The carcinogenicity of radiation therapy.

Ionizing radiation as used for therapy for cancer is probably weakly carcinogenic at worst. The probability that cancers will be induced at a distance from the treatment volume is so small that it can only be inferred from experiences with large populations exposed to much higher radiation doses. The risk of cancer in and adjacent to the treatment volume also appears to be small, especially in adults. Intensive radiotherapy or radiotherapy of children 20 to 30 years ago appears to have induced secondary cancers in about 3 to 4 per cent of those treated, but modern practice has every expectation of reducing this incidence. No precise risk factor can be offered, but it seems likely that less than 3 to 4 per cent is a reasonable projection. The reason for the low carcinogenicity in the treatment volume probably lies in the fact that the irradiation dose is high and many cells are killed rather than transformed. The frequency of the induction of radiogenic cancer adjacent to or near the treatment volume is expected to vary according to the tissue exposed. It is not estimated to exceed a few per cent in the worst instances (for example, breast and thyroid gland) and is much less than 1 per cent in most tissues.