Trends in colorectal cancer incidence rates in New Zealand, 1981-2004

Background: Incidence rates of colorectal cancer (CRC) in New Zealand rank among the highest worldwide. Internationally, there has been evidence of a shift in colon cancer from left- to right-sided. The objective of this study was to determine trends in left- and right-sided colon and rectal cancers incidence by sex, age and ethnicity.

Methods: Using datasets created by linking data from the New Zealand Cancer Registry to the census data, we analysed a total of 47,694 CRCs from 1981 to 2004. Cancers were divided into right-sided colon (cecum to the splenic flexure); left-sided colon (descending and sigmoid colon); and rectal (rectosigmoid junction and rectum).

Results: Left- and right-sided colon, and rectal cancer incidence rates increased by 13-20% among men. In women, colon cancer rates increased by 25% for right-sided cancers, decreased by 8% for left-sided cancers and remained unchanged for rectal cancers. This corresponds with an increase in right-sided cancers from 57% to 65% of total colon cancers in women. The incidence of all CRCs increased at a faster rate among Māori than non-Māori.

Conclusion: We identified a left- to right-sided shift in colon cancer limited to women over the age of 65. While Māori trends in site distribution parallel those of their non-Māori counterparts, the rapid increase in Māori incidence rates is noteworthy. It is unclear why such shifts in CRC site distribution are occurring.