#68 Is there a true “shift” to the right in the incidence of colorectal cancer?

PURPOSE: A recent “shift” in the incidence of colorectal cancer (CRC) to right-sided colon cancers has been reported. Whether this “shift” resulted from a true increase in the incidence of right colon cancers or decline in left colon cancers is unknown. To disentangle these potential explanations, we examined temporal trends in proportion of right colon, left colon, and rectal cancers as well as incidence rates. METHODS: Using data from the NCI SEER program, proportions and age-adjusted incidence rates were calculated for right colon, left colon, and rectal cancers by race over three-year time periods from 1978–1998. Proportion of cases by age at diagnosis was also calculated. RESULTS: Although the proportion of individuals diagnosed with right colon cancer increased for whites (1978–1980 = 34%;1996–1998 = 40%) and blacks (1978–1980 = 37%;1996–1998 = 44%), the proportions with left colon and rectal cancers decreased. Between 1978–1998, age-adjusted incidence rates of left colon cancer declined in whites (1978–1980 = 16/100,000; 1996–1998 = 13/100,000) and blacks (1978–1980 = 18/100,000;1996–1998 = 12/100,000, while the incidence rates of right colon cancer (1978–1980 = 14/100,000;1996–1998 = 15/100,000) did not change significantly. Age-adjusted incidence rates of rectal cancer remained stable for blacks (10/100,000) and declined for whites (1978–1980 = 15/100,000; 1996–1998 = 12/100,000). Overall, 43% of individuals >60 years diagnosed with CRC had right colon cancer. Smaller proportions were diagnosed with left colon (31%) or rectal cancer (26%). Conversely, among individuals <60 years, 28% had right colon, 34% left colon, and 38% rectal. CONCLUSION: 1) The “shift” to right colon cancer is due to a decline in the incidence of left colon and rectal cancers, while the incidence of right colon cancers remains unchanged. 2) In the elderly, the majority of colon cancers are found in the right colon. This has important implications for choice of screening method used in the elderly.