Validation of the International Medullary Thyroid Cancer Grading System and Identification of EZH2 as a Prognostic and Potential Therapeutic Marker in Medullary Thyroid Cancer

<b>Background:</b> Medullary thyroid cancer (MTC) is a heterogeneous disease. While the International MTC Grading System (IMTCGS) provides baseline risk stratification, it lacks therapeutic relevance. In several cancers, EZH2 overexpression harbors an adverse prognosis, with several EZH2 inhibitors undergoing investigation. This study validated the IMTCGS and examined the prognostic value of EZH2 and other biomarkers. <b>Methods:</b> Clinical data were collected and MTC specimens were retrospectively reviewed and morphologically assessed. Immunohistochemistry (IHC) of Ki-67 allowed IMTCGS validation. IHC of EZH2, PD-L1 and PSMA was evaluated on a tissue microarray (TMA). <b>Results:</b> Of 64 MTCs, the median tumor size was 28 mm (IQR 15-40). Coagulative necrosis, ≥5 mitoses, and Ki-67 ≥ 5% was seen in nineteen (30%), three (5%) and seven (11%) cases. Median Ki-67 was 0.9% (IQR 0.4-2.1). Forty-three (67%) and twenty-one (33%) were classified as IMTCGS low- and high-risk, respectively. High-risk tumors were associated with lower distant metastasis-free survival (DMFS) (HR 5.651, <i>p</i> = 0.017), locoregional recurrence-free survival (LRFS) (HR 18.323, <i>p</i> < 0.001) and disease-specific survival (DSS) (HR 10.001, <i>p</i> = 0.002), but not with overall survival (OS) (HR 2.109, <i>p</i> = 0.146). EZH2 expression was identified in 39/46 (85%) cases on the TMA. An expression of ≥10% (9/46, 20%) was predictive for DMFS (HR 4.747, <i>p</i> = 0.030), LRFS (HR 4.242, 0.039), DSS (HR 19.736, <i>p</i> < 0.001) and OS (HR 8.386, <i>p</i> = 0.004). PD-L1 and PSMA had no prognostic value. <b>Conclusions:</b> This study validates the prognostic value of the IMTCGS and identifies EZH2 as a novel prognostic biomarker in MTC patients. The therapeutic potential of EZH2 warrants further investigation in larger cohorts.