Radiobiology and Radiation Protection
Core concepts
Know the deterministic vs stochastic dichotomy: deterministic effects have thresholds (skin erythema, cataract, infertility) and increase in severity with dose, while stochastic effects (carcinogenesis, heritable effects) are modeled as no-threshold and increase in probability. Understand the linear no-threshold model, dose units (Gy, Sv, mSv), and the weighting factors (radiation weighting for type, tissue weighting for organ sensitivity). Review cell survival curves, the shoulder of the curve, and the 5 R's of radiobiology (repair, reassociation, repopulation, redistribution, radiosensitivity). For protection, know ALARA, time-distance-shielding, and the cardinal principles of justification and optimization.
Traps
Questions test whether you know that diagnostic exposures are far below deterministic thresholds, so patient risk is stochastic. Do not confuse absorbed dose (Gy) with equivalent or effective dose (Sv). Fractionation spares normal tissue because of repair and repopulation; it does not spare tumor as much. Children and fetuses are more radiosensitive because of higher mitotic rates. Remember that the fetal dose from a properly performed abdominal CT is generally well below the level associated with deterministic effects, but stochastic concern remains.
Worked example
A worker receives 2 mGy from a gamma source (weighting factor 1) and 1 mGy from alpha particles (weighting factor 20). Equivalent dose = (2 x 1) + (1 x 20) = 22 mSv. If the alpha exposure were omitted, the answer would be only 2 mSv, a classic distractor. Always apply the radiation weighting factor before summing.