Keratin-positive epithelioid angiosarcoma of thyroid. A report of four cases.

We present four cases of a malignant thyroid tumor showing morphologic, immunocytochemical, and ultrastructural features of endothelial cell differentiation. The tumor cells had epithelioid features and displayed strong immunoreactivity for keratin. There was no evidence of follicular or C-cell differentiation in any instance. We interpreted these cases as keratin-positive epithelioid angiosarcomas. The findings presented here support the existence of primary malignant vascular tumors in the thyroid even in the presence of keratin positivity, a marker traditionally regarded as indicative of epithelial differentiation.