The trial is designed as a prospective observational single arm study investigating stage IV non-small cell lung cancer patients who are routinely treated with a PD-1 inhibitor for indications approved by Health Canada. All patients who are selected will be referred for palliative thoracic radiotherapy and treated with a standard dose prescription of 30 Gy in 10 fractions.

Inclusion Criteria:

1. Pathologically confirmed AJCC 7th/8th edition Stage IV adenocarcinoma or squamous cell carcinoma not eligible for curative treatment.
2. Indication and suitability to receive palliative radiotherapy to the thorax (30Gy/10).
3. Receiving or planned to receive nivolumab or pembrolizumab
4. Prior history of systemic chemotherapy is permitted given a washout period of 4 weeks
5. Age 18 or older
6. ECOG Performance Status 0-2
7. Life expectancy greater than 3 months
8. Able and willing to provide informed consent
9. Able to complete patient reported outcome questionnaires

Exclusion Criteria:

1. Contraindications to radiotherapy, including a history of SLE, systemic scleroderma, IPF, ataxia telangiectasia
2. Previous history of thoracic radiotherapy with an overlapping field
3. Previous history of checkpoint inhibitor related pneumonitis or esophagitis
4. Pregnancy

This study will include a population of patients with metastatic NSCLC on first or second line immunotherapy who have an indication for palliative thoracic radiation