The risk of multiple primary breast and thyroid carcinomas: Role of the radiation dose

BACKGROUND. Some studies have suggested that there is an association between breast carcinoma and thyroid carcinoma. Because ionizing radiation is a well known risk factor for breast and thyroid carcinomas, the authors studied the effect of the radiation dose delivered for the treatment of each of these two malignancies on the risk of subsequently developing the other malignancy. METHODS. The risk of developing thyroid carcinoma subsequent to treatment for breast carcinoma was analyzed in 8 patients (cases) and 192 matched control patients (controls) nested in a cohort of 7711 women who were treated at Institut Gustave Roussy between 1954 and 1983. The risk of developing breast carcinoma after treatment for thyroid carcinoma was studied in a cohort of 2365 women who were treated in 3 French cancer centers between 1934 and 1995. RESULTS. Six of 8 patients with breast carcinoma (75%) who developed thyroid carcinoma and 71% of patients in the control group received radiation therapy during their treatment. The median dose to the thyroid was 6.6 grays (Gy) in the case group and 9.4 Gy in the control group. The overall relative risk of thyroid carcinoma associated with radiation therapy was 1.2 (95% confidence interval, 0.2-6.2). No relation was observed between the radiation dose and the risk of thyroid carcinoma (P = 0.8). Among 2365 women who were treated for thyroid carcinoma, 48 women developed a subsequent breast carcinoma. A significant excess of breast carcinoma was observed among women younger than 59 years at the time of diagnosis of breast carcinoma compared with women in the same age group in the general population. The mean absorbed dose delivered to the breasts by ¹³¹I and external radiation therapy was 0.7 Gy. No relation was found between the radiation dose and the risk of breast carcinoma (P = 0.8). CONCLUSIONS. The previously reported excess incidence of breast carcinoma after thyroid carcinoma was not related to radiation treatment with ¹³¹I and/or external radiation therapy. Radiation therapy for breast carcinoma did not increase the risk of subsequent thyroid carcinoma.