Associations of oral contraceptives with mammographic breast density in premenopausal women.

BACKGROUND We investigated the associations of oral contraceptives (OCs) with percent breast density (PD), absolute dense area (DA), non-dense area (NDA), and a novel image intensity variation (V) measure in premenopausal women. METHODS This study included 1,233 controls from a nested case-control study within Nurses' Health Study II cohort. Information on OCs was collected in 1989 and updated biennially. OC use was defined from the questionnaire closest to the mammogram date. PD, DA, and NDA were measured from digitized film mammograms using a computer-assisted thresholding technique; the V measure was obtained with a previously developed algorithm measuring the standard deviation of pixel values in the eroded breast region. Generalized linear regression was used to assess associations between OCs and density measures (square root-transformed PD, DA, and NDA, and untransformed V). RESULTS OC use was not associated with PD (current vs. never: β=-0.06, 95%CI -0.37,0.24; past vs. never: β=0.10, 95%CI -0.09,0.29), DA (current vs. never: β=-0.20, 95%CI -0.59,0.18; past vs. never: β=0.13, 95%CI -0.12,0.39), and NDA (current vs. never: β=-0.19, 95%CI -0.56,0.18; past vs. never: β=-0.01, 95%CI -0.28,0.25). Women with younger age at initiation had significantly greater V-measure (<20 years vs. never β=26.88, 95%CI 3.18,50.58; 20-24 years vs. never β=20.23, 95%CI -4.24,44.71; 25-29 years vs. never β=2.61, 95%CI -29.00,34.23; {greater than or equal to}30 years vs. never β=0.28, 95%CI -34.16,34.72, p-trend=0.03). CONCLUSION Our findings suggest that an earlier age at first OC use was associated with significantly greater V. IMPACT These findings could guide decisions about the age for OC initiation.