The tumor-initiating and -promoting effects of ionizing radiations in mouse skin.

The tumor-initiating and -promoting effects of ionizing radiation were tested, using mouse skin as the target organ and 90Sr-90Y beta-rays as the source of radiation. Neither single 2400 rad beta-irradiation followed by repetitive treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) nor single pretreatment with 7,12-dimethylbenz[alpha]anthracene (DMBA) followed by repetitive 470 rad beta irradiation produced tumors above the level of significance within a period of 210 days, while a positive control, DMBA + TPA, yielded a high incidence of papilloma in a shorter period. DMBA exerted an action antagonistic to beta-rays in the induction of malignant tumors. It is concluded that the tumor-enhancing activity of repetitive radiation is qualitatively different from the promoting activity of TPA.