THERMAL RADIOSENSITIZATION BY 41-DEGREES-C HYPERTHERMIA DURING LOW DOSE-RATE IRRADIATION IN HUMAN NORMAL AND TUMOR-CELL LINES

Three human cell lines (glioma, melanoma and fibroblast) were evaluated for responses to low dose-rate irradiation (LDRI) (0.88 and 0.41 cGy per min) alone or with concurrent heating (41 degrees C) during irradiation. In order to avoid cell cycle redistribution cells were held in plateau phase. The results show that the lowest LDRI gave maximum sparing in the glioma and the fibroblast cell lines while both dose rates achieved approximately the same effect in the melanoma line. The melanoma line was the most heat sensitive and showed the greatest thermal enhancement ratio (TER). For all cell lines TER was greatest at the lowest dose-rate, and in the melanoma the heat plus LDRI curve gave lower survival than the high dose-rate irradiation survival curves. These data show that concurrent mild hyperthermia combined with LDRI used in brachytherapy can enhance the effectiveness of clinical brachytherapy treatments. In addition, this effect was largest in the most resistant cell line indicating the potential for using this combination to overcome radioresistance in brachytherapy.