BACKGROUND A large-scale study was performed to identify the risk factors for developing synchronous and metachronous colorectal cancer (CRC) in gastric cancer (GC) patients, including microsatellite instability (MSI) and p53 overexpression.

METHODS A total of 1041 GC patients who underwent endoscopic resection or surgery and underwent colonoscopy simultaneously or during surveillance for GC were consecutively enrolled. Clinicopathologic characteristics, MSI, and p53 overexpression were compared between the GC patients with and those without synchronous and metachronous CRC.

RESULTS Of the 1041 patients, CRCs were detected in 67 (6.4 %) patients with GC. Forty-six (4.4 %) had synchronous CRC and 21 (2.0 %) had metachronous CRC. Univariate analysis indicated that age ≥63 years (P < 0.001), male sex (P = 0.005), and p53 overexpression (P = 0.040) were significantly associated with a higher incidence of CRC. However, body mass index, smoking, tumor location, tumor multiplicity, tumor histology, TNM stage, and MSI were not significantly associated with the incidence of CRC. Age ≥63 years (OR: 5.881; 95 % CI: 3.083-11.221; P < 0.001) and male sex (OR: 2.933; 95 % CI: 1.307-6.584; P = 0.009) were risk factors for CRC in GC patients according to multivariate analysis.

CONCLUSIONS GC patients who are male and/or ≥63 years old are recommended to receive colonoscopy to detect CRC. MSI and p53 overexpression were not useful molecular markers for predicting CRC in GC.