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 thyroid adenomas dominated by spindle cells with occasional areas of colloid-forming follicular differentiation. The tumors affected a 77-year woman and a 70-year old man; both had a long-history of monoclonal gammopathy of unknown significance (MGUS). One tumor presented as a large cold thyroid nodule and the other was an autopsy finding. The tumors were predominantly composed of fibroblast-like spindled cells. One case showed prominent meningioma-like concentric perivascular arrangement and contained cytoplasmic melanin-like pigment. Stromal hyalinization 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 follicular 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, spindle cell foci in papillary and follicular carcinoma, solitary fibrous tumor and other rare benign and malignant mesenchymal lesions.