Evidence for Increased Susceptibility for Breast Cancer from Exposure to Ionizing Radiation Due to Familial Breast Cancer History: Results from the Swedish Hemangioma Cohort.

Women with breast cancer among family members are at increased risk for breast cancer. However, it is unknown whether familial breast cancer history (FBCH) also increases individual susceptibility for breast cancer from radiation exposure. In this cohort study, 17,200 female Swedish hemangioma patients with 1,079 breast cancer cases between 1958-2013, exposed by ionizing radiation in infancy, were linked to their first-degree relatives. The association between FBCH and radiation-induced breast cancer risk was assessed. Further, the relevance for breast cancer radiotherapy and mammography screening was evaluated. On average, the radiation-induced excess relative (ERR) and absolute risk (EAR) at age 50 was ERR = 0.51 Gy-1 (95%CI: 0.33; 0.71), and EAR = 10.8 (104 PYR Gy)-1 (95%CI: 7.0; 14.6). Radiation risk was higher by a factor of 2.7 (95%CI: 1.0; 4.8, P = 0.05) if one first-degree relative was affected with breast cancer. For whole breast standard radiotherapy at age 40 with a contralateral breast dose of 0.72 Gy, the 20-year radiation-related excess contralateral breast cancer risk was estimated to increase from 0.6% for women without FBCH to 1.7% with FBCH. For biennial mammography screening program between 40-74 years of age, radiation risk until age 80 would increase from 0.11% for women without FBCH to 0.29% with FBCH.