
TG 315
When can an image review be useful for SBRT chart checks?
In theory to just double check a registration used on the machine.
This can be especially vital in the instance where a physician accidentally drags the registration and the numbers change, but nobody notices it got shifted. They would then be treating with a very obvious shift in the images which you can notice in offline review.
What image set do you contour normal structures on for lung SBRT and calc off of? What’s a backup?
Ave-IP from 4DCT is the first choice
If that set is bad, you can contour and calc off free breathing (preferrably non-contrast)
What image set does the physician draw ITV off of for a lung 4DCT? Two methods.
First method: use MIP and draw using what they see
Second method: play the 4DCT and go off the tumor motion to draw the ITV
Per TG-179, what are some typical doses for…
kV planar
MV planar
kV CBCT
MV CBCT
CT Sim
CT On Rails
Fan Beam MVCT
kV planar: 0.5 - 1mGy
MV planar: 1-3cGy
kV CBCT - 0.2 - 2 cGy
MV CBCT - 3 - 10 cGy
CT Sim - 3 -5 cGy
CT On Rails - (likely similar to CT sim but a bit lower. Maybe 2 -4 cGy?)
Fan Beam MVCT- 1 - 3 cGy
In tomotherapy, what energy is typically used for Fan Beam MVCT imaging?
3.5 MV
What has more noise, a MVCT or a kVCT?
MVCT due to dose reduction measures
To get a similar quality image, MVCT would have to pump a lot of dose
Per TG-179, what are 5 important components of OBI to check for an IGRT system during routine QA?
- Geometry
- Image Quality
- Image Dose (annually)
- System operations/communications
- Safety
For kV IGRT systems to be treating SBRT, what is the tolerance for localization accuracy?
1 mm
What are some minimal daily QA IGRT tests that should be performed per TG-179? (4)
Hint: Think to what we do daily
- Safety interlocks
- Warning lights
- Imaging/Laser/Treatment Isocenter coincidence
- Localization and positioning with couch shift
Per TG-179 what are some annual IGRT QA that should be performed, (not including all the checks that are normally dose daily and monthly) (4)
- Imaging Dose
- Generator performance vs spec (kVp, mA, ms, linearity)
- System disk space check
- Geometric accuracy
What are two pros to nonradiographic localization and positioning and two cons?
Pros
1. No additional dose
2. Real-time results
Cons
1. Blocked views or interferences
2. Many systems require surrogates
Give a general rundown of how VisionRT works
Two pods, each pod has a center projector that projects a red pattern onto surface, and two cameras to read the patterns
Based on DIR distortions of the known pattern of the projection, VisionRT can inference a surface geometry
Give a brief rundown of how RPM works
Surrogate with IR reflector placed on patient (either block with 1 reflector or alligator with 4)
Set of IR LEDs aimed from camera bounces off surrogate and position of surrogate is then inferenced by systen
What is a major con of infrared localization systems (such as BrainLab exactrac, freetrack, and RPM)?
System needs time to warmup components, and measured spatial displacement is lower with a cool system vs a a warm system (thermal drift)
Give a general rundown of how ExacTrac works
Radiographic and infrared components
Radiographic component is two floor bounted kV images that meet at isocenter and project onto wall mounted detectors
Infrared component is a single pod with an IR emitter and two IR cameras. Reflective marker placed on patient surface
Which TG report gives QA and commissioning recommendations of non-radiographic localization and positioning systems?
TG 147
How large are calypso beacons and what is their resonant frequency (typically)?
8 mm in length, 2 mm diameter
Resonant frequency depends on make and can be varied. But typically it’s around 10 Hz
Per TG-147, what is recommended daily QA for nonradiographic localization and positioning systems? (4 items)
- Safety (inspection of mounts and possible camera obstructions)
- Static localization of known geometry
- Documentation of daily QA results
- Any other vendor recommended test
Per TG-147, what is recommended monthly QA for nonradiographic localization and positioning systems? (6 items)
- Safety (check for obstructions and mount rigidity)
- Gating capabilities
- Static localization (known object at isocenter should coincide with iso)
- Dynamic localization (moving object should have correctly localized movement)
- Documentation of QA results
- Any other vendor recommended tests
Per TG-147, what is recommended annual QA for nonradiographic localization and positioning systems? (9 items)
- Safety
- System integrity
- Camera stability (camera isn’t slipping and drift is just a natural image drift)
- E2E test
- Extended system performance (predetermined couch shifts accuracy)
- Positioning accuracy
- Gating + tracking
- Data transfer
- Documentation
Per TG-101, what is typically max cross sectional diameter of a tumor to be treated with SBRT?
