CMD CMD Radiation Physics & Radiobiology 2

Question 1

What is the linear energy transfer (LET) of diagnostic X-rays and therapeutic photon beams classified as?

Accepted Answer

Low LET radiation

Answer

Photon beams (X-rays and gamma rays) are low LET radiation because they deposit energy sparsely along their track, unlike heavy charged particles.

Question 2

Which quantity describes the ratio of the biological effect of a given radiation compared to 250 kVp X-rays?

Accepted Answer

Relative Biological Effectiveness (RBE)

Answer

RBE compares the dose of reference X-rays needed to produce the same biological effect as the test radiation, and is greater than 1 for high-LET radiation.

Question 3

In photon beam dosimetry, what does the term 'dmax' refer to?

Accepted Answer

The depth of maximum dose deposition

Answer

Dmax is the depth at which the maximum dose occurs in a water phantom; it increases with beam energy (e.g., ~1.5 cm for 6 MV, ~3.5 cm for 18 MV).

Question 4

What is the oxygen enhancement ratio (OER) primarily relevant to in radiation therapy?

Accepted Answer

Hypoxic tumor cells are more radioresistant

Answer

The OER quantifies how much more dose is required to kill hypoxic cells versus well-oxygenated cells; hypoxic cells are approximately 2.5–3× more radioresistant.

Question 5

Which law states that the intensity of radiation decreases inversely with the square of the distance from the source?

Accepted Answer

Inverse square law

Answer

The inverse square law states that radiation intensity is proportional to 1/d², where d is the distance from the point source.

(CMD) Certified Medical Dosimetrist Practice Test

(CMD) Certified Medical Dosimetrist Practice Test

CMD CMD Radiation Physics & Radiobiology 2