Cumulative incidence of metachronous colorectal cancer.

The incidence of metachronous colorectal cancer has most often been reported as a crude rate: second cancers/index cancers. The reported incidence varies between 0.5 percent and 3.6 percent. However, these calculations do not take into account factors such as length of survival and length of follow-up. The cumulative incidence more accurately reflects the risk for developing a metachronous cancer and was determined in a retrospective analysis of 5,476 patients who were diagnosed with colon or rectal cancer between 1965 and 1985. The cumulative probability was calculated by determining the number of patients developing a metachronous colon cancer vs. the number remaining at risk at that point in time. The calculated annual incidence for metachronous tumors was 0.35 percent per year. The cumulative incidence at 18 years was 6.3 percent. Analysis also demonstrated that metachronous cancers were diagnosed at earlier stages than were index cancers (P = 0.03). Subgroup analysis was performed on patients diagnosed with metachronous cancer before and after 1980. There was a difference in the incidence of metachronous cancers between these two groups (P = 0.04).