Addressing gaps in colorectal cancer screening in Canada : multilevel determinants of screening, pathways to screening inequalities, and program evaluation

Background: Colorectal cancer is the second most diagnosed cancer in Canada and the third most common cause of cancer mortality. Despite the known benefits of early and regular use of preventive screening tests, only 20% to 30% of Canadian adults aged 50 to 74 years participate in regular screening, and social disparities in screening participation exist.  Purpose: The overall aims of the proposed thesis are to contribute to a better understanding of the social determinants of colorectal cancer screening in Canada and to explore potential pathways for intervention—both by identifying mechanisms that explain existing social disparities and by evaluating whether or not current population-level intervention strategies affect screening uptake overall and among vulnerable populations.  The specific objectives of the thesis are as follows: 1) To assess the association between area-level socioeconomic deprivation and colorectal cancer screening participation;  2) To assess whether or not access to primary care physician mediates differences in screening uptake between recent immigrants and non-immigrants in Canada; 3) To evaluate the impact of two types of colorectal cancer screening programs (“systematic mail-based programs” that deliver and collect screening kits via-mail to all residents aged 50 to 74 years, and “patient-reliant programs” that rely on individuals to access test kits via a designed phone-line, website, or their physician, and return kits in-person) on screening participation, and screening disparities.   Methods: Analyses were conducted utilizing data from 50 to 75-year-old respondents from the population-based Canadian Community Health Study (CCHS) who had no known familial or medical risk factors of colorectal cancer (i.e., those at ‘average risk’). CCHS data provided information on respondents’ age, sex, marital status, educational attainment, income, immigration status, visible minority status, access to a primary care physician, area and province of residence, and both lifetime (i.e., ever vs. never) and non-recent colorectal cancer screening (i.e., no stool-based screening in the previous two years or no endoscopic-based screening in the previous five years). Data from the 2006 Canadian Census provided information on respondents’ local area-level income. Different types of analyses were used, including generalized estimating equations (Objective 1), regression- and inverse probability weighting-based mediation analyses (Objective 2), and regression-based difference-in-differences analyses for program evaluation (Objective 3).   Results: First, findings suggest that colorectal cancer screening is determined by both individual- and local area-level or region-wide factors. This thesis observed a gradient in lifetime colorectal cancer screening according to local area-income in Canada independent of individual-level social and economic factors. This finding highlights the potential influence of social and environmental contexts on colorectal cancer screening uptake. Second, access to primary care physicians was not found to mediate the association between recent immigration and colorectal cancer screening. This finding suggests that alternative interventions to reduce immigration-based disparities should be explored. Lastly, both systematic and patient-reliant programs were observed to improve overall screening participation. However, both types of programs did not appear to reduce known social and economic screening inequalities. In fact, the patient-reliant program studied was observed to increase the screening disparity between those with and without a primary care physician. These results indicate that reductions in social inequalities related to colorectal cancer screening may require more targeted strategies.   Conclusions: This thesis contributes to the literature on the social determinants of colorectal cancer screening in Canada by exploring area-level determinants of screening as well as mechanisms that explain screening inequalities and by evaluating the effectiveness of various colorectal cancer screening programs at improving screening uptake and reducing screening inequalities. Findings support the notion that the role of socioeconomic contexts, above and beyond individual-level factors, merit attention both in future research on and surveillance of colorectal cancer screening. It also indicates that modifiable pathways to known screening disparities require continued exploration, and that future work should assess the acceptability, feasibility, and effectiveness of complimentary targeted interventions to promote screening both overall and among those most vulnerable.

Keyphrases: Colorectal neoplasm, Early detection of cancer, Endoscopy, Social Determinants of Health, Inequalities, Residence Characteristics, Epidemiologic Measurements, Epidemiologic Methods, Health Policy, Program Evaluation