Proposal of a novel pND-based staging system for locoregional gastric neuroendocrine carcinoma: A multicenter retrospective study.

624 Background: Gastric neuroendocrine carcinoma (NEC) is a rare, aggressive malignancy lacking a disease-specific staging system. Our previous multicenter analysis showed therapeutic value of D1 lymphadenectomy, limited benefit of D2, and the prognostic impact of nodal area involvement (pND). The UICC TNM classification (7th edition), based on metastatic node count (pN), had poor prognostic discrimination. We aimed to propose a pND-based staging system and compare its prognostic performance with the UICC TNM classification. Methods: Data of 118 patients with gastric NEC (n=79) or mixed adenoneuroendocrine carcinoma (n=39) who underwent gastrectomy with ≥D1 dissection at 29 Japanese centers (2000–2015) were retrospectively analyzed. Patients with metastasis or preoperative therapy were excluded. Overall survival (OS) and recurrence-free survival (RFS) were estimated by Kaplan–Meier. Prognostic factors were assessed by Cox regression. Model performance was evaluated by likelihood ratio tests, concordance index (C-index), Akaike information criterion (AIC), and Bayesian information criterion (BIC). Results: Median age was 69; 83% male. Five-year OS and 3-year RFS were 56.7% and 63.4%. Older age, female sex, and pND2 were independent adverse prognostic factors. The pND-based system was defined as: Stage A = pT1pND0–1 or pT2–4pND0; Stage B = pT2–4pND1; Stage C = pT2–4pND2. Five-year OS and RFS for A/B/C were 63.5/56.3/40.9% and 58.4/43.7/30.7%. Stage C showed significantly worse OS (HR 2.52, p=0.016) and RFS (HR 2.84, p=0.004) versus Stage A. UICC TNM stages I–III overlapped. Model comparison showed better performance of the pND-based system: OS C-index 0.67 vs 0.62, AIC 498 vs 512, BIC 514 vs 528; RFS C-index 0.65 vs 0.60, AIC 476 vs 489, BIC 492 vs 505. LRT confirmed significant improvement when adding the pND-based stage to UICC TNM, but not vice versa. Conclusions: The pND-based Stage A–C system outperformed the UICC TNM classification (7th edition), offering clearer prognostic stratification. This framework may serve as a disease-specific staging system for gastric NEC and inform surgical and multidisciplinary strategies.