The goal of this study is to determine specific perfusion patterns for radiation induced lung changes and residual/recurrent lung malignancies in patients treated with stereotactic body radiotherapy SBRT and thus improve to distinguish radiation changes from residual/recurrent lung cancers. Currently CT is often unable to make a clear differentiation between benign and malignant changes in the lung after SBRT treatment necessitating additional wait time to perform follow up CTs or biopsies. Optimal treatment may be delayed. The investigators want to apply a CT perfusion sequence in addition to the routine follow up CTs with the goal to obtain perfusion values of post treatment lung changes and lung tumours. The investigators' hypothesis is that CT perfusion values will help differentiate benign post radiation changes from residual/recurrent tumour in the lung with higher confidence and may help avoid unnecessary delays in cancer treatment.