Original Article Follicular thyroid adenoma dominated by spindle cells: report of two unusual cases and literature review

Primary spindle cell neoplasms of the thyroid gland are quite rare. They encompass a heterogeneous group of benign and malignant lesions of mesenchymal and epithelial origin. We herein describe two unusual follicular thy- roid adenomas dominated by spindle cells with occasional areas of colloid-forming follicular differentiation. The tu- mors affected a 77-year woman and a 70-year old man; both had a long-history of monoclonal gammopathy of un- known significance (MGUS). One tumor presented as a large cold thyroid nodule and the other was an autopsy find- ing. The tumors were predominantly composed of fibroblast-like spindled cells. One case showed prominent men- ingioma-like concentric perivascular arrangement and contained cytoplasmic melanin-like pigment. Stromal hyaliniza- tion was a prominent feature of both. By immunohistochemistry, the spindled cells expressed vimentin, pankeratin (KL1), thyroglobulin and TTF1 consistent with a follicular differentiation. They did not stain with calcitonin, CEA and other lineage-specific mesenchymal, neuroendocrine and melanocytic markers. There was no evidence of metastasis at autopsy (case 2) or at last follow-up 2 years after surgery (case 1). These cases demonstrate the diversity of follicu- lar thyroid neoplasms and the unusual occurrence of extensive spindle cell metaplasia. These uncommon lesions need to be distinguished from spindle cell medullary carcinoma, paucicellular spindle cell anaplastic carcinoma, spin- dle cell foci in papillary and follicular carcinoma, solitary fibrous tumor and other rare benign and malignant mesen- chymal lesions.