Some problems related to the clinical use of thermal isoeffect doses

The well-known biological isoeffect relationship between treatment time and temperature has been suggested as a basis for a general biological heat dose unit which could be used to compare the effect of different heat treatment schedules. This is frequently expressed as 'equivalent heating time at 43 degrees C'. Such a conversion has in experimental studies been shown to be effective in comparing single heating schedules. However, clinical treatment has some features which may strongly influence the usefulness of an isoeffect heat dose. Firstly, the heat distribution is generally heterogeneous and fluctuates with time, which in some situations results in increased damage due to step-down heating. Secondly, in the situation where hyperthermia and radiation are given simultaneously, the time-temperature relationship may be different from that in the case of heat alone, and from the effect of heat given as an adjuvant to radiotherapy. Thirdly, most clinical treatments are given as fractionated treatments, and it is almost certain that thermotolerance may influence the biological heat effect to some extent. However, with the unknown kinetics of thermotolerance the magnitude of this phenomenon cannot be predicted. A series of experiments in a C3H mammary carcinoma were performed in order to analyse some of these problems.