Hürthle-cell tumor arising in the mediastinal ectopic thyroid and diagnosed by fine needle aspiration. Light microscopic and ultrastructural features.

The light microscopic and ultrastructural features of a Hürthle-cell tumor arising in the ectopic mediastinal ectopic thyroid and diagnosed by fine needle aspiration are presented. Although it lacked clear-cut cytologic features of malignancy, the tumor was clinically malignant, having metastasized to the lungs. Hürthle-cell tumors of the thyroid are unpredictable in their biologic behavior, and the absence of clinical and/or pathologic evidence of malignancy at the time of diagnosis is no guarantee of a benign outcome.