Well-differentiated follicular patterned tumors of the thyroid with high-grade features can metastasize in the absence of capsular or vascular invasion: report of a case.

We report a case of an encapsulated, noninvasive, follicular thyroid neoplasia with high-grade features in a 59-year-old man who developed multiple metastases. The 5-cm lesion was originally categorized by thyroid fine-needle aspiration as "suspicious for a follicular lesion" and as a "follicular adenoma" on the subsequent hemithyroidectomy specimen. The high-grade features were represented by foci of solid, trabecular, and insular pattern (10% of the tumor), necrosis (< 5% of the tumor), and up to 14 mitoses per 10 high-power fields in the solid areas. No immunohistochemical and molecular markers associated to thyroid malignancy were observed. Although encapsulated noninvasive follicular tumors are usually regarded as benign lesions that never metastasize or recur, our case was characterized mainly by an elevated number of mitoses, and the patient progressed to widespread metastatic disease. This uncommon lesion should most likely be considered as an early form of poorly differentiated thyroid carcinoma with a dismal prognosis.