CMD Quality Assurance & Control 2

Question 1

Which TG-51 measurement condition requires the use of a plastic water phantom for electron beam calibration?

Accepted Answer

When measuring beams with energy below 6 MeV where polarity correction is significant

Answer

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?

Accepted Answer

Suspend treatments and investigate immediately

Answer

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?

Accepted Answer

Checking the coincidence of radiation and mechanical isocenters

Answer

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?

Accepted Answer

3% deviation; outside monthly tolerance of 2%

Answer

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?

Accepted Answer

Radiochromic film or small diode detector

Answer

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.

(CMD) Certified Medical Dosimetrist Practice Test

(CMD) Certified Medical Dosimetrist Practice Test

CMD Quality Assurance & Control 2