 The FC-9 study is designed as a phase II, open label, single arm study of the dual immune checkpoint blockade with the combination of durvalumab and tremelimumab following hypofractionated palliative radiation in patients with microsatellite stable MSS metastatic colorectal cancer mCRC who have progressed on chemotherapy. The primary aim is to determine the anti-tumor efficacy of the dual immune checkpoint blockade with durvalumab plus tremelimumab. The secondary aims are to determine the clinical benefit rate, duration of response, tolerability and correlates of response. Tumor response at unirradiated target lesions will be measured at baseline and every 2 cycles using RECIST 1.1. 