Angiosarcoma of the skin and breast is a known complication of chronic lymphedema following mastectomy or external radiation therapy for breast cancer. We report a 68-year-old woman who presented with a 2.5 cm violaceous plaque on the skin of the right breast and a 3 cm mixed mass of the same breast by ultrasound 9 years after MammoSite® balloon brachytherapy. Biopsy of the skin lesion and the breast mass showed an infiltrating high grade angiosarcoma. The tumor cells in the skin and breast showed immunohistochemical reactivity for C-MYC. A total mastectomy confirmed the presence of high grade angiosarcoma in the skin and parenchyma of the breast and radiation changes in the breast parenchyma. Surgical margins were considered negative. The patient had cutaneous recurrence of angiosarcoma three months after the mastectomy. There have been only two other case reports in the literature of angiosarcoma on the skin following MammoSite® therapy. The c-myc mutation has been shown to be a specific mutation for angiosarcoma following radiation treatment. It is not found in atypical vascular lesions following irradiation or angiosarcoma unrelated to radiation treatment.