Duplicate studies were conducted to determine the effects of repeated exposures to low-level X radiation on the incidence, volume, and latent period of DMBA-induced neoplasms in Syrian hamster cheek pouch epithelium. Doses employed were 0.05 ml of 0.1% 7,12-dimethylbenz[a]anthracene (DMBA) in mineral oil twice weekly for 10 weeks and 20-R head and neck X radiation once weekly for 17 weeks. Animals received either radiation alone, DMBA alone, or simultaneous radiation + DMBA beginning together. Appropriate controls were performed. Radiation alone did not result in any detectable pathology. In one study, the incidence (68 vs 40%) and volumes of tumors were significantly greater in animals receiving radiation + DMBA than in animals receiving DMBA alone. The latent period for earliest tumor appearance was shorter in animals receiving radiation + DMBA than in animals receiving DMBA alone (9 weeks vs 24 weeks). In the repeat study, increased tumor incidence in the radiation + DMBA group was again observed, but without the decrease in tumor latent period. These results are compatible with synergy between low-level X radiation and DMBA in terms of numbers of animals developing tumors, and also with low-level irradiation altering the biological progression of these DMBA-induced tumors. The results are suggestive of low-level X radiation acting as a synergistic cocarcinogen.

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