Assess the Efficacy of Pembrolizumab Plus Radiotherapy or Ablation in Metastatic Colorectal Cancer Patients

The radiation therapy or ablation that the patient received as standard therapy treated only the tumors that were radiated or ablated. Radiation therapy or ablation plus pembrolizumab might lead to a stronger immune response that may control or destroy tumors that did not receive radiation therapy or ablation. The purpose of this study is to find out what effects, good and/or bad, pembrolizumab has on the patient, and the cancer that did not receive radiation therapy or ablation.