Answer: Solitary fibrous tumor

Histology: Grossly, the tumor measured 7 cm, had rounded pushing borders, and was confined to the thyroid without invasion of local structures. Microscopically, the tumor is comprised of a uniform proliferation of slightly spindled cells growing in a “patternless” pattern. This monotonous proliferation is interrupted by dilated thin-walled blood vessels. The tumor cells are slender, slightly wavy, and are very bland without pleomorphism. Mitotic figures are difficult to locate. The tumor cells are separated by bundles of collagen that in areas have a kelloid appearance. Follicle formations are not present. The tumor cells are strongly immunoreactive for CD34.

Discussion: Solitary fibrous tumor (SFT) is a tumor that most frequently arises from the pleural surface of the lung. Once regarded as a tumor of mesothelial derivation (hence the old term “localized fibrous mesothelioma”), it is now recognized that SFT is derived from a more ubiquitously distributed CD34+ dendritic cell. As a result, these tumors have been reported in a growing number of anatomic sites unassociated with a mesothelial lining including head and neck sites.

About 20 cases of SFT arising from the thyroid have been reported in the literature. Just like SFTs arising in other sites, these thyroid tumors are recognized by their fairly consistent morphologic appearance (patternless pattern of growth, alternating zones of hyper- and hypocellularity, HPC-like vascular pattern, collagen bundles, etc…) and strong immunohistochemical staining for CD34. Thyroid SFTs are typically circumscribed and non-infiltrative lesions.

SFT can exhibit a range of histologic patterns, and it morphologically overlaps with a broad spectrum of mesenchymal neoplasms. Immunohistochemistry plays a helpful role in the recognition of SFT. SFTs are consistently immunoreactive for the markers CD34 and bcl-2; but they are generally negative (or only weakly positive) for markers of muscle (actin and desmin) and neural (S100) differentiation. When dealing with a primary thyroid neoplasm, additional immunohistochemical stains to rule out the possibility of spindle cell variant of follicular adenoma (thyroglobulin, TTF-1, cytokeratin) and medullary carcinoma (calcitonin, CEA, chromogranin) would be appropriate. SFTs would be negative for these additional markers.

Most authors who have reported on thyroid SFTs have been quick to point out that the clinical behavior of this tumor is currently unknown. Truth be told, none of the reported thyroid SFTs have recurred, metastasized or caused patient death. One can reasonably expect a good outcome for patients with thyroid SFTs provided that microscopic review does not disclose areas of overtly sarcomatous transformation.