We included patients identified in the Danish Lung Cancer Register with stage I lung cancer during 2011–2014 who had received lung cancer surgery with curative intent. This population originated from a study by Christensen et al, which also included patients treated by stereotactic body radiotherapy and other primary cancer treatment regimes.24 (link) The patients were staged according to the Danish lung cancer guidelines, which are in line with international recommendations.25 (link) The majority of the patients with recurrence had a positron emission tomography with 2-deoxy-2-[fluorine-18] fluoro-D-glucose (FDG PET) performed. Additional diagnostic tests performed on suspicion of recurrence were mainly focused on the anatomic site under suspicion and included cerebral magnetic resonance imaging (MRI) or computed tomography (CT), biopsies, endobronchial ultrasound (EBUS) and endoscopic ultrasound (EUS). Patients diagnosed with recurrent lung cancer were identified through a medical record review (performed in May 2016) including imaging results and/or clinical evaluation. Distinction between recurrence and a second primary lung cancer was based on the conclusion from the multidisciplinary team conference that established the diagnostic work-up for the recurrence. Recurrence dates were registered as the number of months from the diagnosis date of the primary lung cancer to the month when the diagnostic procedures for recurrence had been undertaken.