Biological effects of heat.

The biological effects of heat appear to be favorable for its use to treat cancer. Heat kills cells in a predictable and repeatable way. The age response function complements X-rays in that S-phase cells are most sensitive, and at the same time cells that are at low pH or are nutritionally deprived are also more sensitive. This offers the possibility that cycling tumor cells and quiescent cells that have respired to hypoxia may be more sensitive to heat than are the slowly turning over cells of the normal tissues responsible for late effects. Thermotolerance, in general, represents a problem and a complication in clinical practice but may be exploited to advantage. The interaction of heat with ionizing radiation has been studied extensively and is complex; in general, heat inhibits the repair of both sublethal and potentially lethal X-ray damage, but it is not obvious how to exploit this to advantage. By contrast, the potentiation by heat of the action of chemotherapy agents has been relatively neglected. This is a promising area, since local hyperthermia can "target" drug action in a way not otherwise possible. Heat is a weak mutagen and has not been shown to be a carcinogen; this is a most desirable property at a time of increasing concern for the oncogenic potential of agents used to treat cancer.