7 cm
Per TG-101, how much must the CT sim extend past superior and inferior borders of treatment fields for non-coplanar and coplanar fields?
Co-planar: atleast 10 cm
Non-coplanar: atleast 15 cm
Per TG-101, what is recommended calc grid size for SBRT? What do we use in our clinic?
2 mm or finer
We use 1.25 mm
What four options are available for simming and treating mobile tumors?
- 4DCT with MIP for lung and MinIP for liver
- Breath-hold
- Shallow breathing
- Gated CT/treatment
What does ITV stand for?
Internal target volume
What are some considerations that should be addressed when establishing a SBRT program? (9)
- Establishing scope of program (sites and treatment outcome goals, patient selection following guidelines)
- Determining fractionation schemes and planning goals
- Establishing equipment requirements
- Determining personnel needs and roles
- Performining acceptance testing and commissioning of machines and equipment
- Establishing a QA program meeting TG-142 and MPPG9a standards (and other reports for motion managing/tracking QA0
- Establishing CT sim, treatment planning, delivery, and verification SWIs and SOPs
- Conducting personnel training
- FMEA of workflow
per TG-101, what is recommended supervision provided by QMPs and RadOncs?
QMP - must be present for entire first fraction, and direct supervision for all other fractions
RadOncs - should approve image guidance and port films prior to every fraction
Which TG report focuses on management of respiratory motion?
TG 76
What are some treatment sites that may be affected by respiratory motion (don’t need to list all, infact not all are listed here)
Lung
Esophagus
Liver
Pancreas
Breast
Kidneys
Anything upper abdomen
On average, what is respiratory motion magnitude in lung? What is max motion?
On average it’s about 1 cm
Max can be around 5 cm
In which lobes of the lung is tumor motion more drastic?
Lower lobes (those closer to diaphragm) tend to have larger tumor motion than upper lobe tumors
Lower lobe tumors tend to move almost twice as much as upper lobe
What is typical axial movement of tumors due to respiratory motion?
2 mm
How does hysteresis present in tumor respiratory motion?
Tumor location on inhale and exhales may differ by as much as 2 mm for the same tidal volume of air
Caused by lagging of tumor motion vs muscular contraction
When is motion management required?
When expected tumor motion is >= 5 mm
For lung SBRT, what are some potential planning/delivery concerns if tumor motion is not accounted for? (3)
- Large motion = large margins
- Motion may cause contouring and localization errors due to artifacts
- Interplay
What are three possible CT sim options that can take tumor motion into account?
- Slow CT which effectively averages tumor’s motion into composite image
- Taking a full inhale and full exhale CT to see full extent of movement for margins
- 4DCT
What is the difference between a prospective and retrospective 4DCT?
Prospective - acquire images only during specific phases of respiration (less dose, less info)
Retrospective - acquire images during all phases and sort them into groups based on timing (more dose)
What is a “duty cycle”
Fraction of time that the beam is on compared to total treatment time
During inhalation, what direction does tumor travel in axial and saggittal views?
Axial - tumor will travel anteriorly and laterally
Sagittal - tumor will travel inferiorly
What are some pros and cons to gating?
Pros
1. Comfortable for patient (free breathing)
2. Less healthy tissue is irradiated
Cons
1. Longer treatments
2. Worse duty cycle
3. Beam on latency effects
4. Still motion during acceptable window
5. Dosimetric benefites are minimal and don’t justify the cons
Per MPPG 9a, what are some responsibilities for a physicist running/establishing a SBRT program? (list as many as you can)
- Inspection of technical aspects
- Performance of acceptance testing and commissioning for TPS, devices, etc
- Implementation of QA program
- Creating SWIs / SOPs
- Creating safety checklists
- Establishing incident reporting systems
- Supervising treatment planning
- Initial chart checks and validation of secondary dose calcs
- Personal supervision during SBRTs
How are seminal vesicles and penile bulb contoured for prostate SBRT?
Using MRI fused to sim CT
For liver SBRT, what is the mean dose to liver - GTV requirement and what happens if it can’t be met?
Dmean < 15 Gy
If you can’t meet it, lower Rx dose to 35 Gy
If your department is expected to treat only a handful of SBRT patients a year, should you be treating SBRT?
NO!
