Spindle cell foci in the thyroid gland: an immunohistochemical analysis.

Spindle cell proliferations of the thyroid gland are uncommon lesions that encompass a wide spectrum of reactive, hyperplastic, and neoplastic processes. Spindle cells may occur in subsets of papillary carcinomas and follicular adenomas where they are thought to represent metaplastic foci. The goals of the present study are to further characterize the metaplastic nature of spindle cell foci of the thyroid (SCFT), to define their immunohistochemical profiles and to review their differential diagnoses. The study group included: multinodular goiter (2), follicular adenoma (2), and minimally invasive follicular carcinoma (2). SCFTs were composed of elongate cells with thin or slightly plump nuclei with finely granular chromatin and inconspicuous nucleoli. Rare mitotic figures were present but there was no necrosis or inflammation. All cases were positive for thyroglobulin, thyroid transcription factor (TTF)-1, and TTF-2. TTF-1 and TTF-2 had a characteristic nuclear localization although the intensity of staining for TTF-1 was consistently greater than that of TTF-2. Each of the 6 cases was positive for vimentin whereas 5 of the 6 cases were positive for broad-spectrum cytokeratins. None of the cases was positive for high molecular weight cytokeratin, cytokeratin-19, smooth muscle actin, desmin, calcitonin, chromogranin, or synaptophysin. The proliferative rate was less than 1% in all cases. Staining for TTF-1 and TTF-2 provided high specificity for identification of SCFT since these markers were not subject to the same diffusion artifact inherent in thyroglobulin-stained sections. The results of this study further support the hypothesis that SCFT result from metaplastic transformation of follicular cells.