When SCCVII or KHT tumors <tex-math>$(150\ {\rm mm}^{3})$</tex-math> growing in the dorsum of the hind feet of mice were heated in a water bath at 44°C for 60 min, the local control rate was 75 or 5%, respectively. To investigate factors responsible for the differential thermosensitivity between SCCVII and KHT tumors, the intratumor temperature distributions during heating and the thermosensitivities of the tumor cells were studied. Significant temperature heterogeneity was observed in heated tumors. The thermal dose distribution during heating for the sensitive SCCVII tumors was found to be more homogenous than that for the resistant KHT tumors. For cells grown and heated in culture, SCCVII and KHT cells had similar thermosensitivities. However, when heated in vivo, both SCCVII and KHT cells were more sensitive than their counterparts grown in culture and SCCVII cells were more sensitive than KHT cells. If cells dispersed from the tumors were cultured in medium for 6 h and then heated, both types of cells became as resistant as cells grown in culture. One possible reason for tumor cells to be more sensitive to heating in vivo than in vitro, the temperature of unheated tumors, was examined. It was found that the temperature in the same region in unheated tumors varied temporally by several degrees with an average temperature of 31-32°C. We found no evidence that the temperature during tumor growth could greatly influence the thermosensitivity of the tumor cells. Our findings indicate that a more homogeneous distribution of temperature in the tumor during heating and higher in vivo thermosensitivity of the tumor cells are characteristics of the more heat-sensitive tumor.

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