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.

Inclusion Criteria:

• Gender: male or female
• CT ordered for baseline and follow up in patients with lung cancer and lung metastases treated with
• Patient will sign a consent form prior to study entry
• Lung cancer and lung metastases are biopsy proven

Exclusion Criteria:

• Patients who are unable to give valid informed consent
• Patients who are unwilling or unable to undergo a CT exam, including subjects with contra-indications to CT exams or intravenous contrast material administration.