Article Figures & Data
Tables
- Table 1.
Selected characteristics of the participants (N = 102,098) in the Norwegian-Swedish Cohort Study, 1991-2000
Characteristics Norway Sweden Total Age at enrollment, median (range) 41 (34-50) 40 (30-50) 40 (30-50) Number of women 54,321 47,777 102,098 Person-years of follow-up 493,832 430,936 924,768 Median follow-up time in years 9.6 9.3 9.3 Number of invasive breast cancers* 693 547 1,240 Crude breast cancer incidence rate† 140 127 134 Age-adjusted breast cancer incidence rate‡ 131 127 127 Age at breast cancer diagnosis, median (range) 48 (36-57) 49 (30-58) 48 (30-58) Smoking status at cohort enrollment§ Current (%) 29.3 26.6 28.0 Past (%) 38.8 32.7 36.0 Passive (%)∥ 22.9 24.2 23.5 Never (%) 9.0 16.5 12.5 ↵* Incident cases of breast cancer during follow-up through 2000.
↵† Number of incident cases per 100,000 woman-years.
↵‡ Per 100,000 woman-years, adjusted to World Standard population.
↵§ χ2 test for differences between Sweden and Norway, P < 0.001.
↵∥ Women who never smoked, but lived with smokers during childhood or at cohort enrollment.
- Table 2.
Distribution of selected characteristics of study population (N = 102,098) at cohort enrollment, given as mean (±SD) and (%), by smoking status, Norwegian-Swedish Cohort Study, 1991-2000
Characteristics Current smokers (n = 28,624) Past smokers (n = 36,701) Passive smokers (n = 24,030) Never smokers (n = 12,743) P* Age (y) 40.5 ± 5.0 40.1 ± 4.9† 40.6 ± 5.2‡ 40.1 ± 5.5 <0.001 Age at diagnosis (y) 48.6 ± 5.2 47.4 ± 5.2† 48.0 ± 5.4 47.9 ± 5.5 0.04 Education (y) 11.4 ± 2.9 12.1 ± 3.1† 12.6 ± 3.2‡ 13.2 ± 3.2 <0.001 Age at menarche (y) 13.1 ± 1.4 13.1 ± 1.3† 13.1 ± 1.4 13.1 ± 1.3 0.98 Postmenopausal (%) 7.2 4.2† 4.3‡ 3.6 <0.001 Age at menopause (y) 42.4 ± 4.7 41.9 ± 4.7† 42.8 ± 4.7 43.0 ± 4.3 0.09 Family history of breast cancer (%)§ 4.9 4.8 4.9 4.9 0.41 Parous (%) 87.9 90.0† 88.0‡ 84.9 0.001 Age at first birth (y) 22.9 ± 4.2 24.0 ± 4.4† 24.5 ± 4.2‡ 25.4 ± 4.2 0.001 No. of children 2.0 ± 1.1 2.0 ± 1.1† 2.1 ± 1.1‡ 2.0 ± 1.2 0.001 Ever hormonal contraceptive use (%) 76.7 76.8† 68.3‡ 65.4 <0.001 Teetotalers (%) 15.0 16.7† 25.1‡v 33.0 <0.001 Alcohol consumption (g/d)∥ 4.6 ± 6.5 4.2 ± 5.6† 3.3 ± 4.2‡ 2.9 ± 3.6 <0.001 Mean physical activity score¶ 3.0 ± 0.9 3.1 ± 0.9† 3.1 ± 0.9‡ 3.2 ± 0.9 <0.001 BMI** at enrollment 23.1 ± 3.5 23.2 ± 3.4† 23.5 ± 3.6‡ 23.0 ± 3.3 0.02 ↵* Differences between current and never smoking groups.
↵† Differences between current and past smoking groups (P < 0.001).
↵‡ T-test/χ2 test for differences between passive and never smoking groups (P ≤ 0.01).
↵§ Among mother or sister(s).
↵∥ Among drinkers.
↵¶ Leisure time physical activity in the year preceding cohort enrollment (scored as 1-5, low to high level).
↵** BMI: the weight in kilograms divided by the square of the heights in meters.
- Table 3.
