Postmenopausal hormone use, screening, and breast cancer: Characterizationand control of a bias

Previous investigators have suggested that screening-related biases may exp lain associations between postmenopausal hormone use and breast cancer. To investigate these biases, we studied postmenopausal women in the Nurses' He alth Study from 1988 to 1994. Hormone use is associated with increased subs equent screening. Among women not screened in the previous 2 years, the pro bability difference, comparing current hormone users with others, for havin g mammography in the following 2 years is 19.5%; among women previously scr eened, the difference is 4.9%. These differences persist after control for other factors. If the increase in screening is causal, screening by mammogr am could be intermediate in the causal pathway to breast cancer diagnosis. To deal with this problem, we restrict attention to a subset of the cohort in which the effect of postmenopausal hormone use on screening is small (wo men previously screened). In this subset, the rate ratio comparing breast c ancer rates among current postmenopausal hormone users with others is 1.28. In a sensitivity analysis, the bias could not by itself plausibly account for the associations in our data. Our data provide evidence of an associati on between postmenopausal hormone use and breast cancer that is not solely the product of a detection bias.