From: Smoking and risk of breast cancer in the Generations Study cohort
 | Family history of breast cancera | |||||||
---|---|---|---|---|---|---|---|---|
No | Yes | |||||||
Cases | HRb | 95% CI | P value | Cases | HRb | 95% CI | P value | |
Ever previously smoked cigarettesc | ||||||||
 Never | 819 | 1.00 | Baseline |  | 239 | 1.00 | Baseline |  |
 Ever | 554 | 1.07 | 0.96–1.20 | 0.22 | 203 | 1.35 | 1.12–1.62 | 0.0018 |
 | Interaction P = 0.038 | |||||||
Age at start of smoking, yearsc | ||||||||
 Never smoker | 819 | 1.00 | Baseline |  | 254 | 1.00 | Baseline |  |
  < 20 | 427 | 1.17 | 1.04–1.32 | 0.0098 | 138 | 1.26 | 1.02–1.56 | 0.029 |
 20+ | 96 | 0.82 | 0.67–1.02 | 0.076 | 55 | 1.56 | 1.17–2.10 | 0.0028 |
 Age unknown | 16 | 0.75 | 0.46–1.23 | 0.26 | 10 | 1.57 | 0.84–2.96 | 0.16 |
 | Interaction P = 0.0029 | |||||||
 | Trendd P = 0.034 |  | Trendd P = 0.38 | |||||
Trend interaction P = 0.044 | ||||||||
Starting smoking relative to age at menarche, years | ||||||||
 Never smoker | 819 | 1.00 | Baseline |  | 254 | 1.00 | Baseline |  |
 Before or < 5 years | 239 | 1.30 | 1.12–1.51 | 0.0006 | 61 | 1.06 | 0.80–1.41 | 0.68 |
 5+ years | 244 | 0.97 | 0.84–1.13 | 0.70 | 112 | 1.53 | 1.22–1.91 | 0.0002 |
 Interval unknown | 56 | 0.83 | 0.62–1.12 | 0.22 | 30 | 1.51 | 1.02–2.24 | 0.041 |
 | Interaction P = 0.0001 | |||||||
 | Trendd P = 0.0021 |  | Trendd P = 0.40 | |||||
Trend interaction P = 0.0079 | ||||||||
Cigarettes per day age among ever smokers, averaged over years when smokinge | ||||||||
 Never smoker | 819 | 1.00 | Baseline |  | 254 | 1.00 | Baseline |  |
 1–4 | 185 | 1.01 | 0.86–1.18 | 0.95 | 73 | 1.32 | 1.01–1.71 | 0.040 |
 5+ | 210 | 1.28 | 1.09–1.49 | 0.0019 | 69 | 1.42 | 1.09–1.86 | 0.010 |
 Unknown | 144 | 0.94 | 0.79–1.13 | 0.51 | 61 | 1.31 | 0.99–1.74 | 0.059 |
 | Interaction P = 0.13 | |||||||
 | Trendd P = 0.0019 |  | Trendd P = 0.81 | |||||
Trend interaction P = 0.18 | ||||||||
Duration of smoking among ever smokers, yearsf | ||||||||
 Never smoker | 819 | 1.00 | Baseline |  | 254 | 1.00 | Baseline |  |
 1–9 | 129 | 0.94 | 0.78–1.14 | 0.54 | 48 | 1.20 | 0.88–1.64 | 0.24 |
 10+ | 385 | 1.16 | 1.03–1.32 | 0.019 | 140 | 1.39 | 1.13–1.71 | 0.0020 |
 Duration unknown | 25 | 0.76 | 0.51–1.13 | 0.17 | 15 | 1.51 | 0.90–2.55 | 0.12 |
 | Interaction P = 0.094 | |||||||
 | Trendd P = 0.15 |  | Trendd P = 0.86 | |||||
Trend interaction P = 0.37 | ||||||||
Pack-years of smoking among ever smokerse | ||||||||
 Never smoker | 819 | 1.00 | Baseline |  | 254 | 1.00 | Baseline |  |
 1 to < 10 | 208 | 1.02 | 0.87–1.18 | 0.84 | 77 | 1.28 | 0.99–1.65 | 0.060 |
 10+ | 187 | 1.30 | 1.11–1.53 | 0.0015 | 65 | 1.48 | 1.13–1.95 | 0.0051 |
 Unknown | 144 | 0.93 | 0.78–1.12 | 0.46 | 61 | 1.30 | 0.98–1.72 | 0.069 |
 | Interaction P = 0.16 | |||||||
 | Trendd P = 0.055 |  | Trendd P = 0.54 | |||||
Trend interaction P = 0.37 |