Radiation Biology, Radiation Protection, and Safety
Core concepts
Know the stochastic (cancer, hereditary) versus deterministic (threshold) effects and their dose relationships. Understand DNA damage: direct versus indirect action, single- and double-strand breaks, and repair. Learn cell survival curves (shoulder, D0, Dq), the Law of Bergonié and Tribondeau, and radiosensitivity variation across the cell cycle (M most sensitive, late S most resistant). Understand the linear no-threshold model and its role in radiation protection philosophy, plus ALARA and the three protective principles: time, distance, shielding. Know dose quantities - absorbed dose (Gy), equivalent dose (Sv, with radiation weighting factors), effective dose (tissue weighting) - and typical effective doses for common examinations. Understand occupational dose limits (ICRP: 20 mSv/y averaged over 5 years, 500 mSv to extremities/lens historically, now 20 mSv lens), public limits, and pregnancy considerations. Shielding: lead equivalence, half-value layer, and inverse square law.
Traps
Questions conflate absorbed dose with effective dose; effective dose is risk-weighted and cannot be measured directly. Deterministic effects have thresholds; stochastic effects are assumed threshold-free. The inverse square law applies to point sources in air, not to internal scatter. Remember fetal dose is highest in early gestation but teratogenic risk peaks in the 8–15 week window. Distinguish background radiation sources (radon dominates) from medical exposure.
Rule of thumb
Doubling distance from a point source reduces dose to one quarter (inverse square). Use it to justify stepping back during fluoroscopy.