CMD Quality Assurance & Control 2 — Questions and Answers
Question 1: Which TG-51 measurement condition requires the use of a plastic water phantom for electron beam calibration?
- When the field size exceeds 30×30 cm
- When the reference depth is less than 0.5 cm
- When measuring beams with energy below 6 MeV where polarity correction is significant (Correct answer)
- When the linear accelerator output exceeds 600 MU/min
Correct answer: When measuring beams with energy below 6 MeV where polarity correction is significant
For low-energy electron beams (below ~6 MeV), polarity corrections become significant and the TG-51 protocol provides additional guidance for handling these measurements.
Question 2: A daily QA check reveals the radiation isocenter and mechanical isocenter are offset by 2.5 mm. According to TG-142, what action is appropriate?
- Continue treatments; the offset is within tolerance
- Suspend treatments and investigate immediately (Correct answer)
- Increase monitor units to compensate for the offset
- Document and allow treatments only with physician approval
Correct answer: Suspend treatments and investigate immediately
TG-142 specifies that the radiation isocenter coincidence with mechanical isocenter should be within 2 mm; a 2.5 mm offset exceeds tolerance and requires immediate investigation.
Question 3: What is the primary purpose of performing a Winston-Lutz test on a stereotactic radiosurgery system?
- Verifying the output factor for small fields
- Checking the coincidence of radiation and mechanical isocenters (Correct answer)
- Measuring the penumbra width of the collimator
- Confirming leaf transmission through the MLC
Correct answer: Checking the coincidence of radiation and mechanical isocenters
The Winston-Lutz test verifies that the radiation beam passes through the mechanical isocenter during gantry, collimator, and couch rotation, ensuring SRS geometric accuracy.
Question 4: During monthly QA, the output of a 6 MV beam measures 0.97 Gy for a prescribed 1.00 Gy delivery. What is the percentage deviation and is it within TG-142 monthly tolerance?
- 3% deviation; within monthly tolerance of 3%
- 3% deviation; outside monthly tolerance of 2% (Correct answer)
- 1% deviation; within monthly tolerance of 1%
- 0.97% deviation; outside monthly tolerance of 0.5%
Correct answer: 3% deviation; outside monthly tolerance of 2%
A 3% deviation exceeds the TG-142 monthly output tolerance of 2% for photon beams, requiring investigation and potential recalibration.
Question 5: Which detector is most appropriate for measuring output factors in small radiation fields (less than 2×2 cm) used in SRS?
- Farmer-type ionization chamber
- Parallel-plate ionization chamber
- Radiochromic film or small diode detector (Correct answer)
- TLD chips in a phantom
Correct answer: Radiochromic film or small diode detector
Small fields require detectors with high spatial resolution such as radiochromic film, small diodes, or diamond detectors because standard ionization chambers over-average dose in the steep gradient regions.
Question 6: In a photon beam depth-dose measurement, the depth of maximum dose (dmax) shifts proximally when which parameter changes?
- Field size increases
- Source-to-surface distance increases
- Beam energy decreases (Correct answer)
- Collimator rotation changes
Correct answer: Beam energy decreases
Lower energy photon beams have shallower dmax depths; as beam energy decreases, dmax shifts toward the surface.
Question 7: A physicist notices that EPID-based daily output checks consistently read 1.5% higher than the ion chamber measurement. What is the MOST likely cause?
- The EPID gain calibration has drifted (Correct answer)
- The ion chamber has a calibration error
- The linac output has decreased by 1.5%
- The EPID panel is positioned too close to the source
Correct answer: The EPID gain calibration has drifted
EPID response can drift over time due to dark current changes, panel aging, or calibration drift, causing systematic offsets relative to absolute dosimetry.
CMD Quality Assurance & Control 2
Which TG-51 measurement condition requires the use of a plastic water phantom for electron beam calibration?