Spindle cell metaplasia of the thyroid arising in association with papillary carcinoma and follicular adenoma.

Spindle cell proliferations of the thyroid have been described in association with reactive processes and aggressive malignant neoplasms. We describe spindle cell proliferations in 10 patients arising in association with papillary carcinoma and follicular adenoma. The spindle proliferations were 0.3 to 3.0 cm in size, constituted from 1% to 95% of the primary neoplasm, and were either admixed with the neoplastic elements or peripherally located within the primary tumor Cytologically, these proliferations showed bland-appearing spindle cells with fine chromatin and subtle nucleoli. Mitoses were rare, and inflammation was minimal. Immunostains showed reactivity with thyroglobulin, indicating their follicular origin. We believe it is important to recognize these metaplastic proliferations and distinguish them from aggressive malignant neoplasms.