DMBA-induced mammary tumours in locally irradiated rats.

Local X-ray irradiation (400 r) covering 3 abdominal mammary glands revealed no tumours in the irradiated region after 10 months. A similar irradiation 1 or 10 days prior to i.v. DMBA showed a significant increase of tumours in the irradiated region, as compared with the contralateral abdominal region. The two thoracic regions, however, still had a very high tumour incidence. This was equal to, or even higher than that of the irradiated area. The same pattern also prevailed in animals receiving extra-mammary irradiation. The significance of early local factors in carcinogenesis is stressed.