The epidemiology of lung cancer related to interstitial lung disease (ILD) with or without with connective tissue disease (CTD) as well as idiopathic pulmonary fibrosis (IPF) has not been well established yet. Interstitial lung disease has a potential to develop lung cancer in the name of scar cancer. The investigators aimed to explore prevalence of lung cancer in patients with ILD with or without CTD and IPF in comparison with chronic obstructive pulmonary disorder (COPD).