30 patients, recruited from within the PEAR Study (Phase 1 dose escalation of palliative radiotherapy with anti-PD1 antibody pembrolizumab in thoracic tumours) will be scanned both before and after receiving immunotherapy for NSCLC. Immunotherapy induced changes in tumour ADC on DW-MRI (a surrogate of apoptotic tumour cell death) will be compared with changes in near contemporaneous, anatomically co-registered 18FDG PET-CT SUV (a non-specific marker for GLUT expression and hence metabolic activity in both tumour and tumour infiltrating lymphocytes). For treatment response, an increase in ADC (reflecting apoptosis within the tumour) is expected, while 18FDG uptake is expected to decrease compared with baseline tumour uptake. In the presence of significant inflammatory cell recruitment, the investigators hypothesise a similar increase in ADC on DW-MRI but without associated reduction in 18FDG PET-CT SUV. Thus the relationship between the ADC change on DW-MRI and the 18FDG change should distinguish between response and non-response in the presence of inflammation.