CMD CMD Radiation Physics & Radiobiology 2 — Questions and Answers
Question 1: What is the linear energy transfer (LET) of diagnostic X-rays and therapeutic photon beams classified as?
- Low LET radiation (Correct answer)
- High LET radiation
- Intermediate LET radiation
- Neutral LET radiation
Correct answer: Low LET radiation
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?
- Relative Biological Effectiveness (RBE) (Correct answer)
- Oxygen Enhancement Ratio (OER)
- Alpha/Beta Ratio
- Tissue Maximum Ratio (TMR)
Correct answer: Relative Biological Effectiveness (RBE)
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?
- The depth of maximum dose deposition (Correct answer)
- The maximum field size at isocenter
- The maximum output of the linac
- The depth at which scatter is maximum
Correct answer: The depth of maximum dose deposition
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?
- Hypoxic tumor cells are more radioresistant (Correct answer)
- Oxygen increases scatter radiation
- High-oxygen tissues absorb more dose
- Oxygen reduces the alpha/beta ratio
Correct answer: Hypoxic tumor cells are more radioresistant
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?
- Inverse square law (Correct answer)
- Beer-Lambert law
- Bragg-Gray principle
- Compton's law
Correct answer: Inverse square law
The inverse square law states that radiation intensity is proportional to 1/d², where d is the distance from the point source.
Question 6: What is the primary purpose of bolus material in external beam radiation therapy?
- To increase surface dose and reduce skin-sparing effect (Correct answer)
- To reduce scattered radiation
- To improve beam flatness
- To shield critical structures
Correct answer: To increase surface dose and reduce skin-sparing effect
Bolus is placed directly on the patient's skin to bring the dose maximum to the surface, eliminating the skin-sparing effect when superficial targets require full dose.
CMD CMD Radiation Physics & Radiobiology 2
What is the linear energy transfer (LET) of diagnostic X-rays and therapeutic photon beams classified as?