Neoadjuvant chemotherapy for colorectal cancer with synchronous and exclusive peritoneal metastasis: A comprehensive retrospective cohort study.

169 Background: Colorectal cancer (CRC) with synchronous and exclusive peritoneal metastases (PM) is rare and associated with poor prognosis. The role and type of neoadjuvant chemotherapy (NAC) before cytoreductive surgery (CRS) remains controversial. We aimed to characterize this population and evaluate outcomes and prognostic factors after neoadjuvant chemotherapy. Methods: We conducted a retrospective study of CRC patients with isolated synchronous PM (2016–2021) who received NAC followed by surgical reassessment. The primary endpoint was overall survival (OS). Prognostic factors included the peritoneal cancer index (PCI) and Peritoneal Regression Grading Score (PRGS). Results: Eighty-one patients were included (median follow-up 80.7 months). NAC regimens were FOLFOX (70%), FOLFIRI (12%), or FOLFIRINOX (17%); 54% also received targeted therapy (43% anti-EGFR, 57% anti-VEGF). CRS was performed in 54 patients (65%), with complete cytoreduction (R0) achieved in 94%, and median PCI to 10 (0-39). CRS significantly improved mOS (44.1 vs 22.8 months; HR 0.27, 95% CI 0.14 0.53, p