Table 1.
Characteristics of women with contralateral breast cancer (CBC) (cases) and unilateral breast cancer (UBC) controls
| CBC (n = 362) | UBC (n = 403) | |
|---|---|---|
| Mean (SD) | Mean (SD) | |
| Age at mammogram before/at 1st diagnosis (years) | 46 (6) | 46 (6) |
| Age at mammogram after 1st diagnosis (years) | 47 (6) | 47 (6) |
| Follow-up timea (years) | 7.9 (3.8) | 8.0 (4.0) |
| BMI at 1st diagnosis (kg/m2) | 25.2 (5.5) | 25.2 (5.7) |
| BMIb at mammogram after 1st diagnosis (kg/m2) | 25.2 (5.4) | 25.8 (5.7) |
| Percent mammographic density before/at 1st diagnosis | 37.6 (18.1) | 35.8 (18.3) |
| Percent mammographic density after 1st diagnosis | 31.4 (18.3) | 28.0 (16.7) |
| N (%) | N (%) | |
| Study center | ||
| Northern California | 179 (49) | 213 (53) |
| Seattle | 82 (23) | 80 (20) |
| Ontario | 52 (14) | 64 (16) |
| Iowa | 49 (14) | 46 (11) |
| Race | ||
| Non-Hispanic white | 295 (83) | 334 (81) |
| Other | 67 (17) | 69 (19) |
| Histologic type at 1st diagnosis | ||
| Non-lobular | 321 (89) | 353 (88) |
| Lobular | 41 (11) | 50 (12) |
| Stage at 1st diagnosis | ||
| Local | 251 (69) | 254 (63) |
| Regional | 106 (29) | 143 (35) |
| Missing | 5 (1) | 6 (1) |
| Estrogen receptor at 1st diagnosis | ||
| Positive | 213 (59) | 273 (68) |
| Negative | 129 (36) | 105 (26) |
| Missing | 20 (6) | 25 (6) |
| Chemotherapy at 1st diagnosis | ||
| Yes | 236 (65) | 272 (67) |
| No | 126 (35) | 131 (33) |
| Radiation at 1st diagnosis | ||
| Yes | 251 (69) | 179 (69) |
| No | 111 (31) | 124 (31) |
| Tamoxifen at 1st diagnosis | ||
| Yes | 158 (44) | 221 (55) |
| No | 204 (56) | 182 (45) |
| Family historyc of breast cancer | ||
| Yes | 123 (34) | 89 (22) |
| No | 234 (65) | 306 (76) |
| Adopted/missing | 5 (1) | 8 (2) |
| Age at menarche (years) | ||
| <13 | 194 (54) | 193 (48) |
| ≥13 | 167 (46) | 209 (52) |
| Missing | 1 (< 1) | 1 (< 1) |
| Full-term pregnancies | ||
| None | 101 (28) | 95 (24) |
| 1–3 | 241 (67) | 278 (69) |
| ≥4 | 20 (6) | 30 (7) |
SD standard deviation, BMI body mass index
aTime interval between first diagnosis and reference date defined as time of second diagnosis in CBC cases and time of second diagnosis in matched case for UBC controls
bBMI estimated from reported BMI at first diagnosis and reported BMI at reference date using interpolation
cFirst-degree family history