Abstract
BackgroundNo scientific consensus has been reached on whether active tobacco smoking causes breast cancer. We examine the association between active smoking and breast cancer risk in Denmark, which has some of the highest smoking and breast cancer rates in women worldwide.MethodsWe used the data from a nationwide Danish Nurse Cohort on 21,867 female nurses (age > 44 years) who at recruitment in 1993 or 1999 reported information on smoking status, onset, duration, and intensity, as well as breast cancer risk factors. We obtained data on incidence of breast cancer from Danish Cancer Registry until 2013, and used Cox regression models to analyze the association between smoking and breast cancer.ResultsOf 21,831 women (mean age 53.2 years) 1162 developed breast cancer during 15.7 years of follow-up. 33.7% of nurses were current and 30.0% former smokers at cohort baseline. Compared to never smokers, we found increased risk of breast cancer of 18% in ever (hazard ratio and 95% confidence interval: 1.18; 1.04–1.34) and 27% in current (1.27; 1.11–1.46) smokers. We detected a dose-response relationship with smoking intensity with the highest breast cancer risk in women smoking >15 g/day (1.31; 1.11–1.56) or >20 pack-years (1.32; 1.12–1.55). Parous women who smoked heavily (>10 pack-years) before first childbirth had the highest risk of breast cancer (1.58; 1.20–2.10). Association between smoking and breast cancer was not modified by menopausal status, obesity, alcohol or hormone therapy use, and seemed to be limited to the estrogen receptor positive breast cancer subtype.ConclusionsActive smoking increases risk of breast cancer, with smoking before first birth being the most relevant exposure window.
Highlights
No scientific consensus has been reached on whether active tobacco smoking causes breast cancer
Recent large prospective cohort studies [6,7,8,9,10,11,12,13] with detailed data on active smoking consistently report an increased breast cancer risk associated with longer duration and higher intensity of smoking, and indicating that smoking early in life, before first childbirth, is the most relevant exposure window. [6, 7, 9, 11] Still, some inconsistencies exist as not all recent studies linked active smoking to breast cancer. [14, 15] The latest reports from the International Agency for Research on Cancer (IARC) [1] and the US Surgeon General [3] conclude that there was suggestive, but insufficient, evidence to Andersen et al BMC Cancer (2017) 17:556 infer a causal relationship between active smoking and breast cancer, [3] calling for more data
[11] On this basis, we investigated the association between active smoking and breast cancer risk in members of the Danish Nurse Cohort, which is a large, nationwide cohort of female nurses older than 44 years
Summary
The Danish nurse cohort The Danish Nurse Cohort [18] was inspired by the American Nurses’ Health Study to initially investigate the health effects of hormone therapy (HT) in a European population. Statistical analyses We used Cox proportional hazards regression with age as the underlying time, to investigate the association between smoking and breast cancer in a crude model (age adjusted as age is underlying time scale), and in a fully adjusted model, adjusted for age at the time of recruitment, birth cohort (1990–1934; 1935–1944; 1945–1949; 1950–1955), Body Mass Index (BMI) (15 drinks/ week)), leisure time physical activity (low; medium; high), night shift work (yes; no), age at menarche (years), parity (yes; no), number of children, age at first birth (years), menopausal status (yes; no), HT use (never, ever), and OC use (never; ever). No contact has been taken with participating women, relatives or their practicing doctors, and no consent was needed
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