Background Large prospective studies on asthma in relation to the incidence of lung cancer are limited. It is also unclear if the association is explained by smoking, chronic bronchitis or allergy as these conditions commonly occur in asthma individuals. The aim of this prospective cohort study was therefore to explore the association between asthma, levels of asthma control and lung cancer incidence, taking into account the commonly occurring conditions.
Methods We followed 63,103 adults who participated in the second survey of the HUNT Study in Norway from 1995–97 to 2017. None of the participants had known cancer at the time of inclusion. Ever asthma (9.0%), doctor-diagnosed asthma (5.5%) and doctor-diagnosed active asthma (3.7%) were defined based on self-reported information at baseline. Among individuals with doctor-diagnosed active asthma, levels of asthma control were categorized into well controlled and poorly controlled. Incident lung cancer cases were ascertained from the Cancer Registry of Norway. Cox regression models were used to estimate adjusted hazard ratios (HRs) with 95% confidence intervals (CIs) for incident lung cancer in relation to asthma status.
Results In total, 1,013 participants developed lung cancer during a median follow-up of 21.1 years. After adjustment for smoking (classified into detailed categories based on information of smoking status and pack-years), chronic bronchitis, allergy and other confounders, increased overall incidence of lung cancer was associated with ever asthma (HR 1.32, 95% CI 1.09–1.61), doctor-diagnosed asthma (HR 1.32, 95% CI 1.04–1.67) and doctor-diagnosed active asthma (HR 1.40, 95% CI 1.08–1.82). Individuals with ever asthma only and without current smoking, chronic bronchitis or allergy appeared to have an increased incidence of lung cancer compared with those with no ever asthma and no such common condition. Poorly controlled doctor-diagnosed active asthma was associated with an increased incidence of lung cancer (HR 1.57, 95% CI 1.14–2.16), whereas no clear association between well-controlled doctor-diagnosed active asthma and lung cancer was observed (HR 1.16, 95% CI 0.65–2.06).
Conclusion Our study suggested that asthma, in particular poorly controlled asthma, was associated with an increased lung cancer incidence. Smoking, chronic bronchitis and allergy did not seem to explain the association.
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