The clinical benefit of conversion surgery (CS) after chemotherapy remains unclear for stage IV gastric cancer (GC) patients. This study aimed to investigate the prognostic factors used to determine whether CS is a promising therapeutic strategy.

We retrospectively analyzed data from 156 patients diagnosed with unresectable stage IV GC who underwent chemotherapy as the initial treatment, including 40 patients who had R0 resection in CS.

The median survival time of the CS patients was significant longer than that of patients who underwent chemotherapy alone. A multivariate analysis identified only pN3 as an independent prognostic factor in CS patients. Among the differentiated tumor type patients, carbohydrate antigen 19-9 (CA19-9) levels were significantly higher in pN3 patients than in pN0-2 patients before chemotherapy. Among undifferentiated tumor type patients, pN3 patients had a significantly lower tumor size ratio (before chemotherapy/before surgery) than pN0-2 patients.

Although it is clinically difficult to diagnose lymph node metastasis using preoperative examinations, CA19-9 levels and tumor size ratios may be preoperative indicators for predicting pN3, which is associated with a poor prognosis in CS.