Fig. 1.

Mammograms used to define detection mode for first and second cancer events. Most patient risk factors were based on responses to questionnaires completed within 6 months of the mammogram used for initial mode of detection. Breast density was recorded by the interpreting radiologist and was only included in the analysis if it was recorded within ≤2 years of the mammogram used for initial mode of detection. Example Assume a woman has her first breast cancer diagnosed on 1 January 2007. To determine the mode of detection we search for a mammogram from 1 January 2006–1 January 2007. If none is found then we also look ahead until 1 February 2007. Assume work-up of her first cancer finishes on 15 February 2007. We then begin looking for a second breast cancer diagnosis 6 months later (after 16 August 2007)