Physiological mechanisms in hyperthermia: a review

In experimental animal systems, hyperthermia at therapeutic temperature (43-45 degrees C) causes a profound increase in blood flow in normal tissues while it induces only meager and temporal increases in blood flow in tumors. A severe vascular occlusion and hemorrhage usually follows the increase in blood flow in the tumors at the above temperatures. Another pronounced physiological change in tumors by heat is a prompt decrease in intratumor pH. The decrease in intratumor pH would accentuate the thermokilling of tumor cells and also possibly inhibit repair of thermodamage and development of thermotolerance in tumors. The temperature in tumors may rise higher than that in normal tissues during heating because of inefficient heat dissipation from the tumor as a result of decrease blood flow or vascular occlusion. Thus, the differential effects of heat on vascular function and pH in tumors and normal tissues may result in a greater damage in tumors than in surrounding normal tissues. Further investigation is urgently needed to find out whether similar physiological changes occur in human tumors and normal tissues by hyperthermia.