Table 3.

Association between smoking and risk of breast cancer among BRCA mutation carriers

	Person-years (Total 21,098)	Incident cases (N = 420)	Crude HR (95% CI)	Adjusted HR* (95% CI)
Smoking history
Never	11,990	217	Reference	Reference
Ever	9,108	203	1.22 (1.01–1.48)	1.16 (0.96–1.41)
Current smoking history
Past smoker	4,835	104	1.16 (0.92–1.47)	1.04 (0.82–1.32)
Current smoker	4,250	95	1.24 (0.98–1.58)	1.28 (1.00–1.64)
		P for trend =	0.06	0.07
Age at first smoking
≥ 20 years	3,165	75	1.29 (0.99–1.67)	1.15 (0.88–1.51)
18–19 years	2,584	52	1.10 (0.82–1.49)	1.08 (0.79–1.47)
≤ 17 years	3,223	65	1.11 (0.84–1.45)	1.12 (0.84–1.48)
		P for trend =	0.34	0.39
Average packs smoked/week
<2 packs	2,413	47	1.06 (0.77–1.45)	1.04 (0.76–1.43)
2 ≤ packs < 5	3,086	59	1.05 (0.79–1.41)	1.04 (0.77–1.39)
5 ≤ packs	2,783	67	1.32 (1.01–1.74)	1.22 (0.92–1.61)
		P for trend =	0.08	0.22
Smoking duration
≤ 8 years	3,050	39	0.71 (0.50–0.99)	0.76 (0.54–1.07)
8 < duration ≤ 18	2,750	60	1.20 (0.90–1.60)	1.21 (0.90–1.61)
18 < duration	3,172	93	1.59 (1.24–2.02)	1.34 (1.04–1.73)
		P for trend =	< 0.01	0.02
Total pack-years
≤ 2.3 pack-years	2,800	43	0.84 (0.61–1.17)	0.89 (0.64–1.24)
2.3< pack-years ≤ 9.8	2,680	51	1.05 (0.77–1.42)	1.01 (0.74–1.38)
> 9.8 pack-years	2,720	76	1.53 (1.18–1.98)	1.33 (1.02–1.75)
		P for trend =	0.01	0.08
Duration of smoking before the first birth among parous women
Never smoker	7,905	168	Reference	Reference
Initiation after first birth	952	27	1.33 (0.88–1.99)	1.29 (0.85–1.96)
≤ 5 years	2,606	48	0.86 (0.63–1.19)	0.86 (0.62–1.19)
> 5 years	2,858	69	1.14 (0.86–1.50)	1.12 (0.84–1.50)
		P for trend =	0.69	0.77

^*adjusted for age, country, BRCA mutation, pregnancy history, contraceptive drug use, alcohol drinking and obesity using time-dependent Cox regression model