The Role of Effusion Cytology in the Staging of Primary Digestive Tract Malignancies: An Evaluation of 528 Peritoneal Washing Specimens

Summary: Introduction: Cytologic evaluation of peritoneal fluid may be performed as part of surgical staging of primary gastrointestinal tract malignancies (GIMs) and remains an independent prognostic factor. Although conventional cytology is considered the gold standard, limitations include low sensitivity and occasional interpretative difficulty. The aim of this study was to examine the contribution of peritoneal washing specimens to the surgical staging of GIMs at our institution, and to determine the utility of ancillary immunohistochemical studies in the classification of high-risk peritoneal fluid specimens. Materials and Methods: A retrospective query of our pathology database for peritoneal fluid specimens collected during diagnostic laparoscopy (DL) for esophageal, gastric, and pancreatobiliary adenocarcinoma from 1/2015-4/2023 was performed and clinicopathologic data was recorded. Immunohistochemical stains (BerEP-4 and claudin-4) were then performed on contributory cell blocks from high-risk peritoneal fluid specimens (defined as cytologically negative for malignancy but with T4 disease on resection, tumor within 1mm of the adventitial margin (GEJ tumors), or documented peritoneal disease within 100 days of washing). Results: 528 peritoneal washing specimens from 139 patients were included in the study. Patient demographics and clinicopathologic characteristics are summarized in Table 1. Notably, only 68 specimens (13%) from 25 patients were positive; 16 (64%) of these patients had evidence of disease at the time of DL. There were 5 patients with conversion of initial positive washings to negative following neoadjuvant chemotherapy; 3 patients underwent resection and all showed ypT4 disease. Overall, of patients with T4 or nodal disease on resection, only 3 (6%) had positive fluid cytology. For the high-risk peritoneal fluid specimens, 46 cell blocks from 18 patients (15 gastric, 3 GEJ) were considered contributory and stained with Ber-EP4 and claudin-4. Of the 46 specimens, 37 cases were negative for both stains, while 4 cases showed weak non-specific Ber-EP4 staining in mesothelial or inflammatory cells. Five cases demonstrated indeterminate staining (only rare single cells). Conclusions: Cytologic evaluation of peritoneal washings rarely detects microscopic disease and does not accurately predict the stage of primary GIMs. Future consideration of alternative testing/sampling methods should be made to provide a better assessment of surgical candidacy.