Encapsulated medullary carcinoma of the thyroid. A morphologic study including immunocytochemistry, electron microscopy, flow cytometry, and in situ hybridization.

An unusual encapsulated medullary carcinoma of the thyroid is reported. Immunocytochemistry and electron microscopy did not prove conclusively the cytogenesis of the tumor; however, in situ hybridization, by demonstrating the presence of calcitonin mRNA, revealed that the tumor is of C-cell derivation. Because these tumors have an unpredictable biologic behavior, it seems more appropriate to designate them as encapsulated medullary carcinoma than as C-cell adenoma. Immunocytochemistry for calcitonin and carcinoembryonic antigen as well as flow cytometry for DNA ploidy may be valuable techniques to assess prognosis of these tumors.