Abstract
Objective: To examine the effect of smoking on breast cancer risk in a large population-based cohort of women, many of whom started smoking as teenagers.
Methods: We followed 102,098 women, ages 30 to 50 years, completing a mailed questionnaire at recruitment to the Norwegian-Swedish Cohort Study in 1991/1992, through December 2000. We used Cox proportional hazard regression models to estimate relative risk (RR) of breast cancer associated with different measures of smoking initiation, duration, and intensity adjusting for confounding variables. We conducted analyses on the entire study population, among women who had smoked for at least 20 years, among nondrinkers, and separately for each country.
Results: Altogether, 1,240 women were diagnosed with incident, invasive breast cancer. Compared with never smokers, women who smoked for at least 20 years and who smoked 10 cigarettes or more daily had a RR of 1.34 (95% CI, 1.06-1.70). Likewise, those who initiated smoking prior to their first birth (1.27, 1.00-1.62), before menarche (1.39, 1.03-1.87), or before age 15 (1.48, 1.03-2.13) had an increased risk. In contrast, women who had smoked for at least 20 years, but started after their first birth, did not experience an increased breast cancer risk. The increased RR associated with smoking was observed among nondrinkers of alcohol, women with and without a family history of breast cancer, premenopausal and postmenopausal women, and in both countries.
Conclusion: Our results support the notion that women who start smoking as teenagers and continue to smoke for at least 20 years may increase their breast cancer risk.
- Smoking
- breast cancer
- incidence
- cohort
- teenager
- age at first birth
- alcohol
- risk factors
- epidemiology
- women
- Norway
- Sweden
Footnotes
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Grant support: In Norway, the study was supported by grants from the National CancerInstitute of the USA (grant CA 52449), the Norwegian Cancer Society (grant DNK 90050),the Aakre Foundation, and the Norwegian Research Council (Visiting Scientist Award grant 148365/300) for Dr. Gram's work at the IARC, Lyon, France. In Sweden, the Swedish Council supported the study for Planning and Coordination of Research, Swedish Cancer Society,Organon, Pharmacia, Medical Products Agency, AFA, US Army Breast Cancer Research Program, and Schering-Plough.
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- Accepted July 20, 2004.
- Received May 19, 2004.
- Revision received July 8, 2004.