Biological Effects of Radiation and Radiation Biology
Core concepts
Understand deterministic (tissue reaction) vs stochastic effects. Deterministic effects have thresholds (e.g., erythema ~2 Gy, cataract ~0.5 Gy) and increase in severity with dose; stochastic effects (cancer, heritable) are probabilistic with no established threshold. Know DNA damage types (single- and double-strand breaks), repair, cell survival curves (linear-quadratic model), and the 5 R's of radiobiology (repair, repopulation, redistribution, reoxygenation, radiosensitivity). Understand LET, RBE, and the concept of the dose-rate effect. Acute radiation syndrome presents in hematopoietic, gastrointestinal, and cerebrovascular syndromes by dose range.
Traps
Questions test that heritable effects are stochastic and that deterministic effects have thresholds. Don't confuse LET (energy deposition density) with RBE (relative biological effectiveness). High LET (alpha, neutrons) has higher RBE and a survival curve with less shoulder. The linear-quadratic model: survival = e^(−αD−βD²); the α/β ratio describes fractionation sensitivity. Watch fetal effects: highest risk in the first trimester for deterministic effects.
Worked example / rule of thumb
Acute whole-body doses: <0.1 Gy usually no symptoms; 1–2 Gy mild hematopoietic; 4–6 Gy severe hematopoietic; >10 Gy GI/cerebrovascular. Rule of thumb: deterministic effects appear above ~0.5 Gy (cataract) and ~1–2 Gy (skin/hematopoietic).