 This is a phase 1b feasibility study to evaluate the use of PD-1 blockade in combination with ablative radiotherapy for the treatment of metastatic colorectal cancer CRC. This study will examine the sequential combination of stereotactic body radiotherapy SBRT and pembrolizumab for patients for whom the goal is eradicating all known sites of disease. It is very likely that for many patients the SBRT therapy will be completed following other modalities including operative resection or ablation. 