Table 1.
Baseline characteristics of cases and controls, and odds ratios for total breast cancer in the NBCSP case-control study
| Characteristic | No. of controls (%) | No. of breast cancer cases (%) | Odds ratioa(95% CI) | P valueb |
|---|---|---|---|---|
| Screening round (baseline exploration) | ||||
| September 1996 to August 1998 | 3,948 (84.1) | 987 (84.1) | ||
| September 1998 to August 2000 | 408 (8.7) | 102 (8.7) | ||
| September 2000 to August 2002 | 224 (4.8) | 56 (4.8) | ||
| September 2002 to August 2004 | 108 (2.3) | 27 (2.3) | ||
| Age at baseline exploration (years) | ||||
| <50 | 1,733 (37.0) | 433 (37.0) | ||
| 50-54 | 986 (21.0) | 245 (20.9) | ||
| 55-59 | 897 (19.1) | 228 (19.5) | ||
| 60-64 | 870 (18.6) | 217 (18.5) | ||
| ≥65 | 202 (4.3) | 49 (4.2) | ||
| Age at menarche (years) | 0.357c | |||
| ≥15 | 724 (15.5) | 161 (13.7) | 0.95 (0.77-1.18) | |
| 14 | 1,243 (26.5) | 290 (24.7) | 1.00 (reference) | |
| 13 | 1,281 (27.3) | 363 (31.0) | 1.22 (1.03-1.45) | |
| 12 | 867 (18.5) | 225 (19.2) | 1.11 (0.92-1.35) | |
| <12 | 569 (12.1) | 132 (11.3) | 1.00 (0.79-1.25) | |
| Unknown | 4 (0.1) | 1 (0.1) | ||
| Age at first live birth (years) | <0.001 | |||
| <20 | 102 (2.2) | 13 (1.1) | 0.51 (0.28-0.91) | |
| 20-24 | 1,441 (30.7) | 293 (25.0) | 0.81 (0.68-0.95) | |
| 25-29 | 1,823 (38.9) | 457 (39.0) | 1.00 (reference) | |
| 30-34 | 522 (11.1) | 134 (11.4) | 1.03 (0.83-1.28) | |
| ≥35 | 175 (3.7) | 57 (4.9) | 1.31 (0.95-1.80) | |
| Nulliparous | 625 (13.3) | 218 (18.6) | 1.41 (1.17-1.70) | |
| Age at menopause (years) | 0.001 | |||
| ≤45 | 777 (16.6) | 147 (12.5) | 1.00 (reference) | |
| 46-50 | 1,244 (26.5) | 298 (25.4) | 1.25 (1.01-1.56) | |
| >50 | 1,035 (22.1) | 293 (25.0) | 1.48 (1.17-1.87) | |
| Premenopausal | 1,631 (34.8) | 434 (37.0) | 1.44 (1.14-1.83) | |
| Unknown | 1 (0.0) | 0 (0.0) | ||
| Family history of breast cancer | <0.001 | |||
| None | 4,075 (86.9) | 945 (80.6) | 1.00 (reference) | |
| Second-degree relative | 327 (7.0) | 109 (9.3) | 1.43 (1.14-1.80) | |
| First-degree relative ≥50 years | 172 (3.7) | 64 (5.5) | 1.61 (1.19-2.16) | |
| First-degree relative <50 years | 114 (2.4) | 54 (4.6) | 2.06 (1.48-2.88) | |
| Previous breast biopsy | <0.001 | |||
| No | 4,281 (91.3) | 1,001 (85.4) | 1.00 (reference) | |
| Yes | 407 (8.7) | 171 (14.6) | 1.82 (1.50-2.22) | |
| Use of hormone replacement therapy | 0.602 | |||
| No | 4,208 (89.8) | 1,046 (89.2) | 1.00 (reference) | |
| Yes | 480 (10.2) | 126 (10.8) | 1.06 (0.86-1.31) | |
| Mammographic density (%) | <0.001c | |||
| 0-10 | 1,711 (36.5) | 263 (22.4) | 1.00 (reference) | |
| 11-25 | 1,136 (24.3) | 244 (20.8) | 1.52 (1.25-1.85) | |
| 26-50 | 1,158 (24.8) | 375 (32.0) | 2.47 (2.05-2.98) | |
| 51-75 | 529 (11.3) | 215 (18.3) | 3.27 (2.62-4.09) | |
| >75 | 135 (2.9) | 59 (5.0) | 3.74 (2.63-5.32) | |
| Unknown | 19 (0.4) | 16 (1.4) |
aOdds ratios for total breast cancer and 95% confidence intervals (CIs) obtained from conditional logistic regression models that accounted for the matching factors (screening round, year of birth, and place of residence). bP values for homogeneity of breast cancer risk across categories, except in the case of age at menarche and mammographic density. cP values for linear trend using an ordinal variable