Incidence of primary malignancies other than breast cancer among women treated with radiation therapy for benign breast disease.

This report presents data on the long-term risks of developing malignancies other than breast cancer after exposure to scattered doses of ionizing radiation. The estimates were based on a cohort of 3,090 women who were diagnosed clinically with benign breast disease between 1925 and 1961. A total of 1,216 women were treated with radiation therapy. The breasts received a mean absorbed dose of 5.84 Gy. Mean absorbed doses owing to scatter to 14 other organs were also determined. The lung received the highest mean scattered dose (0.75 Gy; range 0.004-8.98 Gy) and rectum the lowest (0.008 Gy; range 0-0.06 Gy). Median age at first exposure was 40 years. The follow-up lasted up to 61 years after treatment (mean follow-up 27 years). End-point data were obtained from population-based registers. With internal reference the relative risk for non-breast solid tumors was 1.2 (95% confidence interval 1.0-1.5). However, this excess was not apparent when comparison with the general population was made (standardized incidence ratio = 0.95; 95% confidence interval 0.84-1.07). Among individual sites no significantly increased risks were observed. For stomach cancer there was a linear increase with dose (ERR/Gy = 1.3; two-sided P = 0.05). No increased risk was observed for leukemias. In two earlier reports, breast cancer incidence has been shown to be significantly increased in this cohort of irradiated women. Our results suggest that the scattered doses from the breast irradiation may have increased the risk of cancers of other sites, but the small number of cases in different locations precludes strong interpretations.