Multivariate RRs with 95% CI of breast cancer according to various measures of smoking exposures among current and ever smokers smoking at least 20 years compared with never smokers (n = 12,743) and the corresponding figures restricted to nondrinkers, with 3,834 never smokers as reference group, Norwegian-Swedish Cohort Study, 1991-2000
Exposure measures Study population Nondrinkers Current smokers (n = 28,624) Ever smokers 20+ y (n = 26,724) Current smokers (n = 4,043) Ever smokers 20+ y (n = 3,778) Multivariate* Multivariate* Multivariate* Multivariate* Cases (n) RR† (95% CI) Cases (n) RR‡ (95% CI) Cases (n) RR§ (95% CI) Cases (n) RR∥ (95% CI) Never smokers 137 1.00 (ref.) 137 1.00 (ref.) 34 1.00 (ref.) 34 1.00 (ref.) Smoking initiation Age (y) 20+ 108 1.02 (0.78-1.35) 84 1.16 (0.86-1.57) 16 1.14 (0.59-2.19) 13 1.35 (0.67-2.71) 15-19 216 1.15 (0.91-1.47) 259 1.23 (0.97-1.55) 26 1.30 (0.72-2.33) 34 1.35 (0.78-2.32) 10-14 36 1.51 (1.00-2.28) 49 1.48 (1.03-2.13) 5 2.46 (0.90-6.68) 7 2.20 (0.92-5.30) P = 0.07¶ P = 0.03¶ P = 0.15¶ P = 0.11¶ Before/around menarche No 282 1.10 (0.88-1.38) 294 1.20 (0.96-1.51) 38 1.26 (0.75-2.12) 41 1.37 (0.82-2.29) Yes 72 1.30 (0.94-1.80) 92 1.39 (1.03-1.87) 7 1.52 (0.63-3.66) 11 1.66 (0.79-3.49) P = 0.13¶ P = 0.03¶ P = 0.28¶ P = 0.13¶ Before first birth* No 91 0.99 (0.72-1.37) 78 0.98 (0.70-1.62) 17 1.36 (0.66-2.79) 12 0.97 (0.44-2.12) Yes 213 1.18 (0.92-1.52) 257 1.27 (1.00-1.62) 26 1.46 (0.81-2.64) 37 1.75 (1.02-2.99) P = 0.14¶ P = 0.03¶ P = 0.20¶ P = 0.03¶ No. of cigarettes per day 1-9 135 0.96 (0.74-1.25) 138 1.10 (0.85-1.43) 23 1.33 (0.74-2.67) 28 1.70 (1.03-3.12) 10+ 225 1.28 (1.01-1.63) 254 1.34 (1.06-1.70) 24 1.25 (0.69-2.26) 26 1.15 (0.65-2.05) P = 0.03¶ P = 0.01¶ P = 0.60¶ P = 0.89¶ No. of years smoked 1-19 68 0.93 (0.68-1.28) - 10 1.07 (0.49-2.32) - 20-24 96 1.09 (0.81-1.45) 158 1.13 (0.88-1.45) 10 0.96 (0.44-2.10) 22 1.32 (0.73-2.38) 25+ 196 1.26 (0.98-1.63) 234 1.36 (1.06-1.74) 27 1.63 (0.90-2.94) 32 1.53 (0.87-2.68) P = 0.05¶ P = 0.02¶ P = 0.15¶ P = 0.13¶ No. of pack-years 0-14 162 0.95 (0.74-1.20) 153 1.01 (0.85-1.40) 27 1.28 (0.73-2.26) 29 1.61 (0.93-2.78) 15-19 90 1.28 (0.96-1.72) 114 1.35 (1.02-1.77) 8 1.11 (0.50-2.49) 12 1.26 (0.63-2.53) 20+ 108 1.48 (1.14-1.96) 125 1.46 (1.11-1.93) 12 1.54 (0.72-3.29) 13 1.22 (0.59-2.53) P = 0.001†† P < 0.02†† P = 0.31†† P = 0.57†† Latency (y) 1-19 48 0.75 (0.52-1.08) 12 0.88 (0.47-1.66) 10 1.33 (0.62-2.87) 2 1.27 (0.29-5.49) 20-24 116 1.20 (0.91-1.58) 125 1.13 (0.86-1.47) 10 0.70 (0.30-1.63) 15 1.02 (0.52-2.02) 25+ 196 1.27 (0.98-1.64) 255 1.37 (1.07-1.76) 27 1.67 (0.93-3.02) 37 1.74 (1.01-3.02) P = 0.02†† P < 0.05†† P = 0.41†† P = 013†† ↵* Adjusted for age at enrollment, menopausal status, number of children, age at first birth, hormonal contraceptive use, BMI (and alcohol consumption for the analysis among all women).
↵† Based upon a model with 446 cases.
↵‡ Based upon a model with 478 cases.
↵§ Based upon a model with 74 cases.
↵∥ Based upon a model with 82 cases.
↵¶ Trend test between levels of smoking categories including never smokers.
** Among parous women.
↵† Trend test between levels of smoking categories excluding never smokers.
- Table 4.
Multivariate RR estimates for breast cancer with 95% CI based upon 478 cases among women who have smoked for at least 20 years compared with never smokers within two (high/low) risk levels of breast cancer risk factors, Norwegian-Swedish Cohort Study, 1991-2000
Confounding variables Cases (n = 478) Multivariate adjusted* (95% CI) P† Age group at enrollment (y) 30-44 253 1.25 (0.93-1.70) 0.82 45-50 225 1.19 (0.86-1.65) Age at menarche (y) <13 145 0.93 (0.63-1.36) 0.08 13+ 333 1.42 (1.08-1.85) Menopausal status‡ Pre 276 1.21 (0.91-1.61) 0.74 Post 198 1.31 (0.92-1.88) Parity No 65 1.18 (0.68-2.06) 0.86 Yes 413 1.25 (0.98-1.58) Age at first birth (y)§ <25 255 1.31 (0.93-1.85) 0.59 25+ 158 1.15 (0.82-1.61) Family history of breast cancer in the mother or sisters No 432 1.23 (0.97-1.55) 0.82 Yes 46 1.35 (0.65-2.79) Hormonal contraceptive use Ever 358 1.21 (0.93-1.56) 0.76 Never 120 1.30 (0.86-1.96) BMI ≤23.9 328 1.30 (0.99-1.70) 0.50 >23.9 150 1.10 (0.75-1.62) Consumption of alcohol Nondrinkers 82 1.43 (0.87-2.33) 0.50 Drinkers 396 1.18 (0.92-1.51) ↵* Adjusted for age at enrollment, menopausal status, number of children, age at first birth, hormonal contraceptive use, and consumption of alcohol where applicable.
↵† Test for heterogeneity between strata Wald χ2 statistics.
↵‡ Missing numbers due to equivocal menopausal status.
↵§ Among parous women.