Table 1.
Distribution of CBC case and unilateral breast cancer controls
| Cases | Controls | P-value* | ||
|---|---|---|---|---|
| Age at (first) diagnosis (%) | ≤45 years | 37 (18) | 37 (18) | |
| 45 to 55 years | 68 (32) | 68 (32) | ||
| 55 to 65 years | 56 (27) | 56 (27) | ||
| ≥65 years | 50 (24) | 50 (24) | - | |
| Calendar period of (first) diagnosis (%) | 1976 to 1980 | 31 (15) | 30 (14) | |
| 1981 to 1985 | 40 (19) | 41 (19) | ||
| 1986 to 1990 | 45 (21) | 41 (19) | ||
| 1991 to 1995 | 51 (24) | 49 (23) | ||
| 1996 to 2005 | 44 (21) | 50 (24) | - | |
| Adjuvant therapy (%)* | No adjuvant therapy | 39 (18) | 39 (18) | |
| Radiotherapy only | 57 (27) | 57 (27) | ||
| Endocrine therapy | 87 (41) | 87 (41) | ||
| Chemotherapy | 28 (13) | 28 (13) | - | |
| Mean follow-up time in years | 8.25 | 8.25 | - | |
| Percent density at (first) diagnosis (%) | ≤5% | 13 (6) | 11 (5) | |
| 5 to 25% | 87 (41) | 87 (41) | ||
| 25 to 50% | 97 (46) | 87 (41) | ||
| ≥50% | 14 (7) | 26 (12) | 0.23 | |
| Quartiles: | ||||
| Dense area at (first) diagnosis (%) | ≤20 cm2 | 55 (26) | 55 (26) | |
| 20 to 34 cm2 | 44 (21) | 56 (27) | ||
| 34 to 53 cm2 | 56 (27) | 49 (23) | ||
| ≥53 cm2 | 56 (27) | 51 (24) | 0.79 | |
| Change in percent density until first | Absolute decrease (>10%) | 40 (19) | 56 (27) | |
| follow-up mammogram (%) | Stable density | 164 (78) | 143 (68) | |
| Absolute increase (>10%) | 7 (3) | 12 (6) | 0.07 | |
| Change in dense area until first | Absolute decrease (>10 cm2) | 61 (29) | 75 (36) | |
| follow-up mammogram | Stable density | 131 (62) | 113 (54) | |
| Absolute increase (>10 cm2) | 19 (9) | 23 (11) | 0.21 | |
| Quartiles: | ||||
| Non-dense area at (first) diagnosis (%) | ≤67 cm2) | 42 (20) | 60 (28) | |
| 67 to 93 cm2 | 58 (27) | 43 (20) | ||
| 93 to 127 cm2 | 50 (24) | 55 (26) | ||
| ≥127 cm2 | 61 (29) | 53 (25) | 0.10 | |
| Mean time until first follow-up mammogram in years (SD) | 1.56 (0.59) |
1.54 (0.57) |
0.56 | |
| Menopause status at diagnosis (%)** | Premenopausal | 89 (42) | 84 (40) | |
| Postmenopausal | 119 (56) | 124 (59) | 0.62 | |
Subdivided by matching variables (age at diagnosis, calendar period of diagnosis, adjuvant therapy and follow-up time), exposure variables (dense area at diagnosis and change of dense area), potential confounding variables (non-dense area at diagnosis and time to first follow-up mammogram), and stratifying variable (menopause status).
* P-value for Chi-square test of association when testing categorical variables and for Student's t-tests when testing the continuous variable (mean time until first follow-up mammogram).
*Endocrine therapy may be with or without radiotherapy, chemotherapy may be with or without radiotherapy and/or endocrine therapy.
** Six patients had uncertain menopause status (for example, hysterectomy).