THERMAL-RESISTANCE IN A SPONTANEOUS MURINE TUMOR

Resistance to subsequent hyperthermia as a result of prior heating was investigated using a spontaneous murine tumor implanted into the feet of C3H/Sed mice. Tumors were treated by immersing the tumor-bearing foot into a constant-temperature hot water bath set at 45.5.degree. C and were given single and split doses of heat. Response was assessed using a tumor-growth time assay. The following 3 aspects of thermally-induced resistance were particularly considered: the time course of development and decay; the importance of the magnitude of the priming dose and the influence of the size of the tumor at the time of treatment. Substantial resistance was induced in this tumor by short priming doses at 45.5.degree. C, rising rapidly 1-2 days after the 1st treatment and then starting to decay. There was no significant difference in the kinetics of thermal resistance induced in tumors treated at 4 mm and those treated at 8 mm in size, although the larger tumors were more sensitive to single doses of heat. Increasing the magnitude of the priming dose of heat resulted in an increase in the magnitude of resistance to the 2nd dose. The results of this study are compared with results of similar studies in this and other laboratories using murine normal tissues and cells in culture. Possible clinical implication [for cancer thermotherapy] are considered.