**1. Introduction**

Medullary thyroid carcinoma (MTC) accounts for 4–10% of all thyroid cancer cases [1,2] and originates from the parafollicular cells of the thyroid or C-cells, physiologically responsible for the secretion of calcitonin; C-cell hyperplasia (CCH) is considered as the first stage of histological progression that evolves into MTC [3,4]. Among MTC cases, around 25% present in the context of an inherited syndrome, whereas the remaining 75% are sporadic [5–7]. Inherited MTC syndromes include multiple endocrine neoplasia type 2 (MEN2A and MEN2B) and familial MTC (FMTC) [8,9]. MTC is associated with pheocromocytoma (PCC) and parathyroid hyperplasia/primary hyperparathyroidism (PHPT) in MEN2A, [10,11], and with PCC, marfanoid habitus and ganglioneuromatosis of the gut and oral mucosa in MEN2B [12]; conversely, FMTC is characterized by multiple cases of MTC in the family, with no other clinical manifestations [13].

Both MEN2 and FMTC are caused by germline, activating variants in the *RET* proto-oncogene following an autosomal dominant pattern of transmission [14,15]. Nevertheless, different *RET* mutations confer different cancer risks [16–19], which has led to recommendations for management of the carriers based on the level of risk associated with the specific mutation [20]. In this regard, the American Thyroid Association (ATA) stratified *RET* mutations into distinct risk levels, in order to define the most appropriate management for each known mutation. Former classification provided for a stratification into four risk levels (from D to A from the highest to the lowest level of risk) [21], while the 2015 guideline revision combined levels A and B for a total of three risk levels: highest (HST), high (H) and moderate (MOD) [22].

The ATA's recommended management consists of prophylactic thyroidectomy to be performed as soon as possible in carriers of HST and H risk level variants (within the first year of life and within 5 years, respectively), while for carriers of MOD risk level variants, it is suggested to perform annual serum calcitonin screening, and prophylactic thyroidectomy should be performed when values become elevated. Screening for PCC (annual dosing of plasma and urinary catecholamines and metabolites) should start at 11 years for carriers of H/HST variants and at 16 years for carriers of MOD variants. For carriers of H and MOD variants, biochemical screening for PHPT is also recommended (annual dosing of serum calcium and parathyroid hormone), starting at 8 and 20 years, respectively [21–23].

Of course, a stratification by risk is possible only for known, recurrent mutations, for which genotype–phenotype correlations have been clearly established. Conversely, the detection of a novel or rare mutation in a family poses serious counselling and management issues, as the associated risk is mostly unknown. In these cases, until genotype–phenotype correlations are clarified, risk and subsequent management of healthy carrier cannot be based on standard guidelines and need to be evaluated case-by-case, possibly on the basis of clinical and family history of the carriers, which however, may be informative only for large families with many well characterized members [24].

The aim of the study was to analyze the experience of our center with *RET* genetic testing in order to (a) explore prevalence, clinical significance and genotype–phenotype correlations of germline *RET* mutations, (b) collect information which could potentially contribute to defining the clinical role of the variants of uncertain significance (VUS) encountered at our center, and (c) discuss the approach to management of carriers of novel and rare *RET* mutations.
