Incidence and Clinicopathological Features of Differentiated High-Grade Thyroid Carcinomas: An Institutional Experience

Differentiated high-grade thyroid carcinoma (DHGTC) is a new entity in the 2022 WHO classification. We aimed to investigate the incidence and clinicopathological features of differentiated HG thyroid carcinoma (DHGTC) and compare the clinicopathological parameters of DHGTC, DTC without HG features, and poorly differentiated thyroid carcinoma (PDTC). A total of 1069 DTCs including papillary thyroid carcinomas (PTCs) and follicular thyroid carcinomas (FTCs) were included in this study. Consecutive 22 PDTCs were also included for comparative purposes. There were a total of 14 (1.3%) cases of DHGTCs, with 13 HGPTCs (1.2% of PTCs) and one HGFTC (6.7% of FTCs). Compared to DTCs without HG features, DHGTCs were associated with larger tumor size, presence of blood vessel invasion, gross extrathyroidal extension, distant metastasis at the time of diagnosis, higher American Joint Committee on Cancer stage, high American Thyroid Association risk, and TERT promoter mutations. DHGTC and PDTC showed a significantly shorter recurrence-free survival (RFS) than DTC without HG features. Multivariate Cox regression analysis revealed that blood vessel invasion, lateral node metastasis, TERT promoter mutations, and HG features were independent prognostic factors (all p < 0.05). When tumor necrosis and increased mitotic count were evaluated separately, tumor necrosis, but not increased mitotic counts, was found to be an independent prognostic factor (p = 0.006). This study confirmed that DHGTC is significantly associated with aggressive clinicopathological features and poor clinical outcomes, similar to PDTC. Although the incidence is low, careful microscopic examination of HG features in DTC is required.