AbstractBackgroundThe purpose of the study was to investigate the ability of new parameters in distinguishing high‐risk patients of recurrence from isthmic papillary thyroid carcinomas (iPTCs).MethodsOne hundred sixteen iPTC patients who underwent total thyroidectomy were identified from 3461 PTC patients from 2014 to 2019. Tumor margin to trachea midline distance (TTD), maximum tumor size (TS), and transverse diameter of trachea (TD) were measured on CT images. Cox proportional hazard models served to identify risk factors associated with recurrence‐free survival (RFS). The iPTC prognostic formula (IPF = TD/(TTD − TS) − TD/TTD) was evaluated to assess the prognosis. RFS was conducted between the different groups using the Kaplan–Meier analysis. The receiver operating characteristic (ROC) curve of each parameter was drawn to predict recurrence.ResultsCentral lymph node metastasis (CLNM) and extrathyroidal invasion in iPTC were 58.6% and 31.0%, respectively. Regional recurrence occurred in 16 (13.8%) patients, and no patient died or had distant metastasis. The 3‐ and 5‐year RFS of iPTC were 87.5% and 84.5%, respectively. Gender (p= 0.001) and PLNM (prelaryngeal lymph node metastasis) (p= 0.010) in cPTC (center of iPTC located between two imaginary lines perpendicular to the surface of the skin from the most lateral points of the trachea) and non‐cPTC (iPTC patients enrolled in this study excluding cPTC) groups differed significantly. A cut‐off point of tumor size >1.1 cm and IPF ≤5.57 were established to have significant differences in prognosis (p= 0.032 andp= 0.005, respectively). Multivariate analysis showed that IPF ≤5.57 was independent prognostic factor for RFS (HR: 4.415, 95%CI: 1.118–17.431,p= 0.034).ConclusionThis study indicated the association between IPF and RFS in iPTC patients and established new models to assess risk factors for recurrence pre‐operation. IPF ≤5.57 was significantly associated with poor RFS and might be promising parameters for predicting prognosis and surgical decision‐making pre‐operation.