[Askenazi (Hurtle) cell tumors of the thyroid]

Oncocytic adenomas have primarily follicular structure; trabeculas, solid areas, necrosis are rare. They may possess malignant potential as their malignant transformation occurs in 35% cases against 5% in adenomas of follicular cells. Oncocytic follicular carcinomas can be hardly distinguished from oncocytic adenomas. Tumors larger than 4-5 cm in diameter are considered to be malignant. Main difference with adenomas is invasion into the capsule surrounding thyroid or into the vessels. They can be well or poorly differentiated or anaplastic. Oncocytic papillary carcinoma and oncocytic medullary carcinoma are rare. The clinical course of oncocytic tumors is more aggressive than that of tumors from follicular cells. Of key importance in differential diagnosis is electron microscopy (EM) and immunohistochemistry with antimitochondrial antibodies. EM may be also useful in determination of the degree of oncocytic tumors maturation.