Dosimetry and Radiobiology
This domain tests dose quantities, measurement methods and biological effects. Expect questions that distinguish absorbed dose, equivalent dose and effective dose, and stochastic from deterministic effects.
Core rules
- Absorbed dose is energy per unit mass, measured in gray (Gy). Equivalent dose applies radiation weighting, in sievert (Sv). Effective dose applies tissue weighting and represents whole-body risk.
- Stochastic effects have no practical threshold and increase in probability with dose; deterministic effects have a threshold and increase in severity.
- Radiosensitivity is higher in rapidly dividing cells and in fetuses. Dose, dose rate, fractionation and cell cycle phase modify response.
- Dosimeters include TLD and OSL badges; dose-area product estimates patient exposure but is not an effective dose.
- The linear no-threshold model is used for radiation protection planning at low doses.
Worked example
An organ receives 2 mGy and has a tissue weighting factor of 0.05. Its contribution to effective dose is 2 mGy × 0.05 = 0.1 mGy, expressed as 0.1 mSv when the radiation weighting is 1. The reasoning is that effective dose sums weighted organ doses; it is not the same as the local absorbed dose.
Traps
Do not interchange Gy and Sv. Do not say deterministic effects are random. Do not treat DAP as effective dose. Do not assume a dosimeter badge measures the patient's dose. Do not ignore fetal and pediatric sensitivity.