Urbanization and breast cancer incidence in North Carolina, 1995-1999

Breast cancer incidence rates are reported to be higher in urban compared with rural areas in the United States. We investigated the relationship between urbanization and breast cancer in North Carolina (1995-1999), and considered hospital characteristics as an explanation.

We calculated age-adjusted in situ and invasive female breast cancer incidence rates stratified by race, urbanization (Urban Influence Codes), and the presence of a hospital with a cancer registry and cancer program approval in a county.

For white women, incidence rate ratios (IRRs) comparing the most urban with the most rural counties were 1.60 for in situ and 1.18 for invasive cancer. For non-white women, IRRs were 1.27 and 0.99, respectively. IRRs for incidence in registry hospital counties versus those without were all > 1.00 and differences were greater for in situ cancer than invasive. For most strata, urban excesses were attenuated when further stratified by registry hospital status.

For most strata, we observed excess incidence in urban counties, but it appeared to be explained through the urban preponderance of registry hospitals. Counties with these hospitals may have higher incidence because of increased detection. Area hospital characteristics should be considered when evaluating geographic patterns of breast cancer incidence.