Experimental radiation therapy and apparent radioresistance of autochthonous tumors subcutaneously induced with 3-methylcholanthrene in mice.

The radiation response of autochthonous tumors induced by s.c. injection of 3-methylcholanthrene in ICR/JCL mice was studied. The tumors were mostly fibrosarcomas and grew with an average volume-doubling time of 2.6 days, independently of the time tumor appearance or dose of the chemical used. The tumors were locally and singly irradiated with 6-MV X-rays through a filter. Autochthonous tumors were similar to their transplants in postirradiation regression and gross cellular radiosensitivity (Do, 400 rads), as estimated from regrowth time. However, most of the autochthonous tumors irradiated with single doses of up to 7.4 kilorads recurred within 120 days, whie in identically irradiated tumor transplants a complete cure was obtained with doses of 6.5 kilorads or more. The 50% tumor control dose with no recurrence within 120 days was 4.4 kilorads for the transplants. Transplantation of 26 irradiated autochthonous fibrosarcomas produced only a few tumors in autochthonous hosts, and they were completely rejected in other, previously tumor-free mice; a 48% recurrence was noted even after resection of the irradiated tumor. Two possibilities for the apparent radioresistance of autochthonous tumors were suggested: (a) existence of radioresistant cells in autochthonous tumors; and (b) induction of a second new tumor by the additive effect of the chemical and radiation. Although our results are preliminary, the second possibility is not favored since autochthonous tumors induced with low doses of 3-methylcholanthrene recurred with high frequency after irradiation. Moreover, irradiation of tumor-free mouse skin that had been treated with the carcinogen produced tumors at a very low frequency.