Dose-response relationships between cigarette smoking and kidney cancer: A systematic review and meta-analysis.

PURPOSE: We aim to provide the most accurate and updated quantification of the effect of cigarette smoking on kidney cancer risk focusing on dose-response relationships. METHODS: We conducted a meta-analysis, using an innovative approach combining an umbrella review and a traditional literature search. RESULTS: Fifty-six original studies were included, providing pooled relative risks (RR) of kidney cancer of 1.39 (95% confidence interval, CI: 1.28-1.51) for current and 1.20 (95% CI: 1.14-1.27) for former compared with never smokers. Kidney cancer risk increased non-linearly with smoking intensity, the RR compared with never smokers being 1.18 (95% CI: 1.11-1.26) for five and 1.72 (95% CI: 1.52-1.95) for 30 cigarettes/day, and increased linearly with smoking duration, the RR being 1.70 (95% CI: 1.10-2.64) after 25 years. The risk linearly decreased with time-since-quitting. CONCLUSIONS: Even smoking few cigarettes per day significantly increases kidney cancer risk. Quitting smoking reduces the risk, the earlier the better.