Part of staff training and competency is to regularly perform a certain procedure. If you’re barely getting SBRTs, you can’t argue that your staff is fully prepared/trained
Per published data, what aspect of clinical workflow results in the most SRS/SBRT accidents?
Lapses in processes and clinical practice
**Note: **The machine itself very rarely is the issue in accidents
Why is an E2E test invaluable?
It’s very difficult to determine uncertainties of individual components of a given process
Your E2E will determine an overall uncertainty of the entire workflow
Why are uncertainties/errors more pronounced in SBRT than conventional fractionation?
Small number of fractions = less opportunity to “average out” setup uncertainties/errors
High dose per fraction also means that if you miss, it will have a larger effect on normal tissues
For Lung SBRTs, which algorithm will give a higher calculated MU? Why?
MC or PB
MC
PB does not account for loss of dose in target due to minimal scatter from surrounding air. Therefore it over-estimates the dose the PTV gets, and therefore thinks the plan needs less MUs to get coverage.
So the result is if you use PB, you’ll under-dose your target
What are some devices we use for patient positioning and immobilization reproducibility?
Wingboard with indexing, head holder and arm hold
Long vac lok
Foam wedge under knee for patient comfort
Give a general rundown of how triggered imaging works
Beacons implanted in patient
At triggered moments, take a single kV image during the treatment delivery and confirm position of beacons vs expected
Triggered is based off one or more of following that oyou can set…
- Breathing motion of patient
- Elapsed time
- MU delivered
- Gantry angle
If decision threshold is exceeded (which you set), the treatment stops and you either make a shift or override
If gating is used in lung planning, what phases are the treatments typically delivered at? And where is planning done if 4DCT
Planning done on Ave-IP
Treat on 40%, 50% and 60% phases
When phasing the breathing cycle, what do 0%, 50% and 100% represent?
0% is beginning of cycle
50% is full expiration
100% is back to full inspiration
For GE and Varian 4DCT, how is the frequency of the CT scan protocol user defined? How does cine mode work?
You first find the breathing cycle using RPM (say 4 seconds)
You enter 4 seconds into CT, and the CT will take an axial image at a given table position at a frequency exceeding user-defined phase binning
That is, an axial image should be obtained AT LEAST every 4/10 seconds
And your axial table position is held for breathing cycle + time required to obtain a single scan (usually around 6 - 7 seconds)
Scans are NOT HELICAL, they are axial
How are 4DCT images correlated to phases?
CT system and RPM system correlate CT data taken during a single binning of time to reflective marker position at that same exact time
Output is a series of CT axial images corresponding to a different phase of the respiratory cycle
For a 4DCT cine mode, approximately what thickness of acquisition is taken per step?
2 cm
What is the pitch of a 4DCT?
In theory it’s 1 since increment of couch = width of collimation
What are the free breathing with and without contrast scans used for in Lung SBRT?
Backups in case the 4DCT looks really bad
Is a free breathing CT scan the same as a Ave-IP? Why or why not?
NO
Free breathing CT takes the scan very quickly, so each slice samples maybe 1 or 2 phases of the cycle
Is a free breathing CBCT the same as an Ave-IP? Why or why not?
It’s fairly close, since CBCTs take a while to scan and scan all slices at once, they will sample enough phases that it can average out to be equivalent to an Ave-IP
True or False
Contouring a structure, including ITV, near the diaphragm using MIP or Min-IP is not reliable. Explain
True
This is due to artifact and blurring.
Ex. if you use MIP to draw the ITV for lung cancer, but the diaphragm goes into where the abdomen was, all of a sudden you can’t distinguish max HU values caused by tumor vs those caused by the diaphragm moving towards where the tumor may have been. So suddenly the tumor in the ITV blends with the diaphragm
Vice versa for Min-IP for liver ^
So to contour targets near abdomen, physician should also watch the breathing play
What’s the main difference between RPM for sim vs treatment?
During sim you use the block with one reflector and a couch mounted cam
During treatment you use the alligator with 4 reflectors and a ceiling mounted cam
True or False,
RPM, if used during treatment, is used exclusively for breath hold
True
Infact, there’s considerations to replace OSMS breath hold with RPM
How do you determine RPM margin for breath holding?
In theory you should see their breath hold capabilities during sim
But a sim is faster than treatments and if therapist forgets to call physics to treatment they may not see the sim. So instead you can also, at first fraction, gauge their breath hold ability
What are some reasons/questions posed that have resulted in gating being less frequently used?
