Prognostic value of baseline peritoneal fluid cytology in 224 patients with unresectable peritoneal metastasis treated with Pressurized IntraPeritoneal Aerosol Chemotherapy (PIPAC)

Patients with peritoneal metastasis (PM) from gastrointestinal and gynecological cancer can be treated palliatively with Pressurized IntraPeritoneal Aerosol Chemotherapy (PIPAC). Only few prognostic markers for stratification of PM-patients treated with PIPAC exist. The prognostic value of peritoneal fluid (PF) cytology has only been examined to a limited extent. Our primary aim was to investigate the prognostic value of baseline PF cytology in unresectable PM treated with PIPAC. We performed a retrospective analysis of prospectively collected data from all PM patients treated with PIPAC at Odense PIPAC Center from 2015 to 2024. Positive baseline PF cytology was defined as malignant cells or cells suspicious of malignancy. Survival data were analyzed using the Kaplan-Meier graphs, log rank test and multivariable Cox proportional hazards regression. Inclusion criteria were fulfilled by 224 patients, of which 139 (61 %) had positive cytology at PIPAC 1. Patients with positive baseline cytology had a significantly higher ascites volume (150 mL vs. 20 mL, p < 0.001), higher peritoneal cancer index (PCI) (15 vs. 3.5, p < 0.001) and higher mean histological Peritoneal Regression Grading Score (PRGS) (3.0 vs. 1.0, p < 0.001), compared to patients with negative baseline cytology. Positive baseline cytology was associated with shorter overall survival (OS) for the entire cohort (P < 0.01), PM from gastric cancer (GC-PM) (P = 0.007), and PM from colon cancer (CC-PM) (P = 0.02), but not for PM from ovarian cancer (OC-PM) or pancreatic cancer (PC-PM). However, at multivariate analysis, baseline PF cytology had no independent prognostic value. The PRGS, on the other hand, showed independent prognostic value in all multivariable models used. In conclusion, our data from this retrospective cohort study indicate that baseline PF cytology holds no independent prognostic value. However, for classification of a given patient as having complete response to treatment, this modality is still recommended, together with histological response assessment (PRGS).