How rapidly does the excess risk of lung cancer decline following quitting smoking? A quantitative review using the negative exponential model

- No other review has modelled the decline in lung cancer risk from quitting smoking.
- We estimated when the excess risk is half that of continuing smoking (half-life H).
- Using 106 independent data blocks we estimated H as 9.93 (95% CI 9.31-10.60) years.
- Estimates of H were somewhat lower for females, and for younger subjects.
- Estimates depended little on the method used to account for reverse causation.

The excess lung cancer risk from smoking declines with time quit, but the shape of the decline has never been precisely modelled, or meta-analyzed. From a database of studies of at least 100 cases, we extracted 106 blocks of RRs (from 85 studies) comparing current smokers, former smokers (by time quit) and never smokers. Corresponding pseudo-numbers of cases and controls (or at-risk) formed the data for fitting the negative exponential model. We estimated the half-life (H, time in years when the excess risk becomes half that for a continuing smoker) for each block, investigated model fit, and studied heterogeneity in H. We also conducted sensitivity analyses allowing for reverse causation, either ignoring short-term quitters (S1) or considering them smokers (S2). Model fit was poor ignoring reverse causation, but much improved for both sensitivity analyses. Estimates of H were similar for all three analyses. For the best-fitting analysis (S1), H was 9.93 (95% CI 9.31-10.60), but varied by sex (females 7.92, males 10.71), and age (<50 years 6.98, 70+ years 12.99). Given that reverse causation is taken account of, the model adequately describes the decline in excess risk. However, estimates of H may be biased by factors including misclassification of smoking status.