A multi-centre randomized controlled open-label trial in medically inoperable patients with biopsy-proven early stage non-small cell lung cancer (NSCLC). Eligible and consenting patients will be randomly allocated to receive stereotactic body radiotherapy (SBRT) or conventional radiotherapy (CRT) in a 2:1 ratio. Radiotherapy will be administered as soon as possible following randomization and subjects will be followed for 5 years post-randomization for cancer recurrence, toxicity and survival. The primary outcome is local control (LC). The trial will be conducted at 16-20 clinical centres throughout Canada.

Inclusion Criteria:

1. T1/T2a N0 M0 NSCLC, either by: (a) histological confirmation (squamous cell, adenocarcinoma, large cell carcinoma, or not specified) and CT thorax and/or PET-CT evidence, or (b) a suspicious growing nodule on serial CT imaging, with malignant PET Fluorodeoxyglucose (FDG) avidity, for which a biopsy would be extremely risky.
2. Deemed medically inoperable (as reviewed by a thoracic surgeon and defined as surgically resectable but, because of underlying physiological medical problems [e.g. chronic obstructive pulmonary disease (COPD), heart disease], surgery is contraindicated) or Radiotherapy is preferred by the patient due to high operable risk.

Exclusion Criteria:

1. Less than 18 years of age.
2. Eastern Cooperative Oncology Group (ECOG) performance status 3 or higher.
3. Prior invasive malignancy within the past 3 years (excluding non-melanomatous skin cancer).
4. History of ataxia telangiectasia.
5. Previous radiotherapy (RT) in the vicinity of the tumour, such that significant overlap could occur.
6. Previous pneumonectomy with Stage I lung cancer in the remaining lung.
7. Diagnosis of idiopathic pulmonary fibrosis and/or interstitial lung disease.
8. Planned for other anticancer therapy (chemotherapy, biological targeted therapy).
9. Female, who is currently pregnant or lactating.
10. Geographic inaccessibility for follow-up.
11. Unable to provide informed consent.