Background The majority of cases of both lung cancer and COPD are attributable to cigarette smoking. Some consider COPD to be an independent risk factor for lung cancer, even after accounting for smoking, with estimates of increased risk up to 9-fold in previous studies. We undertook a large case-control study using prospectively collected data which allowed us to quantify this association in the UK population, whilst carefully controlling for smoking and the impact of timing of diagnoses.

Methods We used The Health Improvement Network, a UK general practise database, to identify incident cases of lung cancer and controls matched on age, sex and the practise with which they were registered. Using conditional logistic regression, we assessed the effects of timing of first diagnoses of COPD, pneumonia and asthma on the odds of lung cancer, adjusting for smoking habit and socioeconomic status.

Results Of 11,888 incident cases of lung cancer, 23% had a prior diagnosis of COPD compared with only 6% of the 37,605 controls. The odds of lung cancer in patients who had COPD diagnosed within 6 months of their cancer diagnosis were eleven-fold those of patients without COPD. However, when restricted to earlier COPD diagnoses, with adjustment for smoking, the effect markedly diminished (for COPD diagnoses >10 years before lung cancer diagnosis OR 2.18, 95% CI 1.87–2.54). The pattern was similar for pneumonia (see table). There was some diagnostic overlap between asthma and COPD but analyses which accounted for this produced similar results.

Conclusion The association between COPD and lung cancer is largely explained by smoking habit, strongly dependent on the timing of COPD diagnosis and not specific to COPD. There is, however, an extremely strong unadjusted relationship of both COPD and pneumonia with lung cancer in the 6 months immediately prior to lung cancer diagnosis. This is useful in a clinical context highlighting the need to consider a diagnosis of lung cancer when making new diagnoses of COPD or pneumonia, and supporting the current NICE recommendation that all patients should have a chest radiograph looking for evidence of lung cancer at the time of COPD diagnosis.