Smoking Cessation and Lung Cancer Mortality in a Cohort of Middle-aged Canadian Women
Introduction
Cigarette smoking is the most important preventable risk factor for lung cancer, which is the leading cause of cancer mortality 1, 2, 3. Cigarette smoking accounts for 80 to 90 percent of lung cancer cases (2). Studies have shown that smokers who quit have a lower risk of lung cancer incidence and mortality relative to that of continuing smokers 4, 5, 6, 7, 8. However, only a few cohort studies have been conducted to determine the impact of smoking cessation on lung cancer risk among women 6, 7, 8, 9, 10, 11, 12. Moreover, most studies of women have assessed the effect of smoking cessation on lung cancer risk solely on the basis of duration of smoking cessation, and only a few studies have determined the impact of smoking cessation according to age at quitting 6, 12. Furthermore, smoking intensity (number of cigarettes per day) and duration (years smoked) may influence the impact of smoking cessation. However, only one study (6) has controlled for the effect of these aspects of smoking history in the analysis.
Although smoking is the leading cause of lung cancer, other factors may play a role in lung cancer etiology as well. Studies have shown that higher education (13), physical activity (14), use of hormone replacement therapy (15), and higher intake of vegetables, fruit, vitamins A, C, and E, lutein, beta carotene, and fiber 16, 17, 18, 19, 20 may be associated with a lower risk of lung cancer, while high intake of fat (21), cholesterol (22), and alcohol (23), and lower levels of body mass index 24, 25 may be associated with an elevated risk of lung cancer. It has been shown that there are significant differences in nutritional exposures by smoking status (26). However, only one cohort study on women has adjusted for some of these potential confounding factors (8) when determining the impact of smoking cessation on lung cancer mortality.
To our knowledge, no single woman cohort study has determined the impact of smoking cessation according to both duration of smoking cessation and age at quitting. Further, no study has assessed the independent effect of smoking cessation on lung cancer risk by controlling for both smoking history and other lifestyle factors including dietary factors, body mass index, and physical activity. Studies that failed to control for these potential confounding factors may have produced biased results. In this study, we analyzed up to 14 years of follow-up data on a large cohort of Canadian women to examine the impact of smoking cessation on lung cancer mortality based on both time since cessation of smoking and age at quitting. The independent effect of cessation on lung cancer mortality was determined, while controlling for smoking history and multiple risk factors.
Section snippets
Study Population
The Canadian National Breast Screening Study (NBSS) is a multicenter randomized controlled trial of mammography screening for breast cancer among women aged 40 to 59 years at enrollment. Participants for the NBSS were recruited between 1980 and 1985 by personal letter, group mailings to employees of large institutions and members of professional associations, advertisements in newspapers, and public service announcements on radio and television 27, 28. Detailed information on the methods of the
Results
A total of 106 women died from lung cancer during the follow-up of 521,622 person-years. The median follow-up time for the cohort is 10.3 years. At baseline, 51% women are never smokers, 22% current smokers and 27% former smokers (Table 1). Among former smokers, the proportion of those quitting smoking with 1 to 9, 10 to 19, and 20 years or more prior to enrollment is 42%, 36%, and 22%, respectively. Current smokers are younger, thinner, less active, less likely to attain university education,
Discussion
This large prospective cohort study provides evidence that women who quit smoking have significantly and substantially lower lung cancer mortality risk than those who continue to smoke. The risk of lung cancer mortality is significantly lower among former smokers, even those with less than 10 years of cessation compared with continuing smokers. Quitting smoking at all ages is associated with a lower risk of lung cancer mortality. Even those who quit in their 50s are at substantially lower risk
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