The peritoneal regression grading score (PRGS) and peritoneal cytology (PC) assess response to chemotherapy in peritoneal metastasis (PM) in a setting of palliative treatment by pressurized intraperitoneal aerosol chemotherapy (PIPAC). Progression has been defined as an increase of PRGS between first and third PIPAC procedures, but failed as positive peritoneal cytology to demonstrate their prognostic impact. These results may be explained by a lack of statistical power. And, it is not known whether the mean or the highest PRGS among taken peritoneal biopsies bears the highest clinical value. We therefore conducted the largest prospective study to investigate the prognostic impact of iPGRS, PC, and their combination, designated as combined progression index (CPI).
Patients with PM who underwent >3 PIPAC (n=112) between December 2016 and February 2019 were prospectively included. A significant difference in OS and PFS according to CPI(highest) was found (OS:CPI-, 83.3, 95%CI[49.8;NA] vs. CPI+, 48.1, 95%CI[38.5;66.4] months; and PFS (respectively, 59.7, 95%CI[43.0;96.0] vs. 33.7, 95%CI[30.4;44.2] months). PRGS or PC had no independent prognostic impact. CPI+ was an independent predictor of worse prognosis, in OS (HR=5.24, 95%CI [2.07; 13.26]), and PFS (HR=4.41, 95%CI [1.40;13.88]).
The CPI based on highest PRGS and PC was found to be independently associated with worse prognosis for OS and for PFS in the setting of peritoneal metastasis. These results indicate that it should be of interest to take systematically peritoneal fluid for cytological examination and to implement the CPI in the therapeutic decision-making process in the context of PIPAC.

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