Solitary fibrous tumor of the thyroid gland: Report of two cases and review of the literature

Presented herein are the first two Japanese cases of solitary fibrous tumor (SFT) of the thyroid gland. The patients were 64- and 41-year-old men, who underwent hemithyroidectomy for thyroid tumors. Histopathologically, the tumors were well circumscribed, and were composed of spindle cells, collagen bundles and entrapped thyroid follicles. Spindle cells and collagen bundles were mostly in a patternless arrangement with an admixture of hypo- and hypercellular areas. Proliferating spindle cells were bland, slender, and occasionally wavy without any atypia and increased mitotic figures. Immunohistochemically, they were strongly positive for CD34, CD99, and bcl-2, but negative for epithelial membrane antigen, keratin, thyroglobulin, calcitonin, thyroid transcription factor-1, alpha-smooth muscle actin, desmin, S-100 protein and CD117. Based on these findings, the two patients were diagnosed as having thyroid SFT. Neither local recurrence nor metastasis has been observed in 5 years in patient 1 or in 4 years in patient 2. Thyroid SFT is extremely rare, and only 20 cases have been reported in the English-language literature including hemangiopericytoma. Although recurrence or metastasis has not been described in any reported cases, the clinical behavior of SFT is still undetermined, therefore long-term follow up seems necessary at present.