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Background To develop a subtype-specific prognostic model for rectal mucinous adenocarcinoma (MAC) treated with neoadjuvant chemoradiotherapy (nCRT), incorporating lymph node regression sum score (LRG-sum) and the highest nodal station of metastasis.Methods We retrospectively analyzed 159 patients with rectal MAC who underwent nCRT followed by surgery. The study workflow, LRG reassessment, and model framework are illustrated in Figure 1 ( IDDF2026-ABS-0511 Figure 1). LRG-sum and the highest metastatic nodal station were pathologically evaluated. Associations between LRG-sum and pathological response variables were assessed (IDDF2026-ABS-0511 Figure 2(A-C)). Kaplan-Meier and Cox regression analyses were used to identify prognostic factors for disease-free survival (DFS). A nomogram was then constructed and validated, and its performance was compared with conventional ypN staging (IDDF2026-ABS-0511 Figure 4(A-E)).Results DFS decreased progressively with increasing LRG-sum, with the high-score group showing the poorest outcome ( P = 0.0018) (IDDF2026-ABS-0511 Figure 3(A)). Survival also differed significantly according to the anatomical extent of residual nodal metastasis, with apical lymph node metastasis indicating the worst prognosis (IDDF2026-ABS-0511 Figure 3(B)). Multivariable analysis identified LRG-sum (HR = 1.016, P = 0.008), apical nodal metastasis (HR = 3.968, P = 0.013), and perineural invasion (PNI) (HR = 2.123, P = 0.010) as independent predictors of DFS. The LRG-based nomogram integrating these factors showed good calibration and discrimination, with 1-, 3-, and 5-year AUCs of 0.779, 0.733, and 0.718, respectively, outperforming conventional ypN staging (IDDF2026-ABS-0511 Figure 4(B-E)). In patients with ypN0 disease, complete nodal regression was associated with a prognosis comparable to that of patients without lymph node involvement (IDDF2026-ABS-0511 Figure 5).Conclusions LRG-sum and the highest nodal metastatic station provide prognostic information beyond conventional nodal staging in rectal MAC after nCRT. Incorporating these metrics with PNI enables more precise risk stratification and may support routine pathological reporting and individualized postoperative management in this aggressive subtype.Abstract IDDF2026-ABS-0511 Figure 1Abstract IDDF2026-ABS-0511 Figure 2Abstract IDDF2026-ABS-0511 Figure 3Abstract IDDF2026-ABS-0511 Figure 4Abstract IDDF2026-ABS-0511 Figure 5