Age dependence of heavy-ion mammary carcinogenesis

Risk of secondary cancers from heavy ion therapy is not well understood. We previously demonstrated that carbon ion irradiation potently induced rat mammary tumors effectively. Because juvenile exposure to ionizing radiation is known to cause high risk of breast cancer, application of heavy ion therapy to childhood cancers should be based on the information on age de¬pen¬dence of secondary cancer induction. In the present study, we irradiated female Spra¬gue-Dawley rats with 290 MeV/u car¬bon ions at 3, 13 and 17 days postcoitum and at 1, 3, 7 and 15 weeks postnatal. Of a total of 893 rats, observation was finished for 492 rats and macroscopic mammary tumors were counted. The preliminary result shows that adulthood (7 and 15 weeks of age) is a more susceptible period than fetal and juvenile periods. Path¬o¬log¬i¬cal analysis is currently in progress.