Peritoneal sampling and histological assessment of therapeutic response in peritoneal metastasis: proposal of the Peritoneal Regression Grading Score (PRGS).

Background: Multimodal therapeutic strategies have improved the outcome of peritoneal metastases (PM). However, objective assessment of therapy response remains difficult in PM, since radiological studies have a poor accuracy for low-volumetric disease. There is an obvious need for a histological gold standard allowing assessment of tumor response to treatment in PM. Content: We propose to perform peritoneal punch biopsies with a diameter of 3 to 5 mm in all four abdominal quadrants. We propose a four-tier Peritoneal Regression Grading Score (PRGS), defined as Grade 1: complete response (absence of tumor cells), Grade 2: major response (major regression features, few residual tumor cells), Grade 3: minor response (some regressive features but predominance of residual tumor cells), Grade 4: no response (tumor cells without any regressive features). Acellular mucin and infarct-like necrosis should be regarded as regression features. We recommend reporting the mean and the worst value of the regression grades obtained. When complete tumor response is suspected intraoperatively, a peritoneal cytology should be sampled. Summary: A generic, unique score for the assessment of histological tumor response to chemotherapy in PM makes sense because of the clinical impact of histological response to therapy and because the organ of metastasis (peritoneum) is the same. By adopting PRGS, different centers will be able to use a uniform terminology and grading that will allow meaningful comparison of their results. Outlook: PRGS has now to be validated in several gastrointestinal and gynecological cancer types and may be useful both in clinical and research settings.