Answer: Angiosarcoma

Histology: The dermis of the skin is infiltrated by irregular vessels which dissect apart the collagen. In one area, the proliferation has an irregular border and extends into the adipose tissue, and there is apparent cystification. On closer examination, the cells lining these vascular spaces have prominent nucleoli, hyperchromatic nuclei, and lack an investing pericytic layer. Their border is irregular. These are the typical features of angiosarcoma of the skin of the breast, associated with radiation therapy. This patient had received radiation therapy for prior breast carcinoma.

Discussion: An atypical vascular lesion should be well circumscribed, lack cytologic atypia, and be confined to the dermis of the skin. Whether atypical vascular lesions after radiation therapy are preneoplastic remains a debated issue. Angiomatosis may be a highly infiltrative process within the breast; however, it should not show cytologic atypia. Kaposi’s sarcoma would feature fascicles of bland spindle cells forming irregular slit like spaces associated with extravasated blood and chronic inflammation.