Arrhenius relationships from the molecule and cell to the clinic

There are great differences in heat sensitivity between different cell types and tissues. However, for an isoeffefct induced in a specific cell type or tissue by heating for different durations at different temperatures varying from 43-44 degrees C up to about 57 degrees C, the duration of heating must be increased by a factor of about 2 (R value) when the temperature is decreased by 1 degrees C. This same time-temperature relationship has been observed for heat inactivation of proteins, and changing only one amino acid out of 253 can shift the temperature for a given amount of protein denaturation from 46 degrees C to either 43 or 49 degrees C. For cytotoxic temperatures < 43-44 degrees C, R for mammalian cells and tissues is about 4-6. Many factors change the absolute heat sensitivity of mammalian cells by about 1 degrees C, but these factors have little effect on Rs, although the transition in R at 43-44 degrees C may be eliminated or shifted by about 1 degrees C. R for heat radiosensitization are similar to those above for heat cytotoxicity, but Rs for heat chemosensitization are much smaller (usually about 1.1-1.2). In practically all of the clinical trials that have been conducted, heat and radiation have been separated by 30-60 min, for which the primary effect should be heat cytotoxicity and not heat radiosensitization. Data are presented showing the clinical application of the thermal isoeffect dose (TID) concept in which different heating protocols for different times at different temperatures are converted into equiv min at 43 degrees C (EM43). For several heat treatments in the clinic, the TIDs for each treatment can be added to give a cumulative equiv min at 43 degrees C, viz., CEM43. This TID concept was applied by Oleson et al. in a retrospective analysis of clinical data, with the intent of using this approach prospectively to guide future clinical studies. Considerations of laboratory data and the large variations in temperature distributions observed in human tumours indicate that thermal tolerance, which has been observed for mammalian cells for both heat killing and heat radiosensitization, probably is not very important in the clinic.(ABSTRACT TRUNCATED AT 400 WORDS)