A Multicenter Collaborative Survey of Peritoneal Lavage Cytology in Gastric Cancer Surgery: Interinstitutional Methodological Differences and Positive Rate.

Objective: This study aimed to clarify the current status of cytological methodologies in high-volume gastric cancer centers and explore the relationship between methodology and positive rates.

Background: International standards for peritoneal lavage cytological methods for collection, handling, and cytopreparation in gastric cancer have not been established yet.

Methods: A questionnaire survey on cytological methodology was conducted in 61 institutions within the Japan Clinical Oncology Group in 2024. Aggregated data from patients with clinical T3 to T4 gastric cancer with cytology from 2017 to 2022 were collected to calculate positivity rates in each institution for the comparison of methodologies between institutions with high- and low-positivity rates.

Results: Thirty-three institutions (64%) collected samples from 2 sites, primarily from the Douglas pouch and left subphrenic area. Fifty-eight institutions (95%) used ≤100 mL of normal saline for injection, and 51 (87%) performed intraoperative rapid cytology. Twenty-five institutions (41%) used additives in samples. Scraping glass slides and centrifugal direct smears were predominant cytopreparation methods in 31 (51%) and 22 (36%) institutions, respectively, and ethanol fixation was employed in 53 (87%). In 61 institutions (11,367 patients), the median cytological positivity rate for clinical T3 to T4 gastric cancer was 8.5% (2.1%-28.3%). Institutions with higher positivity rates more often employed ethanol fixation (97% vs. 77%, P = 0.026) and used ≤50 mL of normal saline for injection (61% vs. 37%, P = 0.074).

Conclusions: Even among Japanese high-volume centers, cytological methodologies for gastric cancer lack uniformity, thereby leading to substantial variability in the proportion of positive cytology.