CMD CMD Radiation Physics & Radiobiology

Question 1

What is the unit of absorbed dose in the SI system used in radiation therapy dosimetry?

Accepted Answer

Gray (Gy)

Answer

The Gray (Gy) is the SI unit of absorbed dose, defined as 1 joule of energy deposited per kilogram of matter.

Question 2

Which interaction of radiation with matter is most responsible for the biological damage caused by therapeutic X-rays?

Accepted Answer

Compton scattering

Answer

Compton scattering dominates at therapeutic energies (1–25 MV), producing energetic recoil electrons that cause ionization and biological damage.

Question 3

In radiobiology, what does the 'α/β ratio' characterize?

Accepted Answer

Tissue sensitivity to fractionation

Answer

The α/β ratio describes how sensitive a tissue is to changes in fraction size; low α/β tissues (≈3 Gy) are more sensitive to large fractions.

Question 4

What is the approximate half-value layer (HVL) concept used to characterize in radiation therapy physics?

Accepted Answer

Beam quality and penetrating power

Answer

The HVL is the thickness of a specified material that reduces beam intensity by 50%, characterizing the beam's penetrating ability and quality.

Question 5

Which factor does the Tissue-Phantom Ratio (TPR) eliminate compared to Percent Depth Dose (PDD)?

Accepted Answer

Source-to-surface distance (SSD) dependence

Answer

TPR is measured at a fixed source-to-detector distance, making it independent of SSD and useful for isocentric treatment planning.

(CMD) Certified Medical Dosimetrist Practice Test