Epidemiology of breast cancer in a screened population

Breast cancer risk factors were examined in a screened population expected to exhibit selection bias. Data were obtained for 1,383 breast cancer cases and 2,543 randomly selected controls screened at the Guttman Breast Diagnostic Institute between 1968 and 1979. Relative risk (RR) estimates were found to be elevated for the usual factors: greater age, greater weight, early menarche, late first live birth or nulliparity, late menopause, and family history of breast cancer. Adjustment for screening variables (number of screenings, breast symptomatology, and year of first screening) removed substantial bias in RR estimates. The unadjusted analysis tended to reduce the magnitude of RR for family history and weight among screenees in general and for age among women screened only once. Although the unadjusted analysis showed no effects for either previous breast biopsy or religion, adjustment for screening variables indicated that among rescreened women, previous biopsy, Jewish and Catholic religion were associated with increased risk; among women screened once only, these characteristics were associated with decreased risk of breast cancer.