The incidence and risk factors of metachronous colorectal cancer: an indication for follow-up.
BACKGROUND
Patients with colorectal cancer are at risk for developing metachronous colorectal cancer. The purpose of posttreatment surveillance is to detect and remove premalignant lesions to prevent metachronous colorectal cancer.


OBJECTIVE
The aim of this study was to investigate the incidence of and predictive factors for metachronous colorectal cancer in patients with newly diagnosed colorectal cancer.


DESIGN AND PATIENTS
The data on all patients with newly diagnosed colorectal cancer between 1995 and 2006 were obtained from the Rotterdam Cancer Registry in The Netherlands and studied for metachronous colorectal cancer.


MAIN OUTCOME MEASURES
The annual incidence rate and the standardized incidence ratios were calculated.


RESULTS
In total, colorectal cancer was diagnosed in 10,283 patients; there were 39,974 person-years of follow-up. The mean annual incidence rate of metachronous colorectal cancer was 314/100,000 person-years at risk during 10 years of follow-up, corresponding with a mean annual incidence of 0.3% and a cumulative incidence of 1.1% at 3 years, 2.0% at 6 years, and 3.1% at 10 years. The incidence of metachronous colorectal cancer after resection of a first colorectal cancer is significantly higher than the incidence of colorectal cancer in an age- and sex-matched general population (standardized incidence ratio 1.3, 95% CI 1.1-1.5). This difference is especially seen during the first 3 years after first colorectal cancer diagnosis (standardized incidence ratio 1.4, 95% CI 1.1-1.8). The presence of synchronous colorectal cancer was the only significant risk factor for developing metachronous colorectal cancer (relative risk 13.9, 95% CI 4.7-41.0).


CONCLUSIONS
Despite the availability of colonoscopy, metachronous colorectal cancer is still seen during follow-up in patients with colorectal cancer; the highest risk is during the first 3 years after initial diagnosis. For this reason, a follow-up colonoscopy is useful at a short-term interval after colorectal cancer diagnosis. The presence of synchronous colorectal cancer at the time of first colorectal cancer diagnosis is the only predictive risk factor for developing metachronous colorectal cancer. Tailored surveillance programs may be considered in patients with a diagnosis of synchronous tumors.