Primary spindle cell lesions of the thyroid gland; an overview.

Spindle cell lesions of the thyroid gland (T-SCL) are not encountered routinely in clinical practice or in the context of thyroid pathology. They commonly are classified as primary or secondary to metastatic disease. Primary T-SCL can be derivedfromfollicular, C-cell (parafollicular), or mesenchymal components and may be the result of reactive or neoplastic processes, including post-fine-needle aspiration spindle cell nodules, Riedel thyroiditis, solitary fibrous tumor, leiomyoma, peripheral nerve sheath tumor, hyalinizing trabecular tumor, spindle epithelial tumor with thymus-like differentiation, follicular dendritic cell tumor, medullary carcinoma, papillary carcinoma, anaplastic carcinoma, sarcoma, squamous cell carcinoma, and carcinoma showing thymus-like differentiation. Because T-SCL may represent the expression of benign and highly malignant neoplasms, distinction among these processes is crucial because it dictates therapy and defines prognosis. The present article reviews the clinical, imaging, pathologic, and immunohistochemical characteristics of primary T-SCL.