Anatomic subsite of primary colorectal cancer and subsequent risk and distribution of second cancers

Individuals with a history of colorectal cancer (CRC) have an increased risk of subsequent cancer. In this study, the authors used cancer registry data to evaluate whether this increased risk of cancer after CRC differed by anatomic subsite of a first CRC.

Individuals diagnosed with a first primary CRC between 1992 and 2009 were identified from 12 Surveillance, Epidemiology, and End Results (SEER) cancer registries. Standardized incidence ratios (SIRs) and 95% confidence intervals (CIs) were calculated by comparing the incidence of subsequent cancers in these patients who had an index CRC versus the cancer incidence rates in the general population. SIRs were calculated for cancers at anatomic sites within and outside the colorectum in analyses stratified by subsite of the index CRC.

Cancer incidence rates were significantly higher in individuals who had a previous CRC than in the general population (SIR, 1.15; 95% CI, 1.13-1.16). Individuals with an index CRC located between the transverse and descending colon experienced the greatest increased risk both overall (SIR, 1.29-1.33) and particularly with respect to the risk of a second CRC (SIR, 2.53-3.35). The incidence of small intestinal cancer was elevated significantly regardless of the index CRC subsite (SIR, 4.31; 95% CI, 3.70-4.77), and the incidence of endometrial cancer was elevated in those who had an index CRC in the proximal colon (SIR, 1.37-1.79).

The risk of second cancer after CRC differs by anatomic site of the first tumor and is particularly pronounced for those with prior CRC located in the transverse to descending colon. The mechanisms underlying this pattern of second cancer risk remain unknown.