2nd Year: Commissioning and General Machine/TPS Knowledge Flashcards
If you take a PDD curve, and the curve is parallel but offset to what gold beam data expects, is this an issue with the Linac? Why or why not?
Most likely not
If energy/beam quality differs, the curve will be skewed in a non-parallel manner (attenuation differences)
If it just appears offset, you most likely just have a bad setup on measurement
If energy of a flattened beam is higher than expected, what happens to the horns?
Horns increase
What beam characteristic is generally the best distinguisher of beam energy?
Flatness (for flattened beams)
Ex. for a 6M beam, a 1% error in energy presents as a 4% error in profile
List some equipment that is required for acquisition of commissioning data
3D Water Tank with large enough dimensions for full scatter
Ion chamber for TG-51
Small field detector
Scanning chambers/detectors
Levels for the tank and gantry head
For water tank scanning, how is a CAX correction applied? How is it measured?
Measured using geometry that extracts inplane offset, crossplane offset and angles based off in-plane and crossplane profiles at 2 or 3 different depths
Translational corrections can be applied to all data collected therafter, but rotational corrections cannot be.
For large fields, not applying rotational corrections does not make a big difference. For small fields, it does so you want rotational CAX measurement to be as close to 0 as possible
For tanks that are unable to measure inplane, crossplane and diagonal profiles for the largest field sizes, what method is used to do this?
Shifted tank method
Center field at a quadrant of the tank, with approx 5 cm from any tank border from crosshair and measure only towards center of tank
With this method you are measuring half the profile, which is fine because in TPS you would just mirror the profile anyway
Why for commissioning do we do 10 x 10 PDD profiles for FFF beams with and without the lead foil?
TG-51 requires it with the lead foil
TPS requires it WITHOUT the lead foil. Which makes complete sense as you never treat a patient with lead foil
At what depths does Eclipse TPS want inplane and crossplane profiles for photons?
max, 5 cm, 10 cm, 20 cm, 30 cm
How do you measure Sp?
You don’t
You measure Scp in a water tank
You measure Sc in air
You then divide Scp/Sc for all respective field sizes to get Sp
How is DLG measured?
Varian created template plan where there’s small slit of opening with MLCs that slides across the field with varying gap sizes
Detector measures integrated dose for each slit size plans
Create a plot of integrated dose vs gap size
Extrapolate down to a gap of 0 mm and that gives you your DLG
What is the difference between inplane and crossplane?
Inplane is along the axis of the linac (LNG)
Crossplane is perpendicular to the axis of the LINAC (LAT)
Why are you allowed during commissioning to assume perfect symmetry of your profiles?
Because you normally check ahead of time during acceptance testing
Afterwards, during all your QA, you ensure symmetry is close to perfect
What is the most ideal small field dosimeter for scanning? What about output factors?
Scanning: diode detector such as Edge or MicroDiamond
OF: W1 or W2
Note: Although W2 has scanning capabilities, it’s not too great. Software kinda stinks. Also scanning requires high doses, and this over time causes plastic yellowing throughout course of commissioning
What criteria is used to compare measured profiles to VRB data?
2% / 2mm gamma analysis for inplane, crossplane and PDD
3% / 3 mm gamma analysis for diagonals
Where does the VRB data come from?
Real measured data which is a composite of multiple sets of data measured across multiple varian truebeams at Duke, with different physicists
What does VRB stand for?
Varian Reference Beam
How is VRB used during commissioning?
Two approaches based off the client
- VRB is used in the TPS to make models, and all commissioning measurements are used to validate the VRB
- The measured data is used in TPS to create the models and are double checked with VRB
In both cases, you need to ensure VRB data is close to measured data anyway. It’s then just a matter of which data do you want to use for your model? Which do you trust more?
How is data processed for PDD curves?
- Shift
- Smooth
- Normalize
- For electrons, software should have a depth dependent PDI to PDD correction for chamber
How is data processed for inplane and cross plan measurements?
Smooth
CAX correction
Make symmetric
The microdiamond and edge detectors are both diode detectors. What is the main compositional difference between them though?
Microdiamond is artifical diamong with a very small dye and is near tissue equivalent
Edge is silicon
Down to what field size is PDD data measured?
1 x 1 cm2
What field sizes are MLC and Jaw collimated small field profiles taken for in plane, crossplane and PDD curves?
1x1 –> 3x3 cm2 for PDD
2x2 and 3x3 for all profiles and PDDs
Down to what field size does eclipse accept in and crossplane data?
3x3 cm2
Anything below will not be accepted by Eclipse
Why for small field PDD can you feel confident measuring directly and not needing a per depth correction factor?
Although best practice is to do a per depth correction, 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