Background: Cigarette smoking is a major cause of lung cancer (LC). Several studies have demonstrated that extent of cigarette smoke inhalation is associated with the risk of LC in European population. However, to date, there are few such studies in Asian populations. The aim of our study is to clarify the relationship between cigarette smoke inhalation and the risk of LC in Japanese population.

Methods: We have conducted a case-control study of cigarette smoking and lung cancer (LC) risk in Aichi Prefecture, the third largest metropolitan area in Japan. Between 1993 and 1998, several hospitals in Aichi prefecture recruited newly diagnosed incident patients with histologically confirmed LC (n=653). The case eligibility included an age of 20–81 years, no previous diagnosis of lung, or other cancer, and ability to participate and provide informed consent. The study included both hospital controls (HC: n=453) and community-based controls (CC: n=828). The eligibility criteria of the two control groups were the same as for cases except that the hospital controls were admitted for non-malignant diseases or conditions unrelated to cigarette smoking. Odds ratios (OR) and 95% confidence intervals (CI) were derived from unconditional logistic regression analysis, adjusted for basic confounding variables such as age, sex, drinking status, family history of LC, occupation (white collar or blue collar), and years of education (less than 12 years, 12 or more than 12years).

Results: The mean of age was 60.7 in cases, 60.1 in controls. Cases had a higher proportion of male, having family history of LC, heavy drinker, and current smoker. The most common histologic types of LC were adenocarcinoma (AD) (50.9%), squamous cell carcinoma (SQ) (22.1%) and small cell carcinoma (SM) (19.2%). Compared to ever smokers in controls, those in cases started smoking at younger age, smoked more cigarettes per day and longer duration. The proportion of smokers who inhale cigarette smoke was greater in cases.

The risk of LC in ever smokers was significantly higher than never smokers, especially in smokers who inhale cigarette smoke. Compared with never smokers, ORs for ever smokers who do not inhale cigarette smoke (inhale-no) and ever smokers who inhale cigarette smoke (inhale-yes) were 1.72(95%CI: 1.15-2.59), 3.28(95%CI: 2.38-4.53), when adjusted for basic confounding variables. Stratified analyses by histologic types of LC showed that these associations were quite strong in SQ and SM (inhale-no: OR 10.1(95%CI: 4.96-20.7), inhale-yes: OR 19.7(95%CI: 10.3-37.6)) On the other hand, such associations seemed to be mild in AD (inhale-no: OR 0.87(95%CI: 0.51-1.48), inhale-yes: OR 1.50(95%CI: 1.01-2.21)). Stratified analysis by pack-year showed that OR of “inhale-yes” group was higher than “inhale-no” group in every categories of pack-year (<20, 20-39, 40-59, 60 or more).

When the analysis was restricted to ever smokers, the OR (adjusted for basic confounding factors and pack-year) of LC risk in ever smokers who inhale cigarette smoke was significantly higher than who do not inhale cigarette smoke. OR for “inhale-yes” group compared with “inhale-no” group was 1.52(95%CI: 1.06-2.18, p=0.021). No heterogeneity was observed across histologic types of LC.

Conclusion: Our case- control study demonstrated that inhalation of cigarette smoke is an independent risk for LC even in the adjustment for pack-year in Japanese population. Cigarette smoke inhalation might have an additive effect toward LC risk.

Citation Format: Koichi Fukumoto, Hidemi Ito, Chiyoung Park, Hideo Tanaka, Keitaro Matsuo, Kazuo Tajima, Toshiro Takezaki. Cigarette smoke inhalation and lung cancer risk: Case-control study in Japan. [abstract]. In: Proceedings of the Twelfth Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2013 Oct 27-30; National Harbor, MD. Philadelphia (PA): AACR; Can Prev Res 2013;6(11 Suppl): Abstract nr C17.