Spindle cell variant of follicular thyroid carcinoma: An extremely unusual case and review of the literature.

Spindle cell proliferations originating in follicular derived thyroid neoplasms are rare and known to cause diagnostic conundrums. We describe a unique case of a spindle cell variant of follicular thyroid carcinoma (FTC) in a 48-year-old female without relevant past medical history, who was being followed for a 1.4 cm left thyroid nodule for the past 15 months. A fine needle aspiration (FNA) of the nodule was interpreted as benign (Bethesda II). On follow-up ultrasound the nodule demonstrated a slight increase in size (to 1.5 cm) and the appearance of coarse calcifications A repeat FNA was performed 12 months later and interpreted as malignant neoplasm (Bethesda VI), containing a population of spindle and epithelioid cells that could not be further classified. A left subtotal thyroidectomy showed an encapsulated tumor mainly composed of fibroblast-like spindle cells, extensive foci of calcifications and focal ossification, with minimal tumor capsule invasion without vascular invasion. Tumor cells expressed vimentin, ERG and SMA (focal), while being negative for pancytokeratin, thyroglobulin, TTF-1, Pax-8, calcitonin, CEA and other lineage-specific mesenchymal, neuroendocrine and melanocytic markers. Importantly, a few residual thyroid follicles were identified within the nodule, and a diagnosis of minimally invasive FTC with extensive spindle cell changes, calcification and osseous metaplasia was rendered. This is only the second cytologic report of a pure spindle cell FTC. The rarity of this neoplasm and its potential broad differential diagnosis create diagnostic difficulties both on cytology and histology.