CMD CMD Radiation Physics & Radiobiology 1 — Questions and Answers
Question 1: What is the unit of absorbed dose in the SI system used in radiation therapy dosimetry?
- Gray (Gy) (Correct answer)
- Rad
- Roentgen (R)
- Sievert (Sv)
Correct answer: Gray (Gy)
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?
- Compton scattering (Correct answer)
- Photoelectric effect
- Pair production
- Rayleigh scattering
Correct answer: Compton scattering
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?
- Tissue sensitivity to fractionation (Correct answer)
- Linear energy transfer of the beam
- Ratio of scattered to primary photons
- Oxygen enhancement ratio
Correct answer: Tissue sensitivity to fractionation
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?
- Beam quality and penetrating power (Correct answer)
- Dose rate at isocenter
- Percentage depth dose at dmax
- Output factor of a field size
Correct answer: Beam quality and penetrating power
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)?
- Source-to-surface distance (SSD) dependence (Correct answer)
- Field size dependence
- Energy dependence
- Scatter dependence
Correct answer: Source-to-surface distance (SSD) dependence
TPR is measured at a fixed source-to-detector distance, making it independent of SSD and useful for isocentric treatment planning.
Question 6: What radiobiological principle describes the repair of sublethal radiation damage between dose fractions?
- The '4 Rs' of radiobiology — Repair (Correct answer)
- Reoxygenation
- Redistribution
- Repopulation
Correct answer: The '4 Rs' of radiobiology — Repair
Repair of sublethal damage is one of the '4 Rs' of radiobiology and is the primary rationale for delivering radiation in multiple fractions over time.
CMD CMD Radiation Physics & Radiobiology 1
What is the unit of absorbed dose in the SI system used in radiation therapy dosimetry?