Multiple primary malignancies involving primary sporadic colorectal cancer in Japan: incidence of gastric cancer with colorectal cancer patients may be higher than previously recognized

Background: Improvement in the prognosis of colorectal cancer (CRC) patients has led to increasing occurrences of multiple primary malignancies (MPMs) alongside CRC but little is known about their characteristics. This study was undertaken to clarify the clinical and pathological features of MPMs, especially those at extra colonic sites, in patients with CRC. Methods: We reviewed 1,111 patients who underwent operations for primary sporadic CRC in Saitama Medical Center, Jichi Medical University between April 2007 and March 2012. Two patients with familial adenomatous polyposis, one with hereditary non-polyposis colorectal cancer, two with colitic cancer, and any patients with metastasis from CRC were excluded. We compared the clinicopathological features of CRC patients with and without MPMs. As a control, we used a database compiled of patients with gastric cancer (GC) detected by mass screening performed in the Saitama Prefecture in Japan 2010 and compared these with CRC patients with synchronous GC. Results: Multiple primary malignancies at extracolonic sites were identified in 117 of 1,111 CRC patients (10.5%). The median age was 68 (range, 29 to 96) versus 71 (50 to 92) (P < 0.001). The incidence of GC (44.4% (52 of 117)) was the highest of all MPMs. All CRC patients with GC were older than 57 years. Synchronous GC was detected in 26 patients. By contrast, out of 200,007 screened people, 225 people were diagnosed as having GC in the Saitama Prefecture. The age-standardized incidence of synchronous GC in CRC patients was significantly higher (0.53%) than in the control group (0.03%) (odds ratio, 18.8; 95% confidence interval, 18.6 to 19.0; P < 0.001). Conclusion: Patients with CRC who were older than 50 years preferentially developed GC synchronously and metachronously. Thus, this patient group should undergo careful perioperative screening for GC.