Income and long-term breast cancer survival: Comparisons of diverse urban places in Ontario and California

Background: Breast cancer survival has been consistently observed to be advantaged in low-income Canadian versus similar American contexts. However, none of the previous studies accounted for between-country case-mix differences on the stage of disease at diagnosis. They also focused on relatively short-term (5-year) follow-up. Previously, we found significant differential effects of socioeconomic status on short-term survival of women with node positive breast cancer. We also discovered a 4-way, country by SES by stage by follow-up interaction. This study examined the differential effects of socioeconomic status (SES) on the long-term survival of women with node negative breast cancer in Canada and the United States. Method: Ontario and California cancer registries randomly provided 800 cases from diverse urban places: Toronto and Windsor, Ontario; and San Francisco and Modesto, California. Chart abstractors reliably enhanced the Ontario registry, replicating stage of disease and treatment variables that were routinely collected by the California registry (average kappa coefficient among three chart abstractors was 0.95). Invasive, node negative breast cancers incident between 1988 and 1990 were followed until 2006. Socioeconomic data were, respectively, taken from 1991 and 1990 population censuses. Census tract-based SES income measures were used to aggregate similarly low-to high-income decile areas within countries. The Ontario and California historical cohorts were compared on their initial surgical and adjuvant treatments as well as their survival experiences using direct age adjustments and Mantel-Haenszel methods. Results: Overall 15-year survival did not differ significantly between-country or between large and small urban places within-countries. SES-survival associations were observed in California, but not in Ontario. Compared with their California counterparts, residents of the lowest income urban areas in Ontario experienced a significant survival advantage (rate ratio [RR] = 1.66 [95% confidence interval 1.00, 2.76]). In these lowest income and other vulnerable, lower-middle- to working-class neighborhoods, significantly more Ontario residents gained access to adjuvant radiation therapy (RR = 1.75 [1.21, 2.53]), and such access was similarly associated with 15-year survival in Ontario and California (RR = 1.36 [1.04, 1.77]). The surgical and chemotherapy experiences of the two cohorts did not differ significantly. Conclusions: In an era of adjuvant radiation therapy innovation, lower income Canadian women with node negative breast cancer seemed to enjoy much greater such access that seemed to matter in terms of their survival. More inclusive health insurance coverage provided by Canada's single payer health care system seems the most plausible explanation for their distinctly more equitable cancer care experiences.