OBJECTIVE: To assess the impact of external iliac lymph node (N1b) metastasis on anal carcinoma (AC) staging and refine the Tumor-Node-Metastasis (TNM) system without modifying existing criteria. METHODS: This retrospective study was performed utilizing the data of 3,815 patients with AC included in the Surveillance, Epidemiology, and End Results (SEER) registry from 2018 to 2021. We compared the TNM8th and 9th editions with our proposed system, focusing on overall survival (OS) and cancer-specific survival (CSS). The Kaplan-Meier survival analysis and time-dependent C-index measures were employed to evaluate the 3 staging systems. RESULTS: The SEER registry identified only 42 patients with solitary N1b metastasis, with lymph node (LN) metastasis rates rising with higher T stages. No significant survival differences were found among N1a to N1c subgroups, yet N1a showed better OS and CSS than N1b+c (hazard ratio [HR] = 1.306, 95% confidence interval (CI): 1.011-1.687, P = .041 for OS
HR = 1.432, 95% CI: 1.088-1.886, P = .011 for CSS). The proposed TNM system, reclassifying T 1N1M0 as stage I and defining T3-T4 with LN status as stages IIIA and IIIB, showed marginally improved predictive accuracy (C-index: 0.684 vs. 0.683 for OS
0.635 vs. 0.634 for CSS). CONCLUSIONS: N1b metastasis minimally affects AC staging. We introduce a simplified TNM system for clinical use: M Staging: Distant metastasis presence as M1. T Staging: T1 as stage I, T2 as stage II, T3-T4 as stage III. N Staging: N status noncontributory for stage I
N negative as stage A (IIA or IIIA), N positive as stage B (IIB or IIIB).