Abstract 3633: Cannabis smoking and lung cancer risk: pooled analysis in the International Lung Cancer Consortium.
Authors: Zhang Li Rita , Zhang Zuo-Feng , Morgenstern Hal , Chang Shen-Chih , Lazarus Philip , Teare M. Dawn , Woll Penella J. , Orlow Irene , Cox Brian , Liu Geoffrey , Hung Rayjean J. 
Published: August 2014

Abstract
Background
Lung cancer remains the leading cause of cancer death worldwide, with tobacco smoking established as the main risk factor. Cannabis smoke contains similar carcinogens as tobacco smoke including the polycyclic aromatic hydrocarbons; animal studies and human case series and histopathologic studies have suggested its potential carcinogenic effect in lungs. However, epidemiologic evidence is limited and conflicting. The present study aimed to examine the role of cannabis smoking in lung cancer risk using a pooled analysis in the International Lung Cancer Consortium (ILCCO).
Methods
Cannabis smoking and putative lung cancer risk factor data on 2131 lung cancer cases and 3075 controls were harmonized and pooled from six case-control studies in US, Canada, UK and New Zealand within the ILCCO. To standardize the definition and to distinguish occasional/non-users from habitual users, cumulative consumption of 1 joint-year (1 joint-equivalent per day for 1 year) or more was used to define habitual vs. non-users. The association between cannabis smoking (habitual vs. non-users, joint-equivalent per day, duration, and total joint-years) and the risk of lung cancer was assessed by odds ratios (OR) and 95% confidence intervals (CI) obtained from unconditional logistic regression in each study, while adjusting for age, sex, sociodemographic factors and tobacco packyears. Pooled risk estimates were calculated using random effect models. To minimize confounding by tobacco smoking, we also conducted analyses restricted to 367 case and 1400 control never tobacco smokers.
Results
The summary OR from the six studies for habitual vs. non-users was 1.15 (95% CI: 0.73-1.82, p for heterogeneity: 0.05). Compared to non-users, the summary OR was 1.28 (95%CI: 0.62-2.63) for individuals who smoked cannabis for 20 years or more and 1.53 (95%CI: 0.57-4.09) for those with 10 joint-years or more cumulative consumption. A lack of significant association between cannabis smoking and lung cancer was also observed in the never tobacco smokers: compared to non-users, the OR was 0.99 (95% CI: 0.49-2.00) for habitual users and 2.13 (95%CI: 0.67-6.78) for those who used 20 years or more.
Conclusion
Our pooled results showed no significant association between the intensity, duration, or cumulative consumption of cannabis smoke and the risk of lung cancer overall or in never smokers. Cannabis use is under international control and its legal status varies, so reporting bias is of concern. However, since the reported prevalence in our data is comparable to nation-specific survey results and not differential between cases and controls, it is unlikely to fully explain the lack of significant association. Our results cannot preclude the possibility that cannabis may exhibit an association with lung cancer risk at extremely high dosage. We will also present data after applying restricted cubic splines to explore non-linear relationships.
Citation Format: Li Rita Zhang, Zuo-Feng Zhang, Hal Morgenstern, Shen-Chih Chang, Philip Lazarus, M. Dawn Teare, Penella J. Woll, Irene Orlow, Brian Cox, Geoffrey Liu, Rayjean J. Hung. Cannabis smoking and lung cancer risk: pooled analysis in the International Lung Cancer Consortium. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3633. doi:10.1158/1538-7445.AM2013-3633