Medullary Thyroid Cancer - Feature Review and Update on Systemic Treatment

Medullary thyroid carcinoma (MTC) is a rare malignancy that originates from parafollicular (C cells) of the thyroid and accounts for 2-4% of all thyroid malignancies. MTC may be sporadic or inherited, the latter as part of the MEN 2 syndromes. Germline mutations in the RET proto-oncogene (REarranged during Transfection) are driver mutations in hereditary MTC, whereas somatic RET mutations and, less frequently, RAS mutations, have been described in tumor tissues of sporadic MTC. Genetic screening for germline mutations in RET proto-oncogene identifies gene carriers of germline mutations. That enables primary prevention (the avoidance of disease onset by total prophylactic thyroidectomy), or at least secondary prevention (early detection) of the disease. Radical surgery with complete tumor resection is still pivotal in attaining cure for MTC. Despite recent advances, the treatment of advanced, metastatic, and progressive MTC remains challenging. Metastatic MTC can have an indolent clinical course; therefore, it is necessary to assess which patient to cure and when to initiate the treatment. Multidisciplinary boards of various specialists involved in the diagnostics and therapy of the patients with MTC in highly specialized centers with a high volume of patients provide optimal patient management. Multikinase inhibitors (MKI) vandetanib and cabozantinib were approved for the treatment of progressive or symptomatic metastatic/unresectable MTC. Although these treatments have been shown to improve progression-free survival (PFS) with higher overall response rates (ORR) compared with placebo, no MKI has been shown to increase the overall survival (OS) yet, except in the subgroup of patients with RETM918T-mutations on cabozantinib therapy. As these drugs are nonselective, significant off-target toxicities may occur. Recently, next-generation small-molecule tyrosine kinase inhibitors (TKIs) have been developed. These highly selective RET-inhibitors are specifically designed for highly potent and selective targeting of oncogenic RET alterations, making them promising drugs for the treatment of advanced MTC. The selective RET-inhibitor selpercatinib has been very recently registered for the treatment of RET-mutated thyroid cancer.