Invited commentary: assessing breast density change--lessons for future studies.

Breast density is one of the strongest predictors of breast cancer risk, and quantitative measurement is fairly reproducible. However, to study change in breast density, other issues should also be considered. Most studies of breast density have relied on one assessment, yet the mammographic features of the breast that constitute breast density change with age and/or menopause. When measuring breast density change, issues related to assessment are of greater concern. In addition, because age-period and cohort effects are codefined, evaluation of age trends must also consider the possible explanations of period and cohort effects. The prevalence of different factors affecting breast density changed dramatically over the last 50 years. In this issue of the Journal (Am J Epidemiol 2008;167:1027-1036), Kelemen et al. evaluate how factors known to be related to breast density influence breast density change with age. These authors are to be complimented on their detailed analysis and consideration of many of these issues. They not only describe the averaged effects of age on breast density changes but also consider whether patterns of density change differ for women with different exposure histories.