The Melbourne colorectal cancer study: incidence findings by age, sex, site, migrants and religion.

During a 12-month period, age, sex, site, country of birth and religion data were obtained on all new cases of colorectal cancer (CRC) in Melbourne (population 2.8 million) yielding 1150 cases, this being the incidence study arm of a large-scale epidemiological and clinico-pathological investigation of CRC in Melbourne. The age, sex, site and subsite findings had, as expected, the characteristics of a high-risk population for CRC. Unexpectedly, rectal cancer rates were some of the highest recorded in the world. Incidence was greater in males than in females except for colon cancer between ages 35 and 58 where there was a female excess. There was an increasing male to female ratio from caecum to rectum and the crossover age to male excess had a monotonic pattern as it became younger from caecum to rectum. This site and subsite sex pattern may represent an aetiological difference and warrants further study. The data on first generation migrants supported the concept that with migration from low-risk to high-risk countries for CRC, such as Australia, there is a transition of rates towards the risk levels of the new country. Migrant rates for CRC were lower for each subsite than in the Australian born, but became closer to the Australian rate the further the distance down the bowel. Jews had rates nearly double those of the Melbourne population, a hitherto unreported finding in Australia. The possible causes of the very high rectal rates, the transition in migrant rates and the high Jewish rate will be reported in the case-control arm of the study.