Establishment of a prognostic score for distant recurrence in follicular thyroid carcinoma

Follicular thyroid carcinoma (FTC) is known to occasionally show distant recurrence (DR); however, no prognostic scoring system has been established. Furthermore, although prognostic factors have been identified, their weights as predictors of poor prognosis remain unclear. In this study, we aimed to establish a weighted scoring system for predicting DR in FTC using conventional prognostic factors and Ki-67 labeling index (LI). We analyzed data of 597 patients with FTC without distant metastasis at initial surgery. Tumor size >4 cm, male sex, age ≥60 years, Ki-67 LI >5%, and vascular invasion (VI) ≥4 (V2) significantly and VI 0-4 (V1) marginally affected DR. Wide capsular invasion was not significantly associated with DR. Stepwise model selection using the Akaike Information Criterion identified age ≥60 years, Ki-67 LI >5%, V1, V2, and tumor size >4 cm as suitable components of the score. According to the beta coefficient value, we assigned a score of 2 to age ≥60 years, Ki-67 LI >5%, and V2, and a score of 1 to tumor size >4 cm and V1, with the sum representing the final score. We categorized patients into three categories according to the DR rates: low-risk (score 0-2; n = 388, 65.0%), intermediate-risk (score 3-4; n = 156, 26.1%), and high-risk (score 5-7; n = 53, 8.9%). The 10-year DR-free survival rates declined from the low- to the high-risk categories (96.0% vs. 90.3% vs. 54.9%). These findings indicate that high-risk patients identified using this prognostic scoring model should be carefully monitored for DR following completion thyroidectomy.