Clinical Implication of World Health Organization Classification in Patients with Follicular Thyroid Carcinoma in South Korea: A Multicenter Cohort Study

BACKGROUND: The study aimed to compare the prognostic value of the 4th edition of World Health Organization classification (WHO-2017) with the previous WHO classification (WHO-2004) for follicular thyroid carcinoma (FTC). METHODS: This multicenter retrospective cohort study included 318 patients with FTC from five tertiary centers who underwent thyroid surgery between 1996 and 2009. We evaluated the prognosis of patients with minimally invasive (MI), encapsulated angioinvasive (EA), and widely invasive (WI) FTC according to WHO-2017. Further, we evaluated the proportion of variation explained (PVE) and Harrell’s C-index to compare the predictability of disease-free survival (DFS) and disease-specific survival (DSS). RESULTS: In total, 227, 58, and 33 patients had MI-, EA-, and WI-FTC, respectively. During a median follow-up of 10.6 years, 46 (14.5%) patients had disease recurrence and 20 (6.3%) patients died from FTC. The 10-year DFS rates of patients with MI-, EA-, and WI-FTC were 91.1%, 78.2%, and 54.9%, respectively (P<0.001, PVE=7.1%, C-index=0.649). The corresponding 10-year DSS rates were 95.9%, 93.5%, and 73.5%, respectively (P<0.001, PVE=2.6%, C-index=0.624). The PVE and C-index values were higher using WHO-2017 than using WHO-2004 for the prediction of DFS, but not for DSS. In multivariate analysis, older age (P=0.02), gross extrathyroidal extension (ETE) (P=0.003), and distant metastasis (P<0.001) were independent risk factors for DSS. CONCLUSION: WHO-2017 improves the predictability of DFS, but not DSS, in patients with FTC. Distant metastasis, gross ETE and older age (≥55 years) were independent risk factors for DSS.