Independent Validation of the International Grading System for Medullary Thyroid Carcinoma: A Single Institution Experience

Medullary thyroid carcinoma (MTC), an uncommon C cell thyroid malignancy, accounts for a disproportionate number of thyroid cancer deaths. To predict MTC clinical behavior, the recent international MTC grading system (IMTCGS) was published combining features from the Memorial Sloan Kettering Cancer Center and Royal North Shore Hospital grading systems that incorporates mitotic count, necrosis, and Ki67 proliferative index (Ki67PI). The IMTCGS appears promising, but independent validation data are limited. Here, we applied the IMTCGS to our institutional MTC cohort and assessed its ability to predict clinical outcomes. Our cohort comprised 87 MTCs (30 germline and 57 sporadic). Slides for each case were reviewed by 2 pathologists and histologic features recorded. Ki67 immunostaining was performed on all cases. Each MTC was graded with the IMTCGS based on tumor necrosis, Ki67PI, and mitotic count. Cox regression analysis was performed to assess the impact of various clinical and pathological data on disease outcomes, including overall survival (OS), disease-free survival, disease-specific survival (DSS), and distant metastasis-free survival. In our MTC cohort, 18.4% (n = 16/87) were IMTCGS high grade. IMTCGS grade was strongly prognostic for OS, disease-free survival, DSS, and distant metastasis-free survival on univariate analysis and multivariable analysis in both the entire MTC cohort and in the sporadic subset. Among the individual IMTCGS parameters, while all 3 were associated with poorer survival outcomes on univariate analysis, necrosis had the strongest association with all survival parameters on multivariable analysis, whereas Ki67PI or mitotic count was associated only with OS and DSS. This retrospective study independently demonstrates that the IMTCGS is valid for grading MTCs. Our findings support incorporating IMTCGS into routine pathology practice. IMTCGS grading may help clinicians to better predict the prognosis of MTC. Future studies may shed light on how MTC grading should impact treatment protocols.