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Table 2 Baseline characteristics of study population by age

From: Mammographic density and risk of breast cancer by age and tumor characteristics

  Age <55 years Age 55 to 64 years Age ≥65 years
  Cases Controls Cases Controls Cases Controls
N 1,533 3,373 1,021 2,031 860 1,795
Percent mammographic density 40.7 (19.6) 33.6 (20.2) 29.2 (18.0) 23.7 (17.6) 24.8 (17.1) 21.1 (16.2)
Age at mammogram 47.3 (4.5) 47.4 (4.4) 59.4 (2.9) 59.4 (2.9) 71.0 (5.2) 71.2 (5.1)
Age at diagnosis 51.8 (5.6) 63.8 (4.1) 75.0 (5.5)
Body mass index 24.4 (6.5) 25.4 (6.3) 26.1 (7.1) 26.2 (5.6) 25.6 (8.5) 26 (5.6)
Body mass index categories (kg/m2)
   <25 845 (55.1%) 1,902 (56.4%) 438 (42.9%) 964 (47.5%) 322 (37.4%) 849 (47.3%)
   25 to 29 432 (28.2%) 831 (24.6%) 332 (32.5%) 642 (31.6%) 262 (30.5%) 602 (33.5%)
   30 to 34 130 (8.5%) 352 (10.4%) 138 (13.5%) 275 (13.5%) 151 (17.6%) 221 (12.3%)
   35+ 76 (5.0%) 249 (7.4%) 84 (8.2%) 140 (6.9%) 71 (8.3%) 110 (6.1%)
   Unknown 50 (3.3%) 39 (1.2%) 29 (2.8%) 10 (0.5%) 54 (6.3%) 13 (0.7%)
Menopausal status
   Premenopausal 948 (61.8%) 2,163 (64.1%) 16 (1.6%) 44 (2.2%) 0 (0%) 0 (0%)
   Postmenopausal 473 (30.9%) 1,032 (30.6%) 1,001 (98.0%) 1,979 (97.4%) 860 (100%) 1794 (99.9%)
   Unknown 112 (7.3%) 178 (5.3%) 4 (0.4%) 8 (0.4%) 0 (0%) 1 (0.1%)
Parity
   Nulliparous 294 (19.2%) 638 (18.9%) 130 (12.7%) 249 (12.3%) 114 (13.3%) 235 (13.1%)
   Parous 1,145 (74.7%) 2,652 (78.6%) 824 (80.7%) 1,686 (83.0%) 674 (78.4%) 1,428 (79.6%)
   Unknown 94 (6.1%) 83 (2.5%) 67 (6.6%) 96 (4.7%) 72 (8.4%) 132 (7.4%)
Postmenopausal hormone therapya
   Not current user 158 (40.9%) 411 (45.7%) 404 (48.6%) 1030 (58.6%) 361 (57.8%) 1,010 (69.1%)
   Current, estrogen 81 (21.0%) 233 (25.9%) 144 (17.3%) 311 (17.7%) 100 (16.0%) 199 (13.6%)
   Current, estrogen + progestin 126 (32.6%) 222 (24.7%) 215 (25.9%) 308 (17.5%) 84 (13.4%) 124 (8.5%)
   Unknown 21 (5.4%) 34 (3.8%) 68 (8.2%) 108 (6.1%) 80 (12.8%) 129 (8.8%)
Family history
   No 1,207 (78.7%) 2,970 (88.1%) 777 (76.1%) 1,719 (84.6%) 627 (72.9%) 1,450 (80.8%)
   Yes 256 (16.7%) 384 (11.4%) 210 (20.6%) 292 (14.4%) 190 (22.1%) 332 (18.5%)
   Unknown 70 (4.6%) 19 (0.6%) 34 (3.3%) 20 (1.0%) 43 (5.0%) 13 (0.7%)
  1. Data presented as mean (standard deviation) or n (%). aAmong postmenopausal women in the Mayo Mammography Health Study, the Nurses’ Health Study, the Nurses’ Health Study II, and the San Francisco Bay Area Breast Cancer SPORE and San Francisco Mammography Registry.