PI Pembro in Combination With Stereotactic Body Radiotherapy for Liver Metastatic Colorectal Cancer

Inclusion Criteria: * Willing and able to provide written informed consent/assent for the trial * Be \>/= 18 years of age on day of signing consent. * Have a diagnosis of histologically confirmed metastatic colorectal cancer to the liver (no other sites of metastatic disease) \* Histologic confirmation of a colorectal primary tumor is acceptable if accompanied by radiographic evidence of metastatic disease * Tumor must be mismatch repair (MMR) proficient as determined by microsatellite instability or immunohistochemistry for for MMR proteins * Microsatellite instability testing must be MSI-stable or MSI-low * Or IHC for MMR proteins must demonstrate intact MMR proteins * Participant must be candidate for SBRT to at least one intrahepatic lesion. There is no limit on the number of intrahepatic lesions the patient may have * Participant must be a surgical candidate with therapeutic goal of eradicating all known disease with one additional surgery. Portal venous embolization is permitted to ensure resectability. * Prior resection of extra-hepatic metastatic disease allowed if completed more than 12 months previous to study enrollment and now new extra-hepatic disease has been found * Have measurable disease based on RECIST 1.1 * Fresh or archived colorectal cancer tissue, preferably from a hepatic metastatic site. Archival tissue is acceptable for enrolled into this study. Participants who have no archival tissue available do not need to undergo a new biopsy solely for …