Is the shape of the decline in risk following quitting smoking similar for squamous cell carcinoma and adenocarcinoma of the lung? A quantitative review using the negative exponential model.

One possible contributor to the reported rise in the ratio of adenocarcinoma to squamous cell carcinoma of the lung may be differences in the pattern of decline in risk following quitting for the two lung cancer types. Earlier, using data from 85 studies comparing overall lung cancer risks in current smokers, quitters (by time quit) and never smokers, we fitted the negative exponential model, deriving an estimate of 9.93years for the half-life - the time when the excess risk for quitters compared to never smokers becomes half that for continuing smokers. Here we applied the same techniques to data from 16 studies providing RRs specific for lung cancer type. From the 13 studies where the half-life was estimable for each type, we derived estimates of 11.68 (95% CI 10.22-13.34) for squamous cell carcinoma and 14.45 (11.92-17.52) for adenocarcinoma. The ratio of the half-lives was estimated as 1.32 (95% CI 1.20-1.46, p<0.001). The slower decline in quitters for adenocarcinoma, evident in subgroups by sex, age and other factors, may be one of the factors contributing to the reported rise in the ratio of adenocarcinoma to squamous cell carcinoma. Others include changes in the diagnosis and classification of lung cancer.