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 Fra ncisco Bay Area has consistently been higher than that in other region s of the United States, The distribution of established risk factors f or breast cancer (i.e., parity, age at first full-term pregnancy, brea st-feeding, age at menarche, and age at menopause) and probable risk f actors (e.g., alcohol consumption) also differs across geographic regi ons, Purpose: A study was planned to explore the extent to which diffe rences ill 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 mere obtained for the San Francisco Bay Area and other regions from the Surveillance, Epidemiol ogy, and End Results (SEER) Program, Risk factor data from January 198 0 through December 1982 were computed from the Cancer and Steroid Horm one Study, a population-based, case-control study of women 22-55 years of age who resided in eight SEER regions. Two different statistical m ethods were used to compute the relative risk (RR) of breast cancer as sociated with residence in the San Francisco Bay Area versus other reg ions, 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 r egions. Nearly all of these differences would be expected to lead to a n elevated incidence of breast cancer in the San Francisco Bay Area. W ith the use of incidence rates adjusted only for age, the RR for San F rancisco Bay Area residence from January 1978 through December 1982 co mpared with residence in seven other SEER areas was 1.14 for white wom en 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 0.7 5-0.83 for black women, after further adjusting for established and pr obable risk factors (parity, age at first full-term pregnancy, breastf eeding, age at menarche, age at menopause, and alcohol consumption), W ithout adjustment for alcohol consumption, the corresponding results w ere 0.97-1.02 for white women and 0.77-0.88 for black women. Conclusio ns. Among both white women and black women, the elevated breast cancer incidence rate in the San Francisco Bay Area can be completely accoun ted for by regional differences in known risk factors.