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PURPOSE Medullary thyroid carcinoma (MTC) is an aggressive neuroendocrine tumor (NET) arising from the calcitonin-producing C cells. Unlike other NETs, there is no widely accepted pathologic grading scheme. In 2020, two groups separately developed slightly different schemes (the Memorial Sloan Kettering Cancer Center and Sydney grade) on the basis of proliferative activity (mitotic index and/or Ki67 proliferative index) and tumor necrosis. Building on this work, we sought to unify and validate an internationally accepted grading scheme for MTC. PATIENTS AND METHODS Tumor tissue from 327 patients with MTC from five centers across the United States, Europe, and Australia were reviewed for mitotic activity, Ki67 proliferative index, and necrosis using uniform criteria and blinded to other clinicopathologic features. After reviewing different cutoffs, a two-tiered consensus grading system was developed. High-grade MTCs were defined as tumors with at least one of the following features: mitotic index ≥ 5 per 2 mm2, Ki67 proliferative index ≥ 5%, or tumor necrosis. RESULTS Eighty-one (24.8%) MTCs were high-grade using this scheme. In multivariate analysis, these patients demonstrated decreased overall (hazard ratio [HR] = 11.490; 95% CI, 3.118 to 32.333; P < .001), disease-specific (HR = 8.491; 95% CI, 1.461 to 49.327; P = .017), distant metastasis-free (HR = 2.489; 95% CI, 1.178 to 5.261; P = .017), and locoregional recurrence-free (HR = 2.114; 95% CI, 1.065 to 4.193; P = .032) survivals. This prognostic power was maintained in subgroup analyses of cohorts from each of the five centers. CONCLUSION This simple two-tiered international grading system is a powerful predictor of adverse outcomes in MTC. As it is based solely on morphologic assessment in conjunction with Ki67 immunohistochemistry, it brings the grading of MTCs in line with other NETs and can be readily applied in routine practice. We therefore recommend grading of MTCs on the basis of mitotic count, Ki67 proliferative index, and tumor necrosis.

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Poorly differentiated carcinoma of the thyroid gland (PDC) represents an heterogeneous group of epithelial neoplasms with morphologic features and clinical characteristics intermediate between well differentiated and anaplastic (undifferentiated) carcinomas. Unlike well differentiated tumors, PDCs are associated with significant morbidity and mortality. The general prevalence of RET/PTC rearrangement in thyroid PDC and its impact on patient outcome are unknown. To address these issues and to identify prognostically relevant clinicopathologic parameters, we have investigated a series of 62 PDCs. RET/PTC rearrangement, analyzed by RT-PCR and immunohistochemistry using antibodies specific for the tyrosine kinase and juxtamembrane portions of the RET protein, was identified in 8/62 (12.9%) PDCs. RET/PTC was more common in cases with histologic evidence indicating coexistence with or possible evolution from a well differentiated papillary carcinoma (5 of 25 tumors, 20%) but did not correlate with other clinicopathologic parameters. The relatively low prevalence of RET activation in PDCs argues against a major role for RET/PTC in the progression from well to poorly differentiated thyroid tumor phenotypes. Survival analysis demonstrates that poor survival in PDC is associated with old age, male sex, invasion of extrathyroidal soft tissues, coexistence in the same tumor of oncocytic features with insular growth pattern, and distant metastases but not RET activation.