Immunotherapies in Combination With Stereotactic Body Radiation Radiotherapy in Microsatellite Stable (MSS) Metastatic Colorectal Cancer (mCRC)

Background: Metastatic colorectal cancer (mCRC) is cancer that has spread beyond the colon and rectum. Most people with mCRC die within 5 years. New immune-based treatments are making progress with some types of colon cancer. But these treatments do little for people with a type of cancer that is microsatellite stable (MSS). MSS is a specific cancer biomarker. Better treatments are needed. Objective: To test 2 drugs (tiragolumab and atezolizumab) combined with radiation therapy in people with MSS mCRC. Eligibility: People aged 18 years and older with MSS mCRC. Design: Participants will be screened. They will have a physical exam with blood tests. They will have imaging scans and a test of their heart function. They will provide a tissue sample from their tumor; if one is not already available, a new sample will be taken. Their ability to perform normal tasks will be assessed. Tiragolumab and atezolizumab are both administered through a tube attached to a needle inserted into a vein. Participants will receive both drugs on day 1 of 3-week treatment cycles. Each study visit should last about 8 hours. Participants will receive radiation therapy on days 1, 3, and 5 of cycle 1 only. Blood samples and rectal swabs will be collected on day 1 of every cycle. Imaging scans will be repeated every 9 weeks. Additional tumor samples may be taken during treatment. Treatment will continue for up to 2 years. Participants will have a follow-up visit 1 month after treatment ends. Follow-up visits will continue every 3 months for 1 more year.