Establishment of novel prognostic groups for papillary thyroid carcinoma using a modified risk classification based on tumor extension in the guidelines of the Japan Association of Endocrine Surgery.

The latest "General Rules for the Description of Thyroid Cancer," published in 2023, introduced depth-based subcategories of tumor invasion, dividing sEx2 into sEx2a, sEx2b, and sEx3. However, the "Clinical Guidelines on the Management of Thyroid Tumors," published in 2024, continue to classify these categories uniformly as high-risk for papillary thyroid carcinoma (PTC). We evaluated the appropriateness of reclassifying sEx2a-high-risk patients as intermediate-risk. A total of 9,247 patients [median age: 52 years (7-93)] who underwent locally curative surgery were enrolled, with a median follow-up of 7.8 years. Cause-specific survival (CSS), distant recurrence-free survival (DR-FS), and local recurrence-free survival (LR-FS) worsened progressively from low-risk to high-risk patients. We compared the prognoses among the patients classified as sEx2a-high-risk, sEx2b, and intermediate-risk. The CSS, DR-FS, and LR-FS outcomes of sEx2b patients were significantly poorer than those of sEx2a-high-risk and intermediate-risk patients. By reclassifying sEx2a-high-risk patients as intermediate-risk, we established a new high-risk and intermediate-risk classification. The number of high-risk patients decreased from 2,274 to 1,132, whereas the number of intermediate-risk patients increased from 2,875 to 4,017. Prognoses in these new groups showed minimal differences compared to the original high- and intermediate-risk classifications. We established novel prognostic groups: favorable (N = 6,398, low-risk and intermediate-risk <55 years), intermediate (N = 2,324, intermediate-risk ≥55 years and high-risk <55 years), and poor (N = 525, high-risk ≥55 years). Prognoses significantly worsened across these groups from favorable to poor (p < 0.001). The reclassification of PTC based on tumor extension and the proposed novel prognostic groups provide a more accurate evaluation of PTC outcomes.