Ectopic thyroid cancer.

Ectopic thyroid tissue may be encountered anywhere from the foramen caecum to the lower neck. It is rarely seen in the mediastinum. True malignant transformation in ectopic thyroid tissue is extremely rare. Such a malignancy is virtually always diagnosed after surgical excision of the lesion at pathological examination. We report on an extremely rare case of true mediastinal thyroid cancer in a 45-year-old woman. The clinicopathologic features and diagnosis of the lesion, with regard to its mediastinal location are discussed.