Retrospective comparison of S-1 plus cisplatin versus S-1 monotherapy for the treatment of advanced gastric cancer patients with positive peritoneal cytology but without gross peritoneal metastasis.

Background: Peritoneal cytology positive for carcinoma cells (CY+) is an independent poor prognostic factor in gastric cancer, and patients with CY+ are diagnosed with stage IV disease. However, there is no standard treatment strategy for CY+ gastric cancer, whereas combination chemotherapy with fluoropyrimidine and platinum has been established as the standard treatment for unresectable advanced gastric cancer or after R2 resection. Herein, we assessed whether adding cisplatin to S-1 (SP) could improve the outcome of CY+ gastric cancer patients, as compared to S-1 monotherapy.

Methods: This retrospective study was conducted at a single Japanese institute between June 2005 and March 2014. Patients diagnosed with CY+ advanced gastric cancer and treated with S-1-based therapy were enrolled. Patients with incurable factors other than CY+ were excluded.

Results: Forty-four patients were enrolled; 25 and 19 were administered S-1 and SP, respectively. The 2-year survival rates were 52.0% [95% confidence interval (CI), 31.2-69.2%] and 52.6% (28.7-71.9%) in the S-1 and SP groups, respectively. The median overall survival (OS) and progression-free survival (PFS) were 28.2 and 15.6 months in the S-1 group and 24.0 and 18.8 months in the SP group, respectively; they were not significantly different. The relative dose intensities were 0.79 (S-1) in the S-1 group and 0.69 (S-1)/0.70 (cisplatin) in the SP group.

Conclusion: Adding cisplatin to long-term S-1 monotherapy did not significantly improve the outcome of CY+ advanced gastric cancer patients.