Background: There are three constitutionally recognized Indigenous populations in Canada, the First Nations, Métis and Inuit. Although historically, Indigenous peoples in Canada were at lower risk for cancer compared to non-Indigenous people, this is no longer true. Data from prior to 2010 suggested that First Nations in Ontario had higher incidence and poorer survival from colorectal cancer (CRC). The current project arose from concerns raised by Indigenous partners about community members being diagnosed with colorectal cancer before screening age-eligibility.

Aim: To examine the age and stage at CRC diagnosis among First Nations people in Ontario (FN) compared to other Ontarians (ON).

Methods: With Indigenous partner support, approvals were obtained to link health administrative datasets at ICES and Ontario Health. Among others, linked datasets included the Ontario Cancer Registry and the Indian Register System, a registry of FN with status, meaning they are on the official record of persons registered under section 6 of Canada’s Indian Act. We created cohorts of FN and ON aged 18-85 from 2000 to 2018, matched 1:5 on sex, age at and year of inception. For the primary analysis, we used a competing risks framework, where death was accounted as a competing event and attained age was selected as the time scale. The primary outcome was CRC diagnosis, the primary exposure was FN vs ON and we adjusted for regular screening test use, comorbidity, remoteness and having a regular primary care provider. Similarly, we examined stage at diagnosis, restricting the cohort to 2007+ due to limitations in the stage data.

Results: Baseline demographics for the 153,300 FN and 766,500 ON included are in the Table. Median (IQR) follow-up was 17 (8-20) and 18 (I8-20) years for FN and ON, respectively (p <0.001). There were 1,156 (0.8%) and 4,249 (0.6%) CRCs diagnosed in FN and ON, respectively (p <0.001). 11,213 (7.3%) FN and 25,732 (3.4%) ON died from other causes (p <0.001). After accounting for the competing event of death, FN were significantly more likely to be diagnosed with CRC at a younger age (HR 1.42, 95%CI: 1.32-1.53) (see Figure) and later stage (HR 1.53, 95%CI: 1.34-1.75) than ON.

Conclusions: FN with CRC were younger and presented at a later stage compared to ON with CRC. Based on these findings, changes to the design of Ontario’s CRC screening program for First Nations people may be warranted. These findings may be of interest to other jurisdictions with large Indigenous populations.

Table. Baseline demographics of First Nations and other Ontarian cohorts at inception

Figure. A. Cumulative incidence of colorectal cancer (primary event) & death (competing event), comparing First Nations (FN) and other Ontarians (ON). B. Focus on cumulative incidence of colorectal cancer (death as competing event, not shown), comparing FN and ON.

Martin Tammemagi

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