Regional Differences in Known Risk Factors and the Higher Incidence of Breast Cancer in San Francisco

BACKGROUND: The age-adjusted incidence of breast cancer in the San Francisco Bay Area has consistently been higher than that in other regions of the United States. The distribution of established risk factors for breast cancer (i.e., parity, age at first full-term pregnancy, breast-feeding, age at menarche, and age at menopause) and probable risk factors (e.g., alcohol consumption) also differs across geographic regions. PURPOSE: A study was planned to explore the extent to which differences in the regional distribution of established and probable risk factors could explain the increased incidence of breast cancer in the San Francisco Bay Area. METHODS: Age-adjusted breast cancer incidence rates for January 1978 through December 1982 were obtained for the San Francisco Bay Area and other regions from the Surveillance, Epidemiology, and End Results (SEER) Program. Risk factor data from January 1980 through December 1982 were computed from the Cancer and Steroid Hormone Study, a population-based, case-control study of women 22-55 years of age who resided in eight SEER regions. Two different statistical methods were used to compute the relative risk (RR) of breast cancer associated with residence in the San Francisco Bay Area versus other regions, after adjusting for regional differences in known risk factors. RESULTS: Substantial differences in the distribution of breast cancer risk factors were found between the San Francisco Bay Area and other regions. Nearly all of these differences would be expected to lead to an elevated incidence of breast cancer in the San Francisco Bay Area. With the use of incidence rates adjusted only for age, the RR for San Francisco Bay Area residence from January 1978 through December 1982 compared with residence in seven other SEER areas was 1.14 for white women and 1.10 for black women. Depending on the statistical method used, the RR was reduced to approximately 0.96-0.99 for white women and 0.75-0.83 for black women, after further adjusting for established and probable risk factors (parity, age at first full-term pregnancy, breast-feeding, age at menarche, age at menopause, and alcohol consumption). Without adjustment for alcohol consumption, the corresponding results were 0.97-1.02 for white women and 0.77-0.88 for black women. CONCLUSIONS: Among both white women and black women, the elevated breast cancer incidence rate in the San Francisco Bay Area can be completely accounted for by regional differences in known risk factors.