Table 1. Baseline Characteristics of Study Participants.
| Characteristic | Participants, No. (%) (N = 19 213)a | |||||
|---|---|---|---|---|---|---|
| Total intervention group (n = 9705) | Total control group (n = 9508) | Intervention group by breast density | Control group by breast density | |||
| Dense (n = 5797) | Nondense (n = 3908)b | Dense (n = 5593)b | Nondense (n = 3915)b | |||
| Age, mean (SD), y | 44.5 (2.9) | 44.6 (2.9) | 44.5 (2.9) | 44.7 (2.9) | 44.5 (2.9) | 44.6 (2.9) |
| Ever undergo breast cancer screening | ||||||
| No | 1687 (17.4) | 1895 (19.9) | 995 (17.2) | 692 (17.7) | 1094 (19.6) | 801 (20.5) |
| Yes | 8018 (82.6) | 7613 (80.1) | 4802 (82.8) | 3216 (82.3) | 4499 (80.4) | 3114 (79.5) |
| Time since most recent breast cancer screening, mo | ||||||
| <12 | 716 (8.9) | 681 (9.0) | 441 (9.2) | 275 (8.6) | 424 (9.4) | 257 (8.3) |
| 12-24 | 2942 (36.7) | 2773 (36.4) | 1759 (36.6) | 1183 (36.8) | 1620 (36.0) | 1153 (37.0) |
| 25-36 | 2916 (36.4) | 2806 (36.9) | 1710 (35.6) | 1206 (37.5) | 1667 (37.1) | 1139 (36.6) |
| >36 | 1391 (17.4) | 1308 (17.2) | 859 (17.9) | 532 (16.5) | 762 (16.9) | 546 (17.5) |
| Unknown or data missing | 53 (0.7) | 45 (0.6) | 33 (0.7) | 20 (0.6) | 26 (0.6) | 19 (0.6) |
| Method of most recent breast cancer screening | ||||||
| Mammography | ||||||
| No | 2342 (29.2) | 2191 (28.8) | 1419 (29.6) | 923 (28.7) | 1308 (29.1) | 883 (28.4) |
| Yes | 5676 (70.8) | 5422 (71.2) | 3383 (70.5) | 2293 (71.3) | 3191 (70.9) | 2231 (71.6) |
| Ultrasonography | ||||||
| No | 6406 (79.9) | 6221 (81.7) | 3800 (79.1) | 2606 (81.0) | 3647 (81.1) | 2574 (82.7) |
| Yes | 1612 (20.1) | 1392 (18.3) | 1002 (20.9) | 610 (19.0) | 852 (18.9) | 540 (17.3) |
| Clinical breast examination | ||||||
| No | 356 (4.4) | 332 (4.4) | 215 (4.5) | 141 (4.4) | 194 (4.3) | 138 (4.4) |
| Yes | 7662 (95.6) | 7281 (95.6) | 4587 (95.5) | 3075 (95.6) | 4305 (95.7) | 2976 (95.6) |
| Age at menarche, y | ||||||
| ≤9 | 25 (0.3) | 25 (0.3) | 12 (0.2) | 13 (0.3) | 9 (0.2) | 16 (0.4) |
| 10-15 | 9559 (98.5) | 9383 (98.7) | 5709 (98.5) | 3850 (98.5) | 5517 (98.6) | 3866 (98.8) |
| ≥16 | 121 (1.3) | 100 (1.1) | 76 (1.3) | 45 (1.2) | 67 (1.2) | 33 (0.8) |
| Menopausal status | ||||||
| Premenopausal | 7353 (75.8) | 7285 (76.6) | 4502 (77.7) | 2851 (73.0) | 4399 (78.7) | 2886 (73.7) |
| Perimenopausal | 1699 (17.5) | 1615 (17.0) | 948 (16.4) | 751 (19.2) | 876 (15.7) | 739 (18.9) |
| Postmenopausal | 652 (6.7) | 603 (6.3) | 347 (6.0) | 305 (7.8) | 315 (5.6) | 288 (7.4) |
| Unknown or data missing | 1 (<0.1) | 5 (<0.1) | 0 | 1 (<0.1) | 3 (<0.1) | 2 (<0.1) |
| Pregnancies, No. | ||||||
| 0 | 813 (8.4) | 742 (7.8) | 606 (10.5) | 207 (5.3) | 520 (9.3) | 222 (5.7) |
