Background and purpose: Gastric cancer is the fifth most common cancer and the fourth cause of death from cancer. CA125 is a useful marker for cancer diagnosis and a predictive marker for metastasis in patients. Since the diagnostic value of this biomarker has not been definitively determined in gastric cancer, the aim of this study is to determine the serum level of the CA125 biomarker in gastric cancer patients before and after treatment.

Methods: In this cross-sectional study, 63 patients with gastric cancer who had confirmed pathology results were included in the study. Before and after the treatment (chemotherapy or surgery), 5 cc blood samples were taken from the patients. The serum level of the CA125 marker was measured with a kit using the ELISA method.

Results: The results showed that Intestinal with 70.5% and Adenocarcinoma with 71.66% are the most dominant types and subtypes of cancer in this study. It was also reported that the difference in CA125 serum levels in men and women before and after treatment is not statistically significant (P = 0.2). The difference in CA125 serum levels before and after treatment based on type (P = 0.4), subtype (P = 0.6) and grade (P = 0.3) of gastric cancer was not statistically significant. the people who received both surgery and chemotherapy a significant difference was observed (P< 0.05).

Conclusion: In this study, it was found that although the difference in the serum level of CA125 marker before and after the treatment was not statistically significant, but there is a significant difference based on the type of treatment, and in people who received both surgery and chemotherapy a significant difference was observed.