Dosimetry and Radiation Biology
Quantities
Absorbed dose (gray, joules per kilogram) describes energy deposited in tissue. Equivalent dose (sievert) applies the radiation weighting factor, which is 1 for x-rays and gamma rays. Effective dose sums tissue-weighted equivalent doses and expresses whole-body stochastic risk; tissue weighting factors are greatest for gonads, red bone marrow, lung, stomach, colon and breast, with the remainder tissues pooled.
Biological effects
Deterministic (tissue reaction) effects have thresholds and rise in severity with dose: skin erythema around 2 Gy, cataract around 0.5 Gy, epilation higher still, and acute radiation syndrome above roughly 1 Gy whole body. Stochastic effects, cancer and heritable disease, are assumed to have no threshold, with probability rising with dose under the linear-no-threshold model. Radiosensitivity follows Bergonié and Tribondeau: rapidly dividing, undifferentiated cells (bone marrow, gut epithelium, gonads, lymphocytes) are most sensitive; nerve and muscle least. Cell-cycle sensitivity runs M > G2 > G1 > late S. Oxygen fixes damage; fractionation spares normal tissue.
Traps
Candidates swap Gy and Sv, or claim a threshold exists for cancer. Foetal risk: below 100 mGy the risk of malformation is regarded as negligible, though childhood cancer risk rises slightly. Diagnostic exposures sit far below deterministic thresholds: a chest radiograph is around 0.02 mSv, an abdominal CT roughly 5–10 mSv.
Worked example
If the whole body receives 10 mGy, effective dose is about 10 mSv. If only the chest (tissue weighting about 0.12) receives 10 mGy, effective dose is about 1.2 mSv. Same absorbed dose, roughly tenfold lower risk estimate.