[Show abstract][Hide abstract] ABSTRACT: Thoracic radiation in the early years of life is a known risk factor for breast cancer later in life. A 21-year-old woman who had received thoracic radiation therapy for Ewing's sarcoma of the vertebra 9 years earlier was referred to our hospital for investigation of a palpable mass in her left breast. Ultrasonography and excisional biopsy showed ductal carcinoma in situ (DCIS) of the left breast, with no detectable pathology in the right breast except that it was more hypoplastic than the left breast. Considering the known risk factors for invasive breast cancer in both breasts, we performed bilateral skin-sparing mastectomy with immediate breast reconstruction using subpectoral implants. The final histopathological diagnosis was bilateral DCIS.