Studies suggest that colorectal cancer incidence increased disproportionately among the Aboriginal population of Ontario relative to the general population. Using an ecological approach, this study examined colorectal cancer incidence for the 1998-to-2009 period among Aboriginal people living in Ontario.
Data and methods
Based on their postal code when they were diagnosed, cases of colorectal cancer identified from the Ontario Cancer Registry were assigned to census geographic areas with high (33% or more) or low percentages of Aboriginal identity residents, using the Postal Code Conversion File Plus (PCCF+). To account for potential misclassification by the PCCF+, Indian reserves for which assignment through postal codes is likely to be accurate were identified. Age-standardized incidence rates and rate ratios were calculated to compare colorectal cancer incidence in high-Aboriginal identity areas or on Indian reserves with incidence in low-Aboriginal identity areas.
Colorectal cancer incidence was significantly higher for residents of high- versus low-Aboriginal identity areas in Ontario (rate ratio for men = 1.44, 95% CI = 1.26-1.63; rate ratio for women = 1.42, 95% CI = 1.23-1.63), a disparity that persisted by age group. When the Aboriginal sample was limited to residents of Indian reserves, the difference was statistically significant only for men and for people aged 50 to 74.
The incidence of colorectal cancer differs across areas of Ontario with high and low percentages of Aboriginal identity residents.
Data linkage, ecological studies, First Nations health
Colorectal cancer is the third most commonly diagnosed cancer, and the second leading cause of cancer death in Canada. Because Aboriginal identity or ancestry (ethnicity) is not routinely captured in cancer registries and mortality databases, little is known about colorectal cancer morbidity and mortality among Aboriginal people. [Full Text]
Stephanie W. Young (Stephanie.Young@cancercare.on.ca), E. Diane Nishri, Elisa Candido and Loraine D. Marrett are with Prevention and Cancer Control, Cancer Care Ontario, Toronto, Ontario. Loraine D. Marrett is also with the University of Toronto, Toronto, Ontario.
- Colorectal cancer is the third most commonly diagnosed cancer and the second leading cause of cancer death in Canada.
- Studies suggest that colorectal cancer incidence is increasing more rapidly in the Aboriginal population than in the general population.
- The incidence of colorectal cancer among Aboriginal people in Ontario cannot be routinely monitored because the Ontario Cancer Registry does not collect information on Aboriginal identity or ethnicity.
- Colorectal cancer incidence was significantly higher in areas of Ontario with a high percentage (33% or more) of residents who self-identified as being an Aboriginal person, compared with areas with a low percentage of Aboriginal identity residents.
- To account for potential misclassification by PCCF+’s population-weighted random assignment, Indian reserves, for which the accuracy of case assignment through postal codes is high, were used to examine health outcomes for a subgroup of First Nations people.
- Although colorectal cancer incidence among residents of reserves was lower than among residents of high-Aboriginal identity areas, rates for men and for people aged 50 to 74 were significantly higher compared with those in low-Aboriginal identity areas.
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