Peritoneal Lavage Cytology and Radiologically Evident Peritoneal Carcinomatosis Within 12 Months in Advanced Digestive Cancer Without Macroscopic Peritoneal Disease: A Single-Centre Prognostic Study

Background/Objectives: Peritoneal dissemination is a major pattern of failure in advanced digestive cancer and is poorly detected by cross-sectional imaging. Outside gastric cancer, peritoneal lavage cytology is rarely performed and carries no staging weight. We examined the association between cytological status and subsequent peritoneal carcinomatosis in patients without peritoneal disease on imaging or at operation. Methods: Retrospective prognostic study of consecutive patients with clinically staged T3–T4 digestive malignancy operated on by one surgical team (January 2019–December 2022). Lavage was performed before tumour manipulation and processed as cell blocks. The exposure was cytological status, positive (CY1) or negative (CY0); the outcome was radiologically evident peritoneal carcinomatosis within 12 months on computed tomography at 3, 6, 9 and 12 months, analysed as interval-censored data. Results: Of 62 patients, 10 (16.1%) had carcinomatosis at operation despite negative imaging. Of the 45 with digestive primaries and no peritoneal disease, 13 (28.9%) were CY1. Carcinomatosis developed in 10 of 13 CY1 and 5 of 32 CY0 patients (76.9% versus 15.6%; relative risk 4.92, 95% CI 2.09–11.62), an association preserved in patients without distant metastases and in those who underwent resection. The hazard ratio from a discrete-time proportional hazards model was 9.07 (95% CI 3.15–29.72; p = 0.00005). No clinicopathological variable examined was significantly associated with cytological status. Conclusions: In this exploratory series, positive lavage cytology was associated with early radiologically evident peritoneal carcinomatosis. The association is unadjusted and derives from a small, heterogeneous single-centre cohort; it does not establish independent prognostic value, nor does it support routine lavage cytology or changes to postoperative surveillance. It warrants prospective evaluation.