- Lots of latency uncertainty
- How do you know if a beam is on at exactly the right time in a cycle
- How do you know that a surragate correlated directly to a phase?
- How do you really QA the system?
Why are we allowed to use breath holding for lung SBRT?
Because tumor motion is slow relative to the motion of MLCs, so interplay is minimized
Why are we allowed to use IMRT for non-SBRT lung?
Over the course of a 30 Fx lung, interplay is minimized due to error averaging reducing error to something not clinically relevant
**Note: **For 3D, there is effectively no interplay
Per RTOG, what is 1 Fx SBRT Spine prescription dose and cord constraints?
16 Gy in 1 Fx
D0.03cc < 14 Gy to cord
D10% < 10 Gy
Cord to be contoured 5.5 mm above and below tumor volume level
What margin is added to SBRT spine GTV?
0 mm
No presumed microscopic extension or setup error accounted for.
Think also, if you add a margin, part of the GTV will go into the cord itself.
Per RTOG guidelines for SBRT of regions of suspected respiratory motion, what is CTV (or ITV) to PTV margin assuming 4DCT and no 4DCT.
If you have a 4DCT to draw ITV, you give 5 mm margin to ITV for PTV
If you don’t have a 4DCT, you give 1 cm margin to CTV I/S and 5 mm axially
For Liver SBRT, what imaging is required for planning?
CT with and without contrast
With contrast is used for target dilineation
Without contrast is used for TPS
In theory, if motion is expected, you can also do 4DCT in addition to the other two. But the other two are required, 4DCT is not
For upper spinal SBRT, what immobilization technique is common use?
Thermoplastic mask system
For prostate SBRT, what is one technique that can be used to minimize rectal uncertainty (we don’t use it, but other sites may)
Rectal balloon that’s inserted and inflated to a reproducible volume during sim and treatment to immobilize prostate and position rectum reproducibly
Water or air may be used. Water is more reproduible, but air gives better rectal wall sparing
What’s the difference between conformal art therapy and dynamic conformal arc therapy?
Conformal arc therapy uses a fixed beam aperature for all angles
Dynamic conformal arc therapy uses changing aperature at angles to conform to projected tumor volume
Per RTOG 0813, what is healthy lung constraint?
V20 Gy < 10%
How does fractionation change for central lung tumors vs non central?
Central: 5 Fx to spare critical structures
Noncentral: less fractions
Which MRI sequence is typically used for spine SBRT TP?
T1
Per RTOG 1112, what is healthy liver dose constraint during liver SBRT?
V15 Gy < 700 cc
For gating treatments (not limited to SBRT), what are typical phases used for lung targets? What about breast or chest wall?
Lung will typically use end exhale phases as it’s repeatable positioning for tumor or end inhale to improve dosimetric parameters of the normal lung
Breast and chest wall will typically use end inhale to increase distance between heart and target
What are three most commonly used surface trackers for respiratory motion?
Exactrac (IR Marker)
RPM (IR Marker)
AlignRT (Optical pattern)
What is active breathing coordination (ABC) device?
Device used to hold patient’s breathing cycle at predetermined threshold.
Comprises mouth piece with a spirometer where the flow of air is stopped once the threshold amount of air has been inhaled
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
In general, do therapists prefer treating rails in or rails out? Why?
Rails in
Less clearance issues
For SBRT, what is better, rails in vs rails out? (Only assuming dosimetry replicability)
Rails in
You can start arcs titled off the PA so you wouldn’t treat through the rails
Additionally, modeling rails in TPS in general has uncertainty due to on the treatment IGRT which may chan ge location of rails relative to patient and isocenter. So if you can start off by generally avoiding the rails PA, thats best.
True or False
Modeling of rails in TPS is never truly accurate
True
Due to IGRT, uncertainty of rail positions relative to isocenter and beam arrangements is not easily modeled. Best practice is treat rails in and start beams off PA.
When would you ever want to treat rails out? (2 scenarios)
- If you have optimal angles PA and not oblique
- When you have very large patients (stress on couch inserts is greater with rails in vs rails out)
What does it mean to “QA patient positioning and immobilization” devices
This is a very vague statement
Essentially it means to do quick safety checks (check for leaks or breaks and ensure everything is functional), and to keep inventory and ensure there is enough to treat patients. The big thing is that this QA is for safety, not accuracy.
