Cigarette smoking and the risk of head and neck cancer in the Japanese population: a systematic review and meta-analysis

Although cigarette smoking is a well-established risk factor for head and neck cancer, the impact of smoking on head and neck cancer might vary among geographic areas. To date, however, no systematic review of cigarette smoking and head and neck cancer in the Japanese population has yet appeared. We conducted a systematic review of previous epidemiological studies for cigarette smoking and head and neck cancer among Japanese. Evaluation of associations was based on the strength of evidence ('convincing', 'probable', 'possible' or 'insufficient') and the magnitude of association ('strong', 'moderate', 'weak' or 'no association'), together with biological plausibility as previously evaluated by the International Agency for Research on Cancer. A meta-analysis was conducted to obtain summary estimates for the overall magnitude of association. We identified five cohort studies and 12 case-control studies. Four of five cohort studies and 11 of 12 case-control studies showed a strong positive association between cigarette smoking and head and neck cancer. Nine of 12 studies indicated a dose-response relationship between cigarette smoking and the risk of head and neck cancer. Meta-analysis of 12 studies indicated that the summary relative risk for ever smokers relative to never smokers was 2.43 (95% confidence interval: 2.09-2.83). Summary relative risks for current and former smokers relative to never smokers were 2.68 (2.08-3.44) and 1.49 (1.05-2.11), respectively. Cigarette smoking is a convincing risk factor for head and neck cancer in the Japanese population.