Breast cancer is the most common cancer in women worldwide, with high mortality. About 5% to 10% of breast cancers are hereditary. Most inherited cases of breast cancer are associated with germline mutations in genes, such as BRCA1, BRCA2 and PALB2. The cumulative breast cancer risk for BRCA1, BRCA2 or PALB2 mutation carriers was high. Besides the increased breast cancer risk for the inherited mutation carriers, the risk of subsequent contralateral breast cancer for the mutation carriers with breast cancer was also significantly increased. Contralateral prophylactic mastectomy was usually recommended to the breast cancer patients with BRCA mutation. However, many breast cancer patients refused the contralateral prophylactic mastectomy, due to the surgical injury, potential surgical complications, deleteriously affected body image and sexuality. Solid evidence validated that radiotherapy after surgery resulted in a reduced local recurrence for three times lower than surgery alone. It is thought that radiation would eliminate the microscopic tumors which may already exist in the breast. Thus, we proposed that for the breast cancer patients with BRCA1, BRCA2 or PALB2 deleterious germline mutations, prophylactic irradiation to the contralateral breast may reduce the risk of subsequent contralateral breast cancer. And we would like to further compare the effect of prophylactic irradiation to the published data from traditional prophylactic contralateral mastectomy.