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Does Breast Size Modify the Association between Mammographic Density and Breast Cancer Risk?

¹ Department of Nutrition, University of Oslo, Oslo, Norway; ² Institute for Health Promotion and Disease Prevention Research, University of Southern California, Alhambra, California; and ³ Department of Preventive Medicine, Keck School of Medicine, University of Southern California/Norris Comprehensive Cancer Center, Los Angeles, California

Requests for reprints: Giske Ursin, Department of Preventive Medicine, Keck School of Medicine, University of Southern California/Norris Comprehensive Cancer Center, 1441 Eastlake Avenue 4407, Los Angeles, CA 90089. Phone: 323-865-0423; Fax: 323-865-0142. E-mail: gursin{at}usc.edu

Background: Both the absolute and the percent of mammographic density are strong and independent risk factors for breast cancer. Previously, we showed that the association between mammographic density and breast cancer risk tended to be weaker in African American than in White U.S. women. Because African American women have a larger breast size, we assessed whether the association between mammographic density and breast cancer was less apparent in large than in small breasts.

Methods: We assessed mammographic density on mammograms from 348 African American and 507 White women, 479 breast cancer patients and 376 control subjects, from a case-control study conducted in Los Angeles County. We estimated odds ratios (OR) for breast cancer with increasing mammographic density, and the analyses were stratified by mammographic breast area.

Results: Median breast size was 168.4 cm² in African American women and 121.7 cm² in White women (P for difference <0.001). For absolute density, adjusted ORs (95% confidence intervals) per increase of 10 cm² were 1.32 (1.13-1.54), 1.14 (1.03-1.26), and 1.02 (0.98-1.07) in the first, second, and third tertiles of breast area, respectively (P for effect modification by breast area = 0.005). The results for percent density were similar although weaker; adjusted ORs per 10% increase (absolute value) in percent density were 1.22 (1.05-1.40), 1.22 (1.06-1.41), and 1.03 (0.90-1.18 P for effect modification by breast area = 0.34).

Conclusion: Our results indicate that the association between mammographic density and breast cancer may be weaker in women with larger breasts. (Cancer Epidemiol Biomarkers Prev 2008;17(3):621–7)