A cohort study of cigarette smoking and risk of fibroadenoma.

BACKGROUND

This cohort study examines the association between cigarette smoking and risk of fibroadenoma.

METHODS

The study was conducted among the 56,537 women in the Canadian National Breast Screening Study (NBSS) who completed self-administered lifestyle and dietary questionnaires. (The NBSS is a randomised, controlled trial of screening for breast cancer in women aged 40-59 at recruitment.) Cases were the 222 women who were diagnosed with biopsy-confirmed incident fibroadenoma. For comparative purposes, a subcohort, consisting of a random sample of 5,681 women was selected from the full dietary cohort. After exclusions for various reasons, the analyses were based on 222 cases and 5,488 non-cases. Incidence rate ratios (IRR) and 95% confidence intervals (CI) were estimated using Poisson regression.

RESULTS

Cigarette smoking was associated with reduced risk of fibroadenoma, the IRR (95% CI) for more than 550 cigarette-years of exposure being 0.66 (0.40-1.10). Although the reduction in risk was evident for all smokers combined (ex-smokers and current smokers), it was indicative largely of an inverse association in current smokers, in whom the IRR (95% CI) for > 300 cigarette-years of exposure was 0.49 (0.24-0.98). The results were mostly the same when examined in strata defined by method of detection of fibroadenoma, menopausal status and history of previous breast disease. There was no evidence for a reduction in risk with time since last use of cigarettes.

CONCLUSIONS

The results of this study suggest that cigarette smoking is associated with reduced risk of fibroadenoma, especially in current smokers.