Cancer after exposure to radiation in the course of treatment for benign and malignant disease.

This review assesses the patterns of radiation-associated relative risks of cancer incidence and mortality in groups exposed to ionising radiation in the course of treatment for various malignant and non-malignant conditions. In general, the relative risks among Japanese survivors of atomic-bomb explosions are greater than those among comparable subsets in studies of medically exposed individuals. Cell sterilisation largely accounts for the discrepancy in the relative risks between these two populations, although other factors may contribute, such as the generally higher underlying cancer risks in the medical series than in the Japanese atomic-bomb survivors, and dose-fractionation effects. The higher underlying cancer risk in some of the medically exposed populations, in particular for those with cancer-prone conditions, implies that the absolute excess risk is sometimes higher than in the Japanese data. For these cancer-prone individuals, the increase in absolute risk for those receiving the large doses of radiation associated with radiotherapy may be quite significant. This increase in risk has to be balanced against the generally high spontaneous cancer risk in these individuals and the benefits accruing from radiotherapy.