Example of devices include vac-loks, masks, couch inserts, water bath, wingboard, slant board, prone breast board, etc
In the workbook, what is “motion management QA” referring to?
It’s the routine QA of our motion management systems (VisionRT and Calypso)
Per TG 147, how often should an end-to-end test be performed for any nonradiographic localization and positioning system?
Annually and at time of acceptance
This is “hidden target test”
Besides artifact reduction, what is one major advantage of MVCT vs CBCT for IGRT?
MVCT has the exact same isocenter as treatment isocenter, meaning you don’t need to QA coincidence
Per TG-179, which of the following yields lowest dose to patient during IGRT treatment setup?
Helical MV CT
kV CT on rails
kV-CBCT
MV-CBCT
kV CT on rails
You’d expect it’s more, however a CT on rails is not equivalent IQ to a CT scan. It only needs to be good enough for localization, and it achieves that standard at a lower dose than a kV CBCT
In addition to respiratory motion, what other sources of movement can affect liver treatment sites?
Peristalsis and skeletal movements
In what scenario would you imagine using Calypso for SBRT Livers?
If lesion is near diaphragm and has expected large motion that you need to track
Ex. For a rare case in our clinic, we planned/treated off a long exhale CT while tracking respiratory motion with the calypso
For SBRT lung, you perform a 4DCT and motion study of the lesion and find it’s moving > 5 mm. What are your options for limiting interplay? Why do we not do anything?
Two options
- Switch to 3D and treat off the ITV–>PTV expansion. This limits interplay and creates uniform dose
- Use either gating, suppressed breathing or breath hold. Breath hold and suppressed breathing will require a re-sim. Gating will not
It was found in a literature search that interplay may not make as much as an affect as expected. This search is the reason why we just treat VMAT to the ITV–>PTV expansion. Especially cause our CT scan is not great at doing RPM with any sort of motion suppression. In theory we could free breath, but this would require re-sim.
For prostate calypso, what are the types of motion you may worry about in the prostate area?
- Patient passes gas
- Patient accidentally urinates cause bladder is full
- Patient moves around
Very common SBRT Prostate prescription is 40 Gy in 5 Fx. Why do we only do 36.25 Gy in 5 Fx?
It’s only recommended to go up to 40 Gy if you can easily visualize the urethra, that’s because urethra is limiting structure.
At our clinic, we MRI, but can’t confidently visualize urethra. What one of our physicians does like to do instead is to SIB up to 40 Gy a subvolume of the PTV that shows visual disease on the MRI
What is fraction frequency for SBRT treatments?
Every other day
For calypso, how do you draw the beacons on the plan before exporting to the calypso station?
- Draw hi-res contours in bone window level for each individual beacon
- Place reference point in center of each individual beacon
- Create a total beacon structure
- Set isocenter to center of the three beacons
- Create a beacon + 2 mm PRV structure (this is what will be used at treatment to ensure alignment of beacons)
Per NRG recommendations, what is urethra max dose constraint for 5 Fx SBRT Prostate.
38.78 Gy
What is MCO? What dose calc algorithm are you forced to use with MCO?
Multi-criteria optimization
It’s a new technology we just recently got that calculates multiple plans and presents a slider bar that allows you to customize the final plan depending on certain criteria you’re looking for
Forced to use Acuros
In our clinic, what are the only times we insrt the couch structure?
- Calypso
- TB1 SBRT patients
It was found in measurement thta all other couches barely affect the dose (~1%)
True or False
In our clinic, you do not always need to insert rail structures for SBRT?
True
Usually we treat off the PA anyway, and we assume for all treatments that rails are in, so we won’t be entering through rails anyway
For Lung SBRT treatment planning, what are the three common dosimetric features of the plan that most sites will look at?
- Gradient index (or visually, how does the 50% line close in? Should be tight, within 2 cm)
- Chestwall dose
- Max hotspot (most sites actually want a hot plan. Most places want 125% minimum hotspot at the center
What are some pros (5) and cons (2) to using 3D over VMAT for SBRTs?
Pros:
1. No interplay effect
2. Uniform dose
3. You can still get desired hotspots
4. Faster treatment time
5. No required patient QA
Cons:
1. Worse dose conformality
2. Higher OAR dose
For what two main scenarios would someone want to use a free-breathing CT to calculate lung SBRT dose, rather than a 4DCT derived Ave-IP?
- If there are a lot of artifacts in the Ave-IP
- Technical limitations (such as a scanner having an image limit per study, and cutting off some volumes)
Why is it okay to treat rails in for VMAT and not include the rail structure?
