Answer: Solitary fibrous tumor of the thyroid

Histology: On cut section there is a solitary round 2 cm nodule. Microscopically, the nodule is sharply circumscribed but non-encapsulated. The nodule is comprised of a proliferation of spindled fibroblastic cells that dissect between follicles. In areas the proliferation exhibits a storiform pattern. The spindled cells have uniform nuclei without atypia, and mitotic figures are not apparent. The spindle cells are intimately entwined between thin and thick bands of collagen.

Immunohistochemical studies were performed. The spindled cells were immunoreactive for CD34 and bcl2; and they were not immunoreactive for AE1:AE3, CAM5.2, calcitonin, TTF1 or thyroglobulin.

A stain for CD34 is shown below.

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 the oral cavity.

To date, about 20 SFTs have been reported in the thyroid gland. In this location, they tend to circumscribed and non-infiltrative although (as demonstrated in this case) they may entrap thyroid follicles at their periphery.

SFT can exhibit a range of histologic patterns, and it morphologically overlaps with a broad spectrum of neoplasms. The differential diagnosis varies widely depending on tumor location. In the thyroid, they must be distinguished from other mesenchymal and epithelial lesions that are characterized by a proliferation of spindle cells. Riedel’s thyroiditis is a permeative fibrosing inflammatory process that does not form a circumscribed nodule. A rare variant of follicular adenoma is characterized by prominent spindling of the follicular epithelial cells, but these spindled cells are immunoreactive for cytokeratin and TTF1. Anaplastic carcinomas often demonstrate a spindled component, but this component invariably exhibits high grade cytologic features and invasive growth. 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.

Although the numbers of reported cases of thyroid SFTs are few with limited clinical follow up, none have recurred or metastasized.