Table 3.

The Age-adjusted Association Between Cigarette Smoking and the Risk of Developing Asynchronous Contralateral Breast Cancer, the Women's Environmental Cancer and Radiation Epidemiology Study, 1985–2001

	Breast Cancer				Rate Ratioa	95% Confidence Interval
	Unilateral		Asynchronous Contralateral
	No.	%b	No.	%
Ever regularly smoked
No	701	49.9	349	49.3	1.0
Yes	698	50.1	359	50.7	1.1	0.9, 1.6
Ever smoked regularly during at-risk periodc
No	1,108	79.6	542	76.9	1.0
Yes	290	20.4	163	23.1	1.2	0.9, 1.5
Lifetime duration of smoking
Never	701	49.9	349	49.4	1.0
<20 years	323	23.4	150	21.2	1.0	0.8, 1.3
≥20 years	374	26.7	207	29.3	1.1	0.9, 1.5
Ptrendd					0.36
Average smoking amount
Never	701	49.9	349	49.7	1.0
≤½ pack/day	358	25.6	180	25.6	1.0	0.8, 1.3
>½ pack/day	337	24.5	173	24.6	1.1	0.8, 1.4
Ptrendd					0.57
Pack-years of smoking
Never	701	50.0	349	49.9	1.0
<6	235	17.2	94	13.4	0.8	0.6, 1.1
6–<19	232	16.4	132	18.9	1.3	1.0, 1.8
≥19	227	16.4	125	17.9	1.1	0.8, 1.4
Ptrendd					0.27
Starting age
Never	701	49.9	349	49.3	1.0
≥20 years of age	225	16.0	104	14.7	1.0	0.8, 1.4
<20 years of age	473	34.1	255	36.0	1.1	0.9, 1.4

^aAccounting for countermatching and adjusted for age at first diagnosis.

^bProportions are weighted for the countermatching.

^cSmoking during the at-risk period was defined as starting prior to or during the period between the first diagnosis and the reference date and stopping during or after the period between the first diagnosis and the reference date.

^dTest for trend across categories.