CELL KILLING AND THE SEQUENCING OF HYPERTHERMIA AND RADIATION

Previous reports have indicated that the greatest cell killing occurs when heat and radiation exposure occur simultaneously. Our studies using Chinese hamster ovary cells comparing cell killing from simultaneous exposure to radiation (500 rad) and heat (either 42.0°C for 60 min or 42.5°C for 40 min) with cell killing from irradiation at 37°C between split heat treatments indicate that these two methods may be equally effective as long as the time between heat treatments remains relatively short (less than 10 min). However, a greater increase in survival with increasing time between split heat doses was observed at 42.0°C than at 42.5°C. While these studies may indicate a more practical clinical method for obtaining maximum interaction between heat and radiation, they do not indicate whether the increased therapeutic ratio that is obtained from in vivo studies when heat and radiation are administered simultaneously will be affected. Investigations comparing the effect of equivalent heat doses (chosen to achieve equal cell killing) at different temperatures on the enhancement of cell killing by radiation either during the heating interval or exactly 3 min after heating indicate that temperatures between 42.5 and 43.5°C are the most effective in enhancing X-ray induced cell killing. Thus, the choice of temperature for the therapeutic use of hyperthermia with irradiation may prove to be an important factor in the success of the treatment. © 1979.