We investigated the characteristics of synchronous and metachronous gastric cancer in patients with colorectal cancer. We reviewed 8,680 patients who underwent operations for primary sporadic colorectal cancer from 1989 to 2008. Synchronous gastric cancer was defined as gastric cancer diagnosed within 6 months of a colorectal cancer diagnosis. Gastric cancer diagnosed more than 6 months before or after colorectal cancer was defined as metachronous. The incidences of synchronous and metachronous gastric cancer were 0.93 and 1.4%, respectively (combined 2.4%). The standardized incidence ratio was 1.199 (95% confidence interval [CI] = 1.005–1.420) when the patients with premetachronous gastric cancer were excluded. Patients with synchronous and metachronous gastric cancer were 5 years older on average compared to the control population without gastric cancer. In addition, multivariate analysis revealed an odds ratio (OR) of 3.6 for being male, OR = 2 for positive family history of solid tumors, OR = 2.2 for colonic lesion, and OR = 4 for MSH2 expression loss compared to patients without gastric cancer. Patients with postmetachronous gastric cancer (when compared to synchronous and premetachronous gastric cancer), a preoperative CEA level of less than 6 ng/ml, and a relatively early stage of colorectal cancer had significantly higher overall (p = 0.016, 0.007, and 0.004, respectively) and disease-free survival rates (p = 0.046, 0.003, and 0.004, respectively), only on univariate analysis. Lymphovascular invasion of colorectal cancer and an advanced stage of gastric cancer were independent poor prognostic factors for both overall (p = 0.018) and disease-free survival (p = 0.028). Gastric cancer surveillance is recommended for patients with colorectal cancer, especially when the patient is old and male, has a positive family history of solid tumors, has a colonic lesion, or lacks MSH2 expression.
