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 | |||
|---|---|---|---|---|---|---|
| Â | 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%) |