| 1 | 1192 (12.3) | 1170 (12.3) | 768 (13.3) | 424 (36.5) | 762 (13.6) | 408 (10.4) |
| 2 | 3659 (37.7) | 3603 (37.9) | 2233 (38.5) | 1426 (36.5) | 2154 (38.5) | 1449 (37.0) |
| 3-4 | 3425 (35.3) | 3328 (35.0) | 1875 (38.5) | 1550 (39.7) | 1757 (31.4) | 1571 (40.1) |
| 5-10 | 372 (3.8) | 400 (4.2) | 159 (2.7) | 213 (5.5) | 212 (3.8) | 188 (4.8) |
| Unknown or data missing | 244 (2.5) | 265 (2.8) | 156 (2.7) | 88 (2.3) | 188 (3.4) | 77 (2.0) |
| Pregnancies delivered, No. | ||||||
| Nulliparous | 130 (1.5) | 124 (1.5) | 96 (1.9) | 34 (0.9) | 85 (1.7) | 39 (1.1) |
| 1 | 1427 (16.5) | 1423 (16.7) | 924 (18.4) | 503 (13.9) | 929 (19.0) | 494 (13.7) |
| 2 | 4499 (52.0) | 4448 (52.3) | 2690 (18.4) | 1809 (50.1) | 2625 (53.7) | 1823 (50.4) |
| 3 | 2270 (26.3) | 2202 (25.9) | 1191 (23.7) | 1079 (29.9) | 1113 (22.8) | 1089 (30.1) |
| 4-8 | 291 (3.4) | 257 (3.0) | 118 (2.3) | 173 (4.8) | 105 (2.2) | 152 (4.2) |
| Unknown or data missing | 31 (0.4) | 47 (0.6) | 16 (2.3) | 15 (0.4) | 28 (0.6) | 19 (0.5) |
| Age at first parturition, y | ||||||
| <20 | 102 (1.2) | 80 (0.9) | 58 (1.2) | 44 (1.2) | 44 (0.9) | 36 (1.0) |
| 20-24 | 1626 (18.8) | 1470 (17.3) | 897 (17.8) | 729 (20.2) | 783 (16.0) | 687 (19.0) |
| 25-29 | 3245 (37.5) | 3184 (37.5) | 1901 (37.8) | 1344 (37.2) | 1811 (37.1) | 1373 (38.0) |
| 30-39 | 1882 (21.8) | 2022 (23.8) | 1132 (22.5) | 750 (20.8) | 1239 (25.4) | 783 (21.7) |
| 40-49 | 48 (0.6) | 47 (0.6) | 36 (0.7) | 12 (0.3) | 27 (0.6) | 20 (0.6) |
| Unknown or data missing | 1745 (20.2) | 1698 (20.0) | 1011 (20.1) | 734 (20.3) | 981 (20.1) | 717 (19.8) |
| Ever breastfed children | ||||||
| Yes | 7754 (90.9) | 7570 (90.4) | 4507 (91.1) | 3247 (90.6) | 4338 (90.4) | 3232 (90.4) |
| No | 758 (8.9) | 792 (9.5) | 428 (8.7) | 330 (9.2) | 455 (9.5) | 337 (9.4) |
| Unknown or data missing | 21 (0.3) | 16 (0.2) | 15 (0.3) | 6 (0.2) | 8 (0.2) | 8 (0.2) |
| First-degree female relatives with breast cancer, No. | ||||||
| 0 | 9214 (94.9) | 9024 (94.9) | 5503 (94.9) | 3711 (95.0) | 5299 (94.7) | 3725 (95.2) |
| 1 | 485 (5.0) | 477 (5.0) | 291 (5.0) | 194 (5.0) | 289 (5.2) | 188 (4.8) |
| >1 | 6 (<0.1) | 7 (<0.1) | 3 (<0.1) | 3 (<0.1) | 5 (<0.1) | 2 (<0.1) |
| Ever had breast surgery | 227 (1.2) | 214 (1.1) | 170 (1.8) | 57 (0.6) | 140 (1.5) | 74 (0.8) |
| Ever had benign neoplasm | 154 (0.8) | 143 (0.7) | 121 (1.3) | 33 (0.3) | 97 (1.0) | 46 (0.5) |
| Ever had breast inflammation | 71 (0.4) | 70 (0.4) | 46 (0.5) | 25 (0.3) | 45 (0.5) | 25 (0.3) |
a
Percentages might not total 100% because of rounding.
b
The 2 least dense categories (almost entirely fatty and scattered fibroglandular tissues) are referred to as nondense, conventionally, the 2 most dense categories (heterogeneously dense and extremely dense) are referred to as dense.