CMD Quality Assurance & Control 3 — Questions and Answers
Question 1: When performing annual QA on a multileaf collimator (MLC), which test specifically evaluates leaf position accuracy under dynamic conditions?
- Picket fence test (Correct answer)
- Starshot test
- Winston-Lutz test
- Beam flatness measurement
Correct answer: Picket fence test
The picket fence test uses a series of narrow, sequential MLC-defined fields to expose film or an EPID, revealing individual leaf positioning errors under dynamic movement.
Question 2: According to TG-40, how frequently should beam flatness and symmetry be measured for a clinical linear accelerator?
- Daily
- Weekly
- Monthly (Correct answer)
- Annually
Correct answer: Monthly
TG-40 recommends monthly measurements of beam flatness and symmetry for clinical linear accelerators as part of standard QA protocols.
Question 3: A radiation therapy department uses IMRT for head and neck treatments. Per TG-142, what is the recommended tolerance for IMRT patient-specific QA gamma analysis (3%/3mm)?
- Greater than 80% of points passing
- Greater than 90% of points passing
- Greater than 95% of points passing (Correct answer)
- Greater than 99% of points passing
Correct answer: Greater than 95% of points passing
TG-142 recommends that IMRT patient-specific QA should achieve greater than 95% of points passing using a 3%/3mm gamma criterion.
Question 4: During a brachytherapy source calibration, the measured air kerma strength deviates by 6% from the vendor-stated value. What is the appropriate action per TG-56?
- Accept the vendor value and proceed with treatment
- Use the measured value and notify the vendor (Correct answer)
- Average the two values for treatment planning
- Return the source and request a replacement
Correct answer: Use the measured value and notify the vendor
TG-56 states that if the measured source strength deviates by more than 5% from the vendor certificate, the measured value should be used and the discrepancy reported to the vendor.
Question 5: What physical quantity is directly measured by a well-type ionization chamber used in brachytherapy source calibration?
- Absorbed dose to water
- Air kerma rate (Correct answer)
- Exposure rate in air
- Dose equivalent rate
Correct answer: Air kerma rate
A well-type chamber calibrated with a NIST-traceable standard measures air kerma rate, which is then used to determine air kerma strength (SK) for brachytherapy sources.
Question 6: Which QA procedure verifies that a treatment plan correctly accounts for tissue heterogeneity in the presence of lung tissue?
- Output factor measurement in a homogeneous phantom
- End-to-end test using an anthropomorphic heterogeneous phantom (Correct answer)
- Point dose measurement in a water phantom at dmax
- Depth-dose curve measurement in solid water
Correct answer: End-to-end test using an anthropomorphic heterogeneous phantom
An end-to-end test using an anthropomorphic phantom with tissue-equivalent lung inserts verifies the complete chain from imaging through dose calculation and delivery in realistic heterogeneous conditions.
Question 7: A dosimetrist reviews a prostate IMRT plan and notices the rectal V70Gy constraint of 20% is exceeded. Which plan modification is MOST appropriate to address this?
- Increase the number of beams to spread dose
- Reduce the prescription dose to the PTV
- Add a rectal avoidance structure and re-optimize with tighter rectal constraints (Correct answer)
- Accept the plan if the PTV coverage exceeds 99%
Correct answer: Add a rectal avoidance structure and re-optimize with tighter rectal constraints
Adding a dedicated rectal avoidance structure with tighter optimization constraints allows the optimizer to better spare the rectum while maintaining target coverage.
CMD Quality Assurance & Control 3
When performing annual QA on a multileaf collimator (MLC), which test specifically evaluates leaf position accuracy under dynamic conditions?