Answer: Angiosarcoma

Histology: The tumor consists of a pleomorphic spindle cell proliferation that dissects native collagen bundles of the reticular dermis. Red blood cells can be seen within the spaces formed by the spindle cells. Nuclei have vesicular chromatin, prominent nucleoli, and mitotic figures are evident. The tumor cells stain for CD31 and CD34.

Discussion: A carcinoma would typically have more epithelioid morphology, and would label intensely for cytokeratin. Atypical vascular lesions that occur after radiation are typically small, circumscribed lesions which lack mitoses, pleomorphism, or solid areas. Cellular angiolipomas are lobulated lesions that typically occur in the subcutaneous tissue. Lesions have a rounded border, with central fibrin thrombi.

Given the patient’s history of breast cancer in the same breast treated by breast conservation therapy, this patient almost certainly received radiation therapy, which predisposes to angiosarcoma of the skin of the breast. Angiosarcoma of the skin of the breast has the distinctive feature of having an extremely short latency (as little as two years) as compared to other radiation-induced sarcomas.