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)