In tissues from patients subjected to gastrectomy or colectomy, the heat sensitivity was determined in the case of 23 neoplastic, 15 gastric, 8 colorectal, and the adjacent normal tissues, using the in vitro succinate dehydrogenase inhibition test. The succinate dehydrogenase (SD) activity of tissue fragments was assayed, following exposure to heat at 43 °C (heat treatment) or 37 °C (control) for 5, 10, 15 or 20 h. The sensitivity to heat treatment was estimated by the percentage of SD activity of the heat-treated cells, compared to that of control cells. The decrease in SD activity varied in the tumor tissue, following expo-sure to heat. The SD activity decreased to a greater extent in the tumor tissue than in the adjacent normal tissue, in each case. The mean ± standard deviation of SD activity, following exposure to heat treatment for 20 h, was 32.1 ± 14.0% for the tumor tissues and 52.4 ± 10.4% for the adjacent normal tissues, with a statistically significant difference (p < 0.01). These results show that the assay of heat sensitivity is meaningful for prediciting the effectiveness of hyperthermia and that hyperthermia has a selectivity for treating a malignant lesion.
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