[Breast density: a biomarker to better understand and prevent breast cancer]

In Quebec, cancer is the principal cause of mortality. This epidemiologic research program includes two components. The first component takes place at the "Institut national de santé publique du Québec" and involves surveillance and evaluation of practices in oncology with the aim of providing the Quebec Ministry of Health with some of the evidence needed to determine its policies in cancer control. The second component takes place at the "Unité de recherche en santé des populations (URESP)" of Laval University and is devoted to studying the etiology and prevention of breast cancer. This paper focuses on this second research component which uses mammographic breast density as an intermediate biomarker to study the causes of breast cancer and strategies to prevent it. Breast cancer risk is much higher among women with very dense breasts than among those with little or no breast density. Recently, we were among the first to show that women with high vitamin D or calcium intakes have less breast density than those with low intakes, especially among premenopausal women. Furthermore, we have confirmed that breast density was increased among premenopausal women with high levels of IGF-I and low levels of IGFBP3 which is consistent with the observed effect of these molecules on breast cancer risk. Studies are now being conducted to assess whether breast density varies according to blood levels of vitamin D and of additional growth factors, as well as to genetic polymorphisms involved in the pathways of vitamin D, calcium and growth factors. The increase in vitamin D and calcium intakes may prove to be a safe and inexpensive approach to breast cancer prevention; this possibility should be carefully examined as quickly as possible.