Breast Cancer Risk Among Women Who Start Smoking as Teenagers

Inger T. Gram; Tonje Braaten; Paul D. Terry; Annie J. Sasco; Hans-Olov Adami; Eiliv Lund; Elisabete Weiderpass

DOI: Published January 2005

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
Age group at enrollment (y)		45-50	225	1.19 (0.86-1.65)
Age at menarche (y)	<13	145	0.93 (0.63-1.36)	0.08
Age at menarche (y)		13+	333	1.42 (1.08-1.85)
Menopausal status^‡	Pre	276	1.21 (0.91-1.61)	0.74
Menopausal status^‡		Post	198	1.31 (0.92-1.88)
Parity	No	65	1.18 (0.68-2.06)	0.86
Parity		Yes	413	1.25 (0.98-1.58)
Age at first birth (y)^§	<25	255	1.31 (0.93-1.85)	0.59
Age at first birth (y)^§		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
Family history of breast cancer in the mother or sisters		Yes	46	1.35 (0.65-2.79)
Hormonal contraceptive use	Ever	358	1.21 (0.93-1.56)	0.76
Hormonal contraceptive use		Never	120	1.30 (0.86-1.96)
BMI	≤23.9	328	1.30 (0.99-1.70)	0.50
BMI		>23.9	150	1.10 (0.75-1.62)
Consumption of alcohol	Nondrinkers	82	1.43 (0.87-2.33)	0.50
Consumption of alcohol		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.