Reanalysis of the Mayo Lung Project data: the impact of confounding and effect modification

To examine whether age at entry, history of cigarette smoking, exposure to non-tobacco lung carcinogens, or previous pulmonary illnesses were confounders or effect modifiers of the relation between screening and lung cancer mortality in the Mayo Lung Project.

The Mayo Lung Project was a randomised, controlled, clinical trial conducted between 1971 and 1986 in 9211 male smokers over the age of 45 in Minnesota (USA). The group screened received chest x ray examination and sputum cytology every four months for six years. The unscreened group were recommended to obtain usual care (annual chest x ray examination and sputum cytology). After follow up, lung cancer mortality was similar in both groups.

Proportional hazard models were used to analyse data. A variable was considered a confounder if its inclusion in a model changed the rate ratio for screening by more than 15%; a variable was considered an effect modifier if its stratum-specific rate ratio for screening differed by a factor of two.

None of the four aforementioned variables changed the rate ratio associated with screening (1.07) by more than 2%. The effect of screening may have differed by years smoked (rate ratio for smoking fewer than 30 years 2.4; rate ratio for smoking 30 or more years 1.0), though we suspect that this result occurred by chance.

Adjustment for or stratification by four established lung cancer risk factors did not alter the original findings of the Mayo Lung Project.