Solitary fibrous tumor of the thyroid: cytopathologic findings and differential diagnosis.

Solitary fibrous tumor (SFT) is an uncommon mesenchymal neoplasm that arises primarily from the pleura. Extrapleural occurrences are rare. To our knowledge, there is no published account of this entity in the thyroid in the cytopathology literature. We report the case of a 61-yr-old man who was evaluated at The Johns Hopkins Hospital for a slow-growing thyroid mass that was present for 2 yr despite thyroid hormone suppression. Thyroid-stimulating hormone (TSH) was within normal limits. The patient underwent ultrasound-guided fine-needle aspiration (FNA), which showed predominantly discohesive slender spindle-shaped cells and fragments of collagenized stromal tissue. After the FNA diagnosis of "thyroid neoplasm" was made, the patient underwent a near-total thyroidectomy, which revealed a SFT. Differential diagnosis of spindle cell lesions in thyroid is also presented.