'SOURCE - LUNG' Stereotactic Ablative Radiation Therapy Of UltRaCEntral LUNG Tumours (SOURCE Lung)

Inclusion Criteria: 1. Written informed consent obtained prior to any study-specific procedures 2. ≥ 18 years of age 3. Life expectancy \>6 months 4. ECOG (Eastern Cooperative Oncology Group) performance status 0-2 5. Histological diagnosis (biopsy or cytology) or radiological diagnosis (PET-positive FDG-avid tumour which has only one lesion to be treated for the purpose of the study / CT-based diagnosis for non FDG-avid tumour) which requires local ablative therapy per Multi-Disciplinary Team (MDT) recommendations) of either: (i) Primary NSCLC (Squamous Cell Carcinoma (SCC), Adenocarcinoma, Large Cell) OR (ii) Single pulmonary oligometastatic lesion to be treated for the purpose of the study 6. Patients with central lung tumours/lesions whose radiotherapy plan meets the following criteria: (i) OAR eligibility constraints are initially exceeded when full PTV coverage is met; (ii) subsequently meets the CTRIAL-IE 18-33 SOURCE OAR Lung constraints and meets CTRIAL-IE 18-33 SOURCE Lung minimum constraints 7. Inoperable (as per MDT) or patient refuses surgery, 8. Females of childbearing potential must not be pregnant or lactating, must be prepared to take adequate contraception methods during treatment. Males whose female partners are of childbearing potential must be prepared to take adequate contraception methods during treatment. Examples of effective contraception methods are a condom or a diaphragm with spermicidal jelly, or oral, injectable or implanted birth control 9. Absence of psychological, familial, sociological or geographical condition, or psychiatric illness/social situation potentially hampering compliance with the study protocol and follow-up schedule Exclusion Criteria: 1. Known co-existing or prior malignancy within the last 5 years (except for adequately treated basal cell carcinoma (BCC) or Squamous Cell Carcinoma (SCC) of the skin)) which is likely to interfere with treatment or assessment of outcomes 2. Tumour/oligometastatic lesion that is abutting the oesophagus 3. Evidence of regional (nodal) or distant metastases or metastatic pleural effusion for patients with primary NSCLC 4. Spinal canal involvement 5. Patients with syndromes or conditions associated with increased radiosensitivity 6. Idiopathic pulmonary fibrosis / usual interstitial pneumonia 7. Chemotherapy and/or other targeted therapy administered within 3 months prior to study radiotherapy or planned for \<6 weeks following radiotherapy for patients with primary NSCLC, or within 1 week prior to study radiotherapy or planned within 1 week following radiotherapy for patients with an oligometastatic lesion 8. Any previous radiotherapy to the thorax or mediastinum (excluding previous breast or chest wall radiotherapy) which is likely to interfere with treatment or assessment of outcomes 9. Any tumour not clinically definable on the treatment planning CT scan (e.g. surrounding consolidation or atelectasis) 10. Patients unable to undergo 4D-CT scan 11. Uncontrolled intercurrent illness that is likely to interfere with treatment or assessment of outcomes 12. Evidence of any other significant clinical disorder or laboratory finding that makes it undesirable for the patient to participate in the study, or if it is felt by the research / medical team that the patient may not be able to comply with the protocol.