<qa>  What is the primary efficacy endpoint of the trial?This is a Phase 2 randomized two arm phase II trial of pembrolizumab alone or sequentially following focal radiation to one of the target lesions, in previously treated patients with stage IV Non-Small Cell Lung Cancer NSCLC. The primary goal of this trial is to compare the efficacy of focal radiation RT to an index lesion as a way of enhancing the anti-tumor immune response to pembrolizumab to that of pembrolizumab alone. The primary efficacy endpoint is overall RECIST-defined response outside the radiation field. To accomplish this goal 66 patients will be randomized 2:1 stratified by histology squamous versus non-squamous and refractory versus non-refractory disease to radiation plus pembrolizumab or pembrolizumab only, respectively. 