This otherwise healthy 76-year-old man developed what he thought was a sunburn or rosacea over his forehead and scalp. Unfortunately, this lesion was unresectable and the patient died 1 year after diagnosis. Primary angiosarcomas are malignant vascular tumors that often develop on the scalp of elderly men. They are very infiltrative, difficult to resect, and not uncommonly metastasize to the lungs. The histology in this case is typical, with hobnail atypical endothelial cells and poorly-formed vascular channels. Immunohistochemistry can be helpful with high grade, poorly differentiated cases.
At medium power, one can appreciate red blood cells within the vascular channels. The endothelial cells are plump, epithelioid, and project into the lumen as "hobnails". They have atypical nuclear features and there are scattered mitoses (not shown).