Histologic Features of Post-radiation Angiosarcoma of the Breast: Histologic features are similar to other cutaneous angiosarcomas. Most cases show a spectrum of findings ranging from well-differentiated areas that mimic a benign hemangioma to poorly-differentiated infiltrative solid foci (with no vascular spaces) resembling high-grade carcinomas or sarcomas. Well-differentiated pattern consists of irregular anastomosing channels lined by endothelial cells with minimal cytologic atypia. The vascular channels infiltrate and dissect dermal collagen, giving a clue to their true malignant nature. Endothelial cells may pile up creating intraluminal papillations and solid nests. Such areas resemble papillary endothelial hyperplasia. Poorly-differentiated foci are present in most cases. They show epithelioid or spindle cells with significant cytologic atypia in a solid growth pattern and devoid of vascular formations. Intracytoplasmic lumina may be seen in some tumor cells. Mitotic activity is increased and necrosis may be present. Necrosis and hemorrhage may create 'blood lakes'. Poorly-differentiated angiosarcomas often show anastomosing vascular channels at their periphery lined by highly atypical plump endothelial cells. The tumors are located in the dermis and rarely involve the breast parenchyma. The recurrence rate following resection is almost 50%. Metastases occur in 40% of cases, generally to the lung, contralateral breast, and bone.