BACKGROUNDAlthough positive peritoneal lavage cytology has been established as an important prognostic factor in gastric and pancreatic cancers, its significance in duodenal cancer remains unclear.METHODSThis study analyzed 74 consecutive patients with peritoneal lavage cytology who underwent surgical resection for duodenal cancer at our institution between 2002 and 2020. (pancreatoduodenectomy, n = 58; partial resection, n = 11; and pancreas-sparing total duodenectomy, n = 5). Clinicopathologic features and survival outcomes were analyzed, focusing on the peritoneal lavage cytology status.RESULTSAmong the 8 peritoneal lavage cytology -positive patients, the primary tumor depth was classified as pT3 in 1 case and pT4 in 7 cases. The recurrence-free survival and overall survival rates were markedly worse in peritoneal lavage cytology-positive patients than in peritoneal lavage cytology-negative patients (5-year recurrence-free survival: 0.0% vs 78.0%, P < .001; 5-year overall survival: 12.5% vs 82.4%, P < .001). In a subgroup analysis of patients with T3 or T4 tumors (n = 38), the recurrence-free survival and overall survival were significantly poorer in patients with peritoneal lavage cytology-positive tumors than in those with peritoneal lavage cytology-negative tumors (5-year recurrence-free survival, 0.0% vs 51.8%, P = .001; 5-year overall survival, 12.5% vs 60.9%, P = .005). A multivariate analysis identified a peritoneal lavage cytology-positive status as an independent prognostic factor for the overall survival (hazard ratio, 3.38; P = .023).CONCLUSIONPeritoneal lavage cytology positivity in patients with duodenal cancer was associated with advanced tumor progression and was a significant prognostic marker. This finding underscores the importance of peritoneal lavage cytology as a valuable tool for identifying patients who may require multidisciplinary treatment approaches.