Spindle cell metaplasia in follicular adenoma of the thyroid gland: case report and review of the literature.

Spindle cell tumors and reactive proliferations of the thyroid gland are rarely reported. In this report, we described a case of follicular adenoma with spindle cell component. The spindle cell proliferation constituted more than 90% of the 4.0 cm lesion and showed fascicular pattern with focal areas of collagenous stroma and hyalinized blood vessels. There was a transition from follicle like glandular structure to fascicular pattern. There was no mitosis and necrosis. Immunoreactivity with thyroglobulin proved that the spindle cell proliferation is of follicular origin. Also strong thyroglobulin and cytokeratin positivity of glandular areas and the gradual decrease in positivity of spindle cells showed epithelial-mesenchymal transformation/spindle cell metaplasia of the thyroid. We thought that it is important to differentiate spindle cell metaplasia in follicular lesions, especially follicular adenoma, from malignant thyroid neoplasms.