Cigarette smoking and postmenopausal breast cancer risk in a prospective cohort.

BACKGROUND

The relationship between cigarette smoking and breast cancer risk has been inconsistent, potentially due to modification by other factors or confounding.

METHODS

We examined smoking and breast cancer risk in a prospective cohort of 186 150 female AARP (formerly American Association of Retired Persons) members, ages 50-71 years, who joined the study in 1995-96 by responding to a questionnaire. Through 2006, 7481 breast cancers were diagnosed. Multivariable-adjusted hazard ratios (HRs) were estimated, overall and stratified by breast cancer risk factors, using Cox proportional hazards regression. Multiplicative interactions were evaluated using the likelihood ratio test.

RESULTS

Increased breast cancer risk was associated with current (HR 1.19, 95% confidence interval (CI) 1.10-1.28) and former (HR 1.07, CI 1.01-1.13) smoking. The current smoking association was stronger among women without (HR 1.24, CI 1.15-1.35) as compared to those with a family history of breast cancer (HR 0.94, CI 0.78-1.13) (P-interaction=0.03). The current smoking association was also stronger among those with later (≥ 15 years: HR 1.52, CI 1.20-1.94) as compared with earlier (≥12 years: HR 1.14, CI 1.03-1.27; 13-14 years: HR 1.18, CI 1.05-1.32) ages at menarche (P-interaction=0.03).

CONCLUSIONS

Risk was elevated in smokers, particularly in those without a family history or late menarche. Research into smoking's effects on the genome and breast development may clarify these relationships.