Clinical objectives and normal tissue responses in combined chemotherapy and radiotherapy.

It is clear that the risk of toxicity to normal tissues in combined therapy is reduced by separating drug administration and radiation exposure in time. The short-term interaction of drugs and radiation aimed at producing enhanced tumour cell kill is difficult to demonstrate in vivo in animal experiments and unlikely to be important in clinical practice. Increased toxicity of normal tissues in man has been manifest both in terms of early and late reactions and probably also in carcinogenesis. In the latter context choice of drug and possibly sequencing may be important in minimising the risk of tumour induction. There are few experimental studies examining moderately long time intervals between drug and radiation exposure. The administration of chemotherapy prior to irradiation may be less toxic than the converse sequence, although clinical data supporting this contention are limited at the present time.