MKI67 and medullary thyroid gland carcinoma: Starting from these observations, a group of internationally recognized experts in thyroid pathology from the USA, Europe, and Australia recently proposed the International Medullary Thyroid Carcinoma Grading System (IMTCGS) that, combining proliferation (mitotic count and Ki67 proliferative index) and necrosis, separates MTC in two different prognostic categories: low and high grade MTC (Fig. 3 and Table 2) [53].