Active and passive smoking and breast cancer in Japan: a pooled analysis of nine population-based cohort studies
Keiko Wada, Chisato Nagata, Mai Utada, Ritsu Sakata, Takashi Kimura, Akiko Tamakoshi, Yumi Sugawara, Ichiro Tsuji, Ren Sato, Norie Sawada, Shoichiro Tsugane, Isao Oze, Hidemi Ito, Tetsuhisa Kitamura, Yuriko N Koyanagi, Yingsong Lin, Keitaro Matsuo, Sarah K Abe, Manami Inoue, Manami Inoue, Sarah Krull Abe, Norie Sawada, Shiori Tanaka, Takashi Kimura, Yumi Sugawara, Tetsuya Mizoue, Shuhei Nomura, Hidemi Takimoto, Hidemi Ito, Keitaro Matsuo, Yingsong Lin, Keiko Wada, Tetsuhisa Kitamura, Ritsu Sakata, Keitaro Tanaka, Hadrien Charvat, Akihisa Hidaka, Mayo Hirabayashi, Motoki Iwasaki, Yuri Kitamura, Nagisa Mori, Michihiro Muto, Chisato Nagata, Mariko Naito, Tomio Nakayama, Yoshikazu Nishino, Atsuko Sadakane, Eiko Saito, Shizuka Sasazuki, Taichi Shimazu, Hiroyuki Shimizu, Kemmyo Sugiyama, Hidekazu Suzuki, Akiko Tamakoshi, Yoshitaka Tsubono, Ichiro Tsuji, Shoichiro Tsugane, Mai Utada, Kenji Wakai, Yoko Yamagiwa, Taiki Yamaji,- General Medicine
- Epidemiology
Abstract
Background
Epidemiological studies have shown inconsistent results regarding the link between smoking and breast cancer risk, despite the biological plausibility of a positive association.
Methods
Participants were 166 611 women from nine prospective cohort studies in Japan which launched in 1984–1994 and followed for 8–22 years. Information on smoking and secondhand smoke was obtained through self-administered baseline questionnaires. Breast cancer was defined as code C50 according to the International Classification of Diseases for Oncology, 3rd Edition or the International Classification of Diseases, 10th Revision. After adjusting for several potential confounders, relative risks for breast cancer were calculated in the individual studies according to the current or previous status of active and passive smoking using Cox regression, followed by a summary estimate of hazard ratios using random-effects meta-analyses.
Results
Of the 60 441 participants who reported being premenopausal and 106 170 who reported being postmenopausal at baseline, 897 and 1168 developed breast cancer during follow-up, respectively. Compared with never smokers, current smokers had a higher risk of developing breast cancer before the age of 50 years. In addition, ever smokers who started smoking at 30 years of age or younger, or who started smoking before first childbirth, had a higher risk of developing breast cancer before the age of 50 years. No association between adulthood or childhood exposure to secondhand smoke and breast cancer was observed.
Conclusion
Smoking may increase the risk of premenopausal breast cancer, and smoking earlier in life might be especially harmful. The impact of secondhand smoke needs further investigation.