Regional variations in breast cancer incidence among California women, 1988–1997

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Abstract
Background: Internationally, California has some of the highest breast cancer rates; these rates also show substantial regional variations within the state. This study describes geographic breast cancer incidence patterns within California and evaluates the degree to which socioeconomic status (SES) and urbanization explain the regional variability. Methods: Invasive breast cancer cases in women ≥20 year of age were identified from the California Cancer Registry, for 1988–1997, then assigned to one of three regions (San Francisco Bay Area, Southern Coastal Area and the rest of California), based on residence at diagnosis. Neighborhood SES and urbanization were derived from U.S. Census data. Rate ratios (RR) and 95% confidence intervals (CI) were computed using Poisson regression. Analyses were conducted for all invasive breast cancer cases (n=176,302) and by selected histologic subtypes: ductal (n=121,619); lobular (n=13,410); mixed ductal and lobular (n=9744). Results: Compared to block groups with the lowest quartile of SES, rates were highest in block groups with high SES. Rates also were higher in suburban and city areas than in small town/rural areas. Compared to the rest of California, age- and race-adjusted rates for all breast cancer were approximately 20% higher in the San Francisco Bay Area and 10% higher in the Southern Coastal Area. After adjusting for SES and urbanization the rate ratios were reduced to near unity (RR=1.06, 95% CI: 1.03–1.09 for San Francisco Bay Area; RR=1.02, 95% CI: 0.99–1.04 for Southern Coastal Area). Rates ratios for ductal carcinomas mirrored those for all cases. For lobular cases, rate ratios remained elevated after adjustment for age, race/ethnicity, neighborhood SES and urbanization (RR=1.18, 95% CI: 1.11–1.27 for San Francisco Bay Area; RR=1.10, 95% CI: 1.04–1.17 for Southern Coastal Area). For the subset of cases with mixed ductal and lobular histologies, the rate ratio for the San Francisco Bay Area was no longer elevated after adjusting for age, race/ethnicity, SES and urbanization (RR=0.92, 95% CI: 0.84–1.01); the adjusted rate ratio for the Southern Coastal Area, however, remained elevated (RR=1.22, 95% CI: 1.12–1.32). Conclusions: Regional differences in neighborhood SES and urbanization appear to largely explain regional rate differences in California for all breast cancers and ductal carcinomas but do not fully explain geographic patterns of breast cancer with a lobular component.