Protean cytological, histological and immunohistochemical appearances of medullary thyroid carcinoma: current updates

Medullary thyroid carcinoma (MTC) accounts for only 0.5-3% of all malignant diseases, but is responsible for more deaths every year than all the other endocrine malignancies taken together. Approximately 75-80% of MTCs occur sporadically, while the inherited forms of MTC are responsible for the rest of the cases. The heritable MTC results from a germline mutation in the rearranged during transfection (RET) proto-oncogene and is included into the multiple endocrine neoplasia 2 (MEN2), being associated with other endocrine abnormalities and clinical features. MTC is a neuroendocrine tumor that releases a wide range of secretory products that are responsible for a variety of symptoms, making it difficult to be diagnosed. For this reason, the pathological analysis is of vital importance to ensure that the correct diagnosis is made. This review presents the main data from the contemporary literature related to the pathological diagnosis of a patient with MTC and highlights the wide range of tumor cytological features, the many histological variants, as well as the particular tumor immunophenotype. It also reveals the new approach to this type of cancer in the new World Health Organization (WHO) Classification of Thyroid Tumors (2017) and the reassessment of MTC tumor category in the new American Joint Committee on Cancer∕Tumor, Node, Metastasis (AJCC∕TNM) Staging (2017).