The challenge of evaluating annual mammography screening for young women with a family history of breast cancer.

It has been recommended that women aged 40-49 years with a significant family history of breast cancer should be offered annual mammography screening (http://www.nice.org.uk). An observational study known as FH01 (http: //www. screening services.org/btw/fh01/index.asp) is evaluating this policy in a cohort of 6000 women at moderately increased risk of breast cancer due to family history. The main aims are to assess the likely impact on breast cancer mortality and cost-effectiveness. Measuring these outcomes is challenging in an environment where a randomized trial is not feasible and there is no natural comparison group. In this paper, we present some approaches to estimating effectiveness and the planned analyses. These involve comparison of disease stage and likely consequent breast cancer mortality in the cohort offered screening with that estimated in the absence of screening. The estimation uses observed outcomes in external populations and estimated outcomes for the hypothetical situation where screening had not taken place.