Biological Effects of Radiation
Core concepts
Understand the mechanisms of radiation damage: direct action on DNA and indirect action via radiolysis of water producing free radicals. Distinguish deterministic (tissue reaction) effects, which have thresholds and increase in severity with dose, from stochastic effects (cancer, heritable), which are probabilistic with no established threshold. Know the linear no-threshold (LNT) model and its use in radiation protection. Recognize acute radiation syndrome (hematopoietic, gastrointestinal, neurovascular syndromes) and their dose ranges, plus late effects (cataracts, fibrosis, cancer induction). Understand radiosensitivity (law of Bergonié and Tribondeau), cell-cycle dependence, and factors modifying response (dose rate, fractionation, LET, RBE, oxygen effect).
Traps
Questions often conflate deterministic and stochastic effects, or misstate thresholds. Another trap is treating LNT as a proven biological mechanism rather than a conservative regulatory assumption. Remember that high-LET radiation has higher RBE and less dose-rate sparing.
Worked example
A dose of ~1 Gy whole-body produces mild hematopoietic changes; ~4–6 Gy causes severe hematopoietic syndrome; >10 Gy is generally fatal even with support. Rule of thumb: deterministic effects appear above roughly 0.5 Gy (cataracts) and rise steeply; stochastic risk is assumed proportional to dose with no threshold.
Practical points
Fractionation spares normal tissue by allowing repair. The oxygen enhancement ratio is higher for low-LET radiation.