Physical principles of local heat therapy for cancer.

Local hyperthermia therapy for cancer can produce selective heating of solid tumors on the basis of known physical laws. If energy is deposited in the general region of the tumor, temperature tends to develop in the tumor higher than that in surrounding normal tissues. The goal of therapy is to achieve cytotoxic temperature elevations in the tumor for an adequate period of time, without damaging nearby normal tissues. Several modalities exist for local heat treatment, of which radiofrequency and ultrasound offer the most promise for controlled, localized heating at depth. A paucity of blood flow in the tumor compared to that in adjacent normal tissues can enhance selective tumor heating considerably. The tumor types that have reduced flow in their central regions are especially vulnerable to heat therapy, both because they can be heated more efficiently and because hypoxic and acidotic tumor tissues are more susceptible to damage by heat. This effect is more pronounced in larger tumors, which have smaller surface-to-volume ratios and so lose heat less rapidly by thermal diffusion. Selective heat treatment of larger tumor masses with low blood perfusion, therefore, is physically practical and rational therapy. Vigorous research efforts are now underway at many centers to optimize this approach.