Pembrolizumab in Combination With Radiotherapy in Locally Advanced Non-Small Cell Lung Cancer (NSCLC)

Inclusion Criteria: * Histologically or cytologically confirmed NSCLC * Unresectable stage III NSCLC not suitable for concurrent chemoradiotherapy i.e; * Patient unsuitable for cisplatin (eg poor renal function); * Large volume of disease with predicted dose to thoracic organs at risk that are likely to exceed the constraints for concurrent chemoradiotherapy, in the opinion of a clinical oncologist specialised in lung cancer * Stage IV NSCLC with dominant chest symptoms and low burden of metastatic disease who may benefit from thoracic RT * Patient considered suitable for radical radiotherapy * If chemotherapy has been given previously, the maximum interval between the last day of chemotherapy and the start of radiotherapy must be 6 weeks. The minimum interval between the last day of chemotherapy and the start of Pembrolizumab must be one week * Age ≥ 18 * Life expectancy estimated to be greater than 6 months * Performance status (ECOG) 0 or 1 (see Appendix 1) * MRC dyspnoea score \< 3 (see Appendix 2) * FEV1 ≥ 40% predicted and DLCO ≥ 40% predicted; Lung V20 ≤ 30% in the dose finding part of the study and ≤ 35% in the expanded cohort * No prior thoracic radiotherapy (excluding patients that have had RT for Breast cancer providing that the overlap is minimal as per local investigators discretion or as discussed and agreed by CI as required) or T cell modulating antibodies (including anti-PD-1, anti-PD-L1, PD-L2, anti-CD137 and anti-CTLA4, including ipilimumab or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways) * Measurable disease based on RECIST 1.1 * Patient willing to undergo a repeat biopsy post RT * Written informed consent must be given according to GCP and national regulations. * Adequate organ function within 7 days of study treatment as defined in the protocol. Exclusion Criteria: * Mixed non-small cell and small cell tumours * Participation in a study of an investigational agent or using an investigational device within 4 weeks prior to the anticipated start of treatment. * Current or previous malignant disease within 3 years except CIN, non-melanoma skin cancer and low grade, low stage prostate cancer found as incidental finding and not requiring treatment * History of interstitial pneumonitis (to include diffuse alveolar damage, non-malignant causes of pneumonitis, acute respiratory distress syndrome, alveolitis, cryptogenic organising pneumonia, obliterative bronchiolitis, non-malignant causes of pulmonary fibrosis) * Presence of brain metastases confirmed by CT or MR brain (unless suitable for local treatment such as SRS or Neurosurgery) * History of autoimmune disease requiring steroids or immunosuppressive medication * Uncontrolled hypothyroidism or hyperthyroidism * Other diseases requiring immunosuppressive therapy greater than 28 days prior to the anticipated first dose of trial treatment. * Other diseases requiring systemic glucocorticoid (doses \<=10 mg prednisolone or equivalent) prior to the first dose of trial treatment. * Received a prior autologous or allogeneic organ or tissue transplantation. * Chronic GI disease likely to interfere with protocol treatment. * Testing positive for human immunodeficiency virus, active hepatitis B or C infection. * Treatment with live vaccine within 30 days prior to the first dose of trial treatment. * Patients of reproductive potential who are unable to comply with effective contraception if sexually active during the study and for up to 120 days after the last dose of Pembrolizumab * Women who are pregnant or breastfeeding. Women of childbearing potential must have a negative serum or urine pregnancy test * Any psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule; those conditions should be discussed with the patient before registration in the trial