Answer: Follicular adenoma, spindle cell type

Histology: The nodule is sharply circumscribed and thinly encapsulated. It is comprised of sheets of elongated spindle cells. The spindle cells have uniform nuclei that lack significant pleomorphism, and mitotic figures are not noted. In areas the spindled cells round up, become more epithelioid, and transition with the epithelium lining microfollicles. Immunohistochemical studies were performed. The spindle cells are immunoreactive for TTF-1 and thyroglobulin; and they are not immunoreactive for chromogranin, calcitonin or CD34.

Discussion: Follicular adenomas can rarely undergo spindle cell metaplasia (follicular adenoma with spindle cell metaplasia). When this metaplastic change is extensive throughout much of the lesion, these adenomas can easily be mistaken for more ominous neoplasms such as medullary carcinoma, anaplastic carcinoma, and papillary carcinoma with spindle cell change. Careful microscopic evaluation will often demonstrate areas of transition between follicles and the spindle cells. The spindle cells are cytologically bland. They lack the overtly malignant cytologic features of anaplastic carcinoma, and they do not display the nuclear alterations that characterize papillary carcinoma. Immunohistochemistry plays a critical role in confirming the follicular epithelial origin of this adenoma variant. Despite its mesenchymal appearance and resemblance to solitary fibrous tumor of the thyroid, the spindle cell adenoma retains its immunoreactivity for cytokeratin, TTF-1 and thyroglobulin.

The presence of prominent spindling within a follicular adenoma represents a metasplastic change that by itself is of no prognostic significance. This is a benign lesion that should be treated like conventional follicular adenoma.