The World Health Organization (WHO) classifies follicular thyroid carcinoma (FTC) into three categories: minimally invasive (mFTC), encapsulated angioinvasive (eaFTC), and widely invasive (wFTC). This study investigated whether this classification is appropriate. We enrolled 523 patients who underwent initial surgery at Kuma Hospital between 1998 and 2015 and were diagnosed with FTC. Capsular invasion (CI) was classified as none, minimal (microscopic), or wide (macroscopic) invasion. Vascular invasion (VI) was divided according to the number of invasive foci into three degrees: VI(-), VI(1+), and VI(2+). For 507 M0 patients, age ≥55 years (p = 0.004), non-oxyphilic histology (p = 0.043), and male sex (p < 0.001) predicted poor distant recurrence-free survival (DR-FS) on univariate analysis; however, tumor size >4 cm and wide CI did not. The DR-FS rates significantly decreased from VI(-) to VI(2+) in a step-by-step fashion, including VI(-) vs. VI(1+) (p = 0.011) and VI(1+) vs. VI(2+) (p = 0.014). Multivariate analysis revealed that older age (p = 0.0004), non-oxyphilic histology (p = 0.041), male sex (p = 0.0052), VI(1+) (p = 0.017), and VI(2+) (p < 0.001) independently predicted distant recurrence. The DR-FS rates did not significantly differ among mFTC, wFTC/VI(-), and eaFTC/VI(1+). The DR-FS rate of eaFTC/VI(2+) was worse than that of eaFTC/VI(1+) (p = 0.042), but did not differ from that of wFTC/VI(1+/2+). Our findings suggest that subclassifying eaFTC according to the degree of VI and restricting wFTC to VI-positive cases would be better in the WHO classification. Revising the definition for wide CI is recommended.
Keywords: Capsular invasion; Follicular thyroid carcinoma; Prognosis; Vascular invasion.