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Changes in Thermosensitivity of Mouse Mammary Carcinoma following Hyperthermia in Vivo¹

Radiobiology Laboratory, Department of Therapeutic Radiology, University of Minnesota Medical School, Minneapolis, Minnesota 55455

The time course of change in thermosensitivity of SCK tumor cells in vivo was investigated following preheating at 43.5° for 30 min. At varying times after the preheating with water bath, tumors were subjected to graded doses of second heating at 43.5° in vivo, and cell survival was determined by using in vitro cloning method. Thermosensitivity of the tumor cells in vivo [duration of heating for exponential inactivation of cells to 1/e (D₀): 15.5 min] gradually increased after the preheating, reaching a maximum increase at 5 hr (D₀: 7.5 min), and then decreased thereafter, due probably to a development of thermotolerance. Maximum thermotolerance was observed at 12 hr after the preheating, leading to a 3-fold increase in D₀ (48 min). The thermotolerance gradually decayed for several days therafter. The initial increase in thermosensitivity might be attributed to the acidic and nutritionally deprived intratumor environment as a result of vascular damage in heated tumor. It appears that thermotolerance gradually develops as time elapses and that it eventually overcomes the thermal sensitization of tumor cells in vivo.

¹ This work was supported by NIH Grant CA13353, Department of Health, Education and Welfare.

² To whom requests for reprints should be addressed.

Received 3/ 1/82. Accepted 8/ 6/82.