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Table 3 Relative risk of breast cancer in relation to smoking in relation to age at start of smoking, by oestrogen receptor status

From: Smoking and risk of breast cancer in the Generations Study cohort

  Oestrogen receptor status All breast cancera
Positive Negative  
Cases HRb 95% CI P value Cases HRb 95% CI P value Cases HRb 95% CI P value
Age started smoking, yearsc
 Never smoked 898 1.00 Baseline   167 1.00 Baseline   1073 1.00 Baseline  
  < 17 218 1.25 1.08–1.46 0.0032 42 1.22 0.87–1.72 0.25 261 1.24 1.08–1.43 0.0023
 17–19 249 1.13 0.98–1.30 0.093 54 1.32 0.97–1.80 0.077 304 1.15 1.01–1.31 0.030
 20+ 122 0.95 0.79–1.15 0.62 28 1.25 0.84–1.87 0.28 151 1.00 0.84–1.18 0.96
 Age unknown 22 0.94 0.62–1.44 0.79 3 0.73 0.23–2.30 0.59 26 0.94 0.64–1.39 0.76
  Trendd P = 0.17 Trendd P = 0.74 Trend among ever smokersd P = 0.18
Trend interaction P = 0.30  
Cigarettes per day at ages 16–24 years (among those aged ≥ 25 years at entry)
 Never smoked 895 1.00 Baseline   166 1.00 Baseline   1069 1.00 Baseline  
 1–4 77 1.10 0.87–1.39 0.42 21 1.65 1.05–2.60 0.032 99 1.19 0.97–1.46 0.10
 5–9 131 1.23 1.02–1.48 0.027 18 0.92 0.57–1.50 0.75 149 1.16 0.99–1.40 0.069
 10–14 170 1.11 0.94–1.31 0.22 31 1.07 0.73–1.57 0.73 203 1.10 0.95–1.29 0.21
 15+ 169 1.13 0.96–1.34 0.14 42 1.44 1.02–2.03 0.038 212 1.18 1.01–1.37 0.034
 Started at age 25+ 31 0.91 0.64–1.31 0.62 7 1.22 0.57–2.60 0.61 38 0.95 0.69–1.31 0.75
 Unknown 31 0.97 0.68–1.39 0.87 7 1.32 0.62–2.79 0.47 38 1.01 0.73–1.40 0.95
  Trendd P = 0.70 Trendd P = 0.65 Trendd P = 0.78
Trend interaction P = 0.81  
Starting smoking relative to age at menarche, years
 Never smoked 898 1.00 Baseline   167 1.00 Baseline   1073 1.00 Baseline  
 At or before menarche 32 1.60 1.12–2.28 0.010 2 0.50 0.12–2.02 0.33 34 1.40 0.98–1.99 0.061
Years started after menarche
  1–4 217 1.21 1.04–1.41 0.015 48 1.38 1.00–1.91 0.049 266 1.23 1.07–1.41 0.0040
  5–9 239 1.09 0.94–1.27 0.23 54 1.35 0.99–1.84 0.060 295 1.13 0.99–1.29 0.071
  10–14 36 1.03 0.74–1.44 0.86 7 1.13 0.53–2.43 0.75 43 1.04 0.76–1.41 0.82
  15+ 12 0.64 0.36–1.13 0.12 6 1.92 0.85–4.34 0.12 18 0.81 0.51–1.30 0.39
  Interval unknown 75 1.02 0.79–1.32 0.88 10 0.74 0.38–1.41 0.35 86 0.99 0.77–1.27 0.92
  Trendd P = 0.041 Trendd P = 0.80 Trendd P = 0.031
Trend for interaction P = 0.42  
Starting smoking relative to age at thelarche, years
 Never smoked 898 1.00 Baseline   167 1.00 Baseline   1073 1.00 Baseline  
 At or before thelarche 16 1.45 0.88–2.39 0.14 1 0.45 0.06–3.24 0.43 17 1.27 0.79–2.06 0.33
Years started after thelarche
  1–4 160 1.17 0.99–1.39 0.075 31 1.17 0.79–1.72 0.43 192 1.17 1.00–1.37 0.056
  5–9 238 1.06 0.92–1.22 0.47 60 1.42 1.06–1.91 0.020 300 1.11 0.98–1.27 0.10
  10–14 37 0.98 0.70–1.36 0.90 11 1.60 0.87–2.95 0.13 48 1.07 0.80–1.43 0.66
  15+ 18 0.94 0.59–1.50 0.79 6 1.83 0.81–4.14 0.14 24 1.06 0.71–1.59 0.77
Interval unknown 142 1.22 1.02–1.46 0.031 18 0.87 0.53–1.42 0.59 161 1.17 0.99–1.38 0.074
  Trendd P = 0.58 Trendd P = 0.35 Trend among ever smokersd P = 0.82
Trend for interaction P = 0.28  
Starting smoking relative to first childbirth, among parous (follow-up time only from first childbirth)
 Never smoked 772 1.00 Baseline   148 1.00 Baseline   927 1.00 Baseline  
 After first childbirth 34 1.20 0.85–1.71 0.30 9 1.85 0.94–3.67 0.076 43 1.28 0.94–1.76 0.12
 Interval from starting smoking to first childbirth, years
  1–4 73 1.12 0.88–1.44 0.35 15 1.28 0.74–2.21 0.38 89 1.14 0.91–1.45 0.23
  5–9 177 1.06 0.90–1.25 0.51 42 1.35 0.96–1.90 0.089 220 1.10 0.94–1.28 0.22
  10–14 130 1.08 0.89–1.30 0.42 22 0.89 0.57–1.40 0.62 152 1.04 0.87–1.24 0.65
  15+ 77 1.17 0.91–1.51 0.21 16 1.13 0.66–1.92 0.65 94 1.17 0.93–1.47 0.19
  Interval unknown 22 1.07 0.70–1.64 0.75 2 0.54 0.13–2.17 0.38 25 1.02 0.69–1.52 0.92
  Trendd P = 0.72 Trendd P = 0.33 Trend among ever smokersd P = 0.97
Trend for interaction P = 0.27  
  1. aIncludes 21 breast cancers with unknown or unmeasured oestrogen receptor status
  2. bAdjusted for attained age (Cox regression time scale); time since recruitment to cohort (0, 1–2, 3+ years); birth cohort (1908–1939, 1940–1949, 1950–1959, 1960–1969, 1970–1996); benign breast disease (yes/no); family history of breast cancer in first-degree relatives (yes/no); socio-economic score (Acorn score as trend, missing); age at menarche (trend, missing); age at first pregnancy (trend, missing); parity (trend, missing); duration of breastfeeding (trend, missing); current oral contraceptive use before menopause (yes/no); alcohol consumption (never regular, trend current drinker 1 to < 60 g/day, current drinker ≥ 60 g/day, past drinker, drinker with unknown details); physical activity [log(metabolic equivalent) trend, missing]; pre-menopausal body mass index at age 20 years (trend, missing); post-menopausal body mass index (trend, missing); menopausal hormone therapy use (never used, ex-user, current oestrogen-only user, current oestrogen plus progestogen user, current user of other types, missing); menopausal status (pre- or post-menopausal) and age at menopause (trend, missing)
  3. cTime updated through follow-up period
  4. dTrend excludes never smoker and unknown group