The aim of this study is to use a new diagnostic test to detect ALK rearrangements using next generation sequencing, which will improve the diagnosis and treatment of ALK-positive NSCLC patients. Furthermore, this test will be performed on blood samples, making it minimally invasive for the patients. It is our believe that circulating tumor DNA (ctDNA) in blood can be employed as an easy accessible and comprehensive source of information to diagnose ALK-positive disease, but also as a means of monitoring patient response during treatment. Quantitation of the the amount of ALK rearrangement will give information about which patients benefit from treatment and when treatment is no longer effective. The project will be a multicenter study where blood samples will be collected every 6 weeks from patients treated at four major hospitals in Denmark.