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.

中文翻译：

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日本的主动和被动吸烟与乳腺癌：九项基于人群的队列研究的汇总分析。


背景流行病学研究表明，尽管吸烟与乳腺癌风险之间存在生物学上的合理性，但吸烟与乳腺癌风险之间的联系却存在不一致的结果。方法 参与者为来自日本 9 项前瞻性队列研究的 166 611 名女性，该研究于 1984 年至 1994 年启动，并随访了 8 至 22 年。有关吸烟和二手烟的信息是通过自我管理的基线问卷获得的。根据《国际肿瘤学疾病分类》第 3 版或《国际疾病分类》第 10 版修订版，乳腺癌被定义为代码 C50。在调整了几个潜在的混杂因素后，在个别研究中根据当前或以前的主动和被动吸烟状态，使用 Cox 回归计算了乳腺癌的相对风险，然后使用随机效应荟萃分析对风险比进行了总结估计。结果 在基线时报告为绝经前的 60 441 名参与者和报告为绝经后的 106 170 名参与者中，分别有 897 人和 1168 人在随访期间患上乳腺癌。与从不吸烟的人相比，目前吸烟的人在 50 岁之前患乳腺癌的风险更高。此外，在 30 岁或更年轻时开始吸烟或在第一次生育前开始吸烟的吸烟者在 50 岁之前患乳腺癌的风险较高。没有观察到成年或儿童时期接触二手烟与乳腺癌之间存在关联。结论 吸烟可能会增加绝经前乳腺癌的风险，并且生命早期吸烟可能尤其有害。二手烟的影响需要进一步调查。