Angiosarcoma with predominant hobnail cells mimicking hobnail hemangioma

Angiosarcoma is a rare, highly aggressive malignant vascular tumor with a poor prognosis, most frequently involving the skin and superficial soft tissues. Its varied clinical and histopathological presentations often pose significant diagnostic challenges. Hobnail hemangioma, also known as targetoid hemosiderotic hemangioma, is a benign vascular lesion characterized by a biphasic pattern and prominent hobnail endothelial cells. We report a diagnostically challenging case of cutaneous angiosarcoma with predominant hobnail endothelial morphology, closely mimicking hobnail hemangioma both clinically and histologically. A 46-year-old previously healthy male presented with multiple asymptomatic erythematous indurated plaques with surrounding satellite lesions over the scalp for three months. Initial histopathological examination revealed superficial dermal vascular spaces lined by hobnail endothelial cells, suggestive of hobnail hemangioma. 
However, due to strong clinical suspicion of malignancy, a repeat biopsy with immunohistochemical analysis was performed. This demonstrated irregular, anastomosing vascular channels, sheets of atypical epithelioid endothelial cells with nuclear pleomorphism, brisk mitotic activity, and cytoplasmic vacuoles containing erythrocytes. Immunohistochemistry showed positivity for CD34 and D2-40, with a high Ki-67 proliferation index (~50%), while melanocytic markers were negative. These findings confirmed the diagnosis of angiosarcoma. This case highlights the importance of clinicopathological correlation and the need for careful evaluation of vascular lesions with hobnail features, as angiosarcoma may closely mimic benign entities such as hobnail hemangioma. Early recognition and appropriate diagnostic workup are crucial to prevent delayed diagnosis and adverse outcomes.