A few reasons
- We can start arcs off the PA to avoid angles shooting through rails
But even if we don’t do that,
- The fraction of time treating through the rails across an entire arc is not too large
When do we insert couch structures for SBRTs? In theory the answer should be “always”, but in reality we don’t, why?
Insert for…
1. patients treated on TB1
2. Calypso patients
Why not all patients?
1. Not all support structures are available in eclipse
2. You can in theory create your own with a CT scan, but that presents uncertainty
3. It’s found that The dose max couch (tennis racket), which we use for most SBRTs, barely affects dose unless the lesion is super shallow posteriorly
Although not common, when may a physician want to call a certain extremity treatment an SBRT extremity? (two reasons)
- They want to escalate dose above usual
- They want increased supervision during the procedure
For spine SBRTs, when do you want to spare the cord/cauda and when don’t you?
Spines will usually be accompanied by a MRI
If MRI shows disease in the cord or cauda region, you want to limit the hotspot in the structure but still give it prescription dose or close
If MRI shows no disease in cauda/cord or there is prior radiation to cord/cauda, you want to spare the structure even further and don’t worry about giving prescription
For curative vs palliative SBRTs, what is the general rule of thumb for hotspots? Give an example of one curative and one palliatiive SBRT
Curative: You want very high hotspot in the center. Lung is example
Palliative: You want to limit the hotspot. For palliatives, the objective is to first and foremost “do no harm”. So you want to avoid hotspots on the chance that the hotspot can move due to setup error and wind up in a critical structure. Spines and bone mets are examples
For Calypso SBRT, what is our flexible tolerance and hard tolerance for beacon pitch?
Flexible but ideal: 5 deg
Hard absolute limit: 10 deg
For Calypso SBRT, what is the offset tolerance we use?
2 mm in any one direction
When calypso displays LAT, VRT and LNG offsets, what is that in reference to?
Centroid of beacons offset from isocenter
True or False
After you align using calypso, then verify beacons are still in place with CBCT, you can make shifts with calypso anytime without having to retake a CBCT?
True
For positioning, we trust calypso more than CBCT. The only thing we use CBCT for is to confirm that the beacons have not migrated. If they have, THEN we would only shift off CBCT and track relative with Calypso
What is the ICRU defined heterogeneity index?
The ratio between the highest dose received by 5% of the PTV and the lowest one received by 95%
How does cine time relate to breathing period for 4DCT?
Cine time = breathing period + 1.5 seconds
How many total images are taken during 4DCT?
3000
What margins do we give all our SBRTs besides spine?
5 mm
What slice thickness do we use for SBRTs?
If we did 4DCT, we use 2.5 mm
Everything else we use 1.25 mm
Where does interplay tend to be an issue if anywhere? GTV or PTV?
Edges of PTV
Barely even effects the GTV
True or False
Unless margins = 0 mm, interplay barely affects target coverage?
True
If you do use 0 margin however, it may affect coverage
Fill in the blanks,
More arcs = ________ interplay
More fractions = ________ interplay
More modulation = ________ interplay
Higher dose rate = ________ interplay
Large movement amplitudes = ________ interplay
Across the board, all studies showed a less than ____% change in dose to target structures is found due to interplay in SBRT
More arcs = less interplay
More fractions = less interplay
More modulation = more interplay
Higher dose rate = more interplay
Large movement amplitudes = more interplay
Across the board, all studies showed a less than 1% change in dose to target structures is found due to interplay in SBRT
True or False
Hypoxic region of lesions/tumors are not always in the same place?
True
For SBRT/SRS lesions, they are typically in the center, but this is not a law that they must follow
Higher hot spots in SBRTs leads to….
[] conformity index
[] fall-off
[]normal tissue dose
Higher hot spots in SBRTs leads to….
better conformity index
faster fall-off
lower normal tissue dose
True or false
For equal fractionation, a plan with more dose heterogeneity in target will lead to improved NTCP AND TCP versus a more homogenous target dose?
True and False
NTCP yes, it will improve
TCP it improves, but slightly
TCP improves more significantly though when the decreased NTCP allows for dose escalation, allowing you to increase dose per fraction. But for equal fractionations it improves only a little bit
Above how much hotspot do you start to see a minimal advantage in raising dose heterogeneity to improve plans?
125%
Above 125% the improvements to plan quality for SBRTs is minimal
