A Randomized, Multicenter Phase II Basket Study of Hypofractionated Radiotherapy/Stereotactic Body Radiotherapy Followed by Immunotherapy-Based Systemic Therapy +/- L. Rhamnosus M9 for the First-Line Treatment of Advanced Digestive System Malignancies.

This study is testing a treatment approach for advanced digestive system cancers that combines focused radiation with immunotherapy, and also looks at whether adding a probiotic (a beneficial gut bacteria called L. rhamnosus M9) improves outcomes. **You may be eligible if...** - You have been diagnosed with an advanced cancer of the digestive system — including stomach, liver, bile duct, or colorectal cancer — that cannot be surgically removed - You have not received any previous cancer treatment - You are between 18 and 75 years old and in good general health - You have at least one measurable tumor - You are able to receive chemotherapy and/or immunotherapy **You may NOT be eligible if...** - You have brain metastases - You have already received treatment for your cancer - You have major heart or blood vessel problems - You have a condition that prevents you from receiving chemotherapy or immunotherapy - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.