Diagnostic controversies in vascular proliferations of the thyroid gland.

Vascular lesions are one of the most controversial issues in thyroid pathology. The differential diagnosis includes benign lesions on one side, i.e., endothelial reactive hyperplasia (Masson's "hemangioma") in goiter and hemangioma, and malignant tumors on the other, i.e., angiosarcomas and undifferentiated (angio)sarcomatoid carcinomas. Benign reactive endothelial hyperplasia with atypias mimicking malignant tumors may occur in long-standing nodular goiter, as a result of spontaneous hemorrhage followed by granulation tissue and fibrous organization. Alternatively, it may follow a fine-needle aspiration biopsy (FNAB) procedure. Angiosarcoma is a rare primary malignant thyroid tumor, mainly observed in endemic goiter areas displaying morphologic and phenotypical similar to those typical of angiosarcomas in other locations (including focal cytokeratin expression). The distinction between angiosarcoma and (angio)sarcomatoid anaplastic carcinoma is difficult and the true existence of angiosarcoma has been challenged. Other extremely rare vascular lesions or mimics in the thyroid include benign hemangioma and pseudo-angiosarcomatous variant of medullary carcinoma. The differential diagnosis between benign and malignant vascular conditions in FNAB material is extremely challenging, and the cytopathology report should be remarkably cautious, especially in poorly cellular and highly hemorrhagic cases: atypias in endothelial cells are not per se indicative of malignancy, being a common feature of reactive endothelial hyperplasia in infracted goiter nodules as well.