Open Access Open Badges Research article

Mammographic density and breast cancer: a comparison of related and unrelated controls in the Breast Cancer Family Registry

Linda Linton1, Lisa J Martin1, Qing Li1, Ella Huszti1, Salomon Minkin1, Esther M John2, Johanna Rommens3, Andrew D Paterson4 and Norman F Boyd1*

Author Affiliations

1 Campbell Family Institute for Breast Cancer Research, Ontario Cancer Institute, 610 University Avenue, Toronto, ON M5G 2M9, Canada

2 Cancer Prevention Institute of California, 2201 Walnut Avenue, Fremont, CA 94538, USA, and Division of Epidemiology, Department of Health Research and Policy, Stanford University School of Medicine,150 Governor's Lane, Stanford, CA 94305, USA, and Stanford Cancer Institute, 265 Campus Drive, Stanford, CA 94305, USA

3 Program in Genetics and Genomic Biology, The Hospital for Sick Children, 555 University Avenue, Toronto, ON M5G 1X8, Canada

4 Dalla Lana School of Public Health, University of Toronto, 155 College Street, Toronto, ON M5T 3M7, Canada

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Breast Cancer Research 2013, 15:R43  doi:10.1186/bcr3430

Published: 25 May 2013



Percent mammographic density (PMD) is a strong and highly heritable risk factor for breast cancer. Studies of the role of PMD in familial breast cancer may require controls, such as the sisters of cases, selected from the same 'risk set' as the cases. The use of sister controls would allow control for factors that have been shown to influence risk of breast cancer such as race/ethnicity, socioeconomic status and a family history of breast cancer, but may introduce 'overmatching' and attenuate case-control differences in PMD.


To examine the potential effects of using sister controls rather than unrelated controls in a case-control study, we examined PMD in triplets, each comprised of a case with invasive breast cancer, an unaffected full sister control, and an unaffected unrelated control. Both controls were matched to cases on age at mammogram. Total breast area and dense area in the mammogram were measured in the unaffected breast of cases and a randomly selected breast in controls, and the non-dense area and PMD calculated from these measurements.


The mean difference in PMD between cases and controls, and the standard deviation (SD) of the difference, were slightly less for sister controls (4.2% (SD = 20.0)) than for unrelated controls (4.9% (SD = 25.7)). We found statistically significant correlations in PMD between cases (n = 228) and sister controls (n = 228) (r = 0.39 (95% CI: 0.28, 0.50; P <0.0001)), but not between cases and unrelated controls (n = 228) (r = 0.04 (95% CI: -0.09, 0.17; P = 0.51)). After adjusting for other risk factors, square root transformed PMD was associated with an increased risk of breast cancer when comparing cases to sister controls (adjusted odds ratio (inter-quintile odds ratio (IQOR) = 2.19, 95% CI = 1.20, 4.00) or to unrelated controls (adjusted IQOR = 2.62, 95% CI = 1.62, 4.25).


The use of sister controls in case-control studies of PMD resulted in a modest attenuation of case-control differences and risk estimates, but showed a statistically significant association with risk and allowed control for race/ethnicity, socioeconomic status and family history.

Mammographic density; case-control study; overmatching; case control