The investigators will evaluate the prevalence of lung cancer associated with interstitial lung disease (ILD) and idiopathic pulmonary fibrosis (IPF) utilizing the Korean Health Insurance Review and Assessment Service (HIRA) database, spanning the period from January 2011 to December 2011. The database (HIRA-NPS-2011-0001) was based on random sampling of outpatients from whole population. Patients with ILDs, IPF, connective tissue disorder (CTD), and COPD were identified based on the International Classification of Disease-10 (ICD-10) diagnostic codes.