Body mass index and breast cancer survival in relation to the introduction of mammographic screening.

Aims: Mammographic screening reduces mortality in breast cancer. It is not known if this reduction is more pronounced in certain groups. Obesity has been associated with worse survival following breast cancer diagnosis. This study investigates BMI in relation to breast cancer mortality, and if this association is affected by invitation to mammographic screening.
Methods: In 1976, a randomised mammographic screening trial, inviting 50% of all women aged 45-69 years (n=42 283), was set up in Malmö, Sweden. BMI in relation to breast cancer mortality was examined separately in women invited or not invited to screening in the trial. The analyses also included a historical control-group diagnosed before the screening trial. The study included 2974 women diagnosed in 1961-1991. Relative risks (RR) with a 95% confidence interval was obtained from a Cox proportional hazard analysis and in the analysis of all women, follow-up was limited to 10 years.
Results: Obese women (BMI> or =30) not invited to mammographic screening had a higher adjusted RR of dying of breast cancer as compared to normal weight women (2.08:1.13-3.81) in the 10-year follow-up. In women invited to screening there was no association between BMI and breast cancer mortality. In the historical control group, mortality was increased in overweight women (BMI: 25-30), RR=1.27:0.99-1.62, and obese women, RR=1.32:0.94-1.84, but these associations totally disappeared in the multivariate analysis, following adjustment for tumour size and stage.
Conclusions: Overweight and obese women may be a group that profit from mammographic screening to more than normal weight women.