Encapsulated follicular variant of papillary thyroid carcinoma with bone metastases

Although true follicular thyroid carcinoma is known to metastasize via the bloodstream and give rise to bone and lung metastases, such a pattern of sp read is rare in papillary thyroid carcinoma. The follicular variant of papi llary thyroid carcinoma (FVPTC) is believed to behave in a clinical manner similar to usual or classical papillary cancer and to follow a similar Indo lent course. There have been a few reports of "aggressive" FVPTC wherein follicular patt erned tumors with nuclear features of papillary carcinoma have metastasized hematogenously; these neoplasms have been diffusely invasive or multicentr ic in the thyroid. We report five cases of FVPTC, which were encapsulated and simulated grossl y and microscopically follicular adenomas. In two of these, the primary was discovered after clinical presentation of bone metastases. In three others , bony metastases (without other nonosseous metastases) arose 7 to 17 years after thyroid lobectomy for lesions initially diagnosed as follicular aden oma. In retrospect, these three encapsulated lesions had vascular invasion. We wish to bring attention to these innocuous-appearing lesions, which, al though sharing nuclear features of papillary cancer, behave clinically in a n unexpectedly malignant fashion.