Endocrine Abstracts

Introduction: The 2022 WHO Classification categorizes oncocytic (OTC) and follicular thyroid carcinoma (FTC) based on the degree of capsular and vascular invasion into minimally invasive (MI), encapsulated angioinvasive (EA) and widely invasive tumors (WI). While associations with clinical outcomes have extensively been studied in FTC, for OTC robust clinical data is lacking. The aim of this study is to investigate the newly defined categorization by the 2022 WHO Classification on clinical outcomes in OTC compared to FTC.

Methods: All adult FTC and OTC patients treated at a tertiary referral hospital between 2000 and 2016 were retrospectively included (n = 141). All tumors were thoroughly revised independently by two pathologists applying the 2004 and 2022 WHO Classification. Kaplan-Meier curves were used to study the association of the 2004 and 2022 WHO Classification with overall survival (OS), disease-specific survival (DSS), recurrence-free survival (RFS) and incidence of radioactive iodine (RAI-) refractory disease.

Results: 52 OTC and 89 FTC patients were included. OTC patients were older at diagnosis (61.7 years vs 51.7 years), and more often male (50% vs 23.6%). After revision, 28.8% of OTC tumors were reclassified (7 from MI to EA and 8 from WI to EA), resulting in 5 MIOTC, 15 EAOTC and 32 WIOTC. In FTC, 38.2% tumors were reclassified (20 from MI to EA and 14 from WI to EA), resulting in 32 MIFTC, 34 EAFTC and 23 WIFTC. Compared to the 2004 WHO Classification, the 2022 WHO classification showed a better risk stratification for DSS, with an intermediate prognosis in EAOTC and EAFTC. Ten-year DSS with the 2022 WHO Classification were 100% for MIOTC, 92.3% for EAOTC and 56.5% for WIOTC, compared to 100% (MIOTC) and 64.2% (WIOTC) following the 2004 WHO Classification. Similar trends were observed for RAI-refractory disease, but not for OS and RFS.

Conclusion: To our knowledge, our study is the first to show that classification of OTC and FTC into three subcategories based on the extent of invasiveness (i.e. MI, EA and WI), as defined by the 2022 WHO Classification, substantially improves discrimination between low, intermediate and high risk patients, especially for DSS and RAI-refractory disease.