Second cancer following chemotherapy and radiotherapy. An epidemiological perspective.

Both ionizing radiation and alkylating chemicals are used to treat cancer, and both are themselves carcinogenic. Epidemiological methods of study have provided a substantial amount of information on the long-term risks of the two types of treatment. Radiotherapy appears to increase the risk of a wide range of cancers, but the size of the increase is not substantial. Chemotherapy, in contrast, produces a very large increase in the risk of acute leukaemia, and does not generally appear to produce other types of cancer. Leukaemias following both treatment modalities appear rapidly, while the risk of solid tumours induced by radiotherapy is not manifested until a decade or more has elapsed. There does not seem to be synergy between the two types of therapy in their carcinogenic action. Continued use of the epidemiological approach is essential in risk-benefit evaluation of cancer therapy.