Obesity has long been associated with an increased risk and severity of breast cancer in postmenopausal women. New research now shows that this risk is not the result of differences in the use or accuracy of mammography screening but instead may actually be the result of biologic differences in tumor development. The results of this study were published in the December 3, 2008 issue of the Journal of the National Cancer Institute.
Approximately 200,000 women are diagnosed with breast cancer every year in the United States alone. The majority of breast cancers are diagnosed among postmenopausal women. Previous research has indicated that postmenopausal women who are overweight or obese are at an increased risk of developing breast cancer. Furthermore, being overweight or obese has been associated with larger tumor size and more advanced disease at diagnosis.
Researchers have been uncertain about the reasons for the association between obesity and increased breast cancer risk, especially because obese women are more likely to be diagnosed with advanced disease. Could this mean that obese women are less likely to be screened for breast cancer and therefore the cancer has more time to develop into advanced-stage disease prior to diagnosis? Or could the association be related to the accuracy of mammography? Obese women are more likely than normal weight women to have fatty breast tissue, which increases the visibility of tumors on mammography. However, obese women also have larger breasts and it can be difficult to get accurate images of the entire breast, which could result in missed cancers. Finally, could the increase in breast cancer incidence in this population be the result of biologic differences?
In order to evaluate this association, researchers pooled data from seven mammography registries associated with the Breast Cancer Surveillance Consortium. The data was collected from 1996-2005 and included 287,115 postmenopausal women not using hormone therapy (HT) who underwent 614,562 mammograms. The women were divided into five standard BMI categories: underweight, normal weight, overweight, obese I, obese II/III. The rates of overall breast cancer increased across the BMI groups, as did the rates of advanced disease, advanced-stage, and high nuclear grade. The rate of larger, more advanced cancers increased between 1.3- to 1.8-fold with each higher BMI category. The rates increased across BMI groups, regardless of mammography use.
The researchers noted that overweight and obese postmenopausal women not using HT had higher rates of breast cancer and advanced-stage disease than normal weight women, regardless of mammography use. They concluded that “patterns of mammography use and mammography accuracy are not the primary reasons for higher rates of advanced breast cancer among overweight and obese postmenopausal women not using HT; thus, biologic differences in breast tumor development and/or progression may be important.”
Research will likely be ongoing to examine the relationship between BMI and cancer risk. Body weight is one modifiable factor that women can change in order to decrease their risk of developing breast cancer.
 Kerlikowske K, Walker R, Miglioretti DL, et al. Obesity, mammography use and accuracy, and advanced breast cancer risk. Journal of the National Cancer Institute. 2008; 100: 1724-1733.
 Eliassen A, Colditz G, Rosnter B, et al. Adult weight change and risk of postmenopausal breast cancer. Journal of the American Medical Association. 2006;296:193-201.
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