Neoadjuvant Chemoradiotherapy Plus Tislelizumab With or Without Probio-M9 in pMMR/MSS Locally Advanced Rectal Cancer

This study tests whether adding a probiotic supplement (Probio-M9) to standard treatment—which combines chemotherapy, radiation, and an immunotherapy drug (tislelizumab)—can improve outcomes for patients with locally advanced rectal cancer. Researchers think that gut bacteria may influence how well the immune system responds to cancer treatment. **You may be eligible if...** - You are 18 or older with confirmed rectal cancer (a type of colon cancer at the end of the bowel) - Your cancer is locally advanced (stage II or III) based on MRI, and has not spread to distant organs - Your tumor is within 10 cm of the anal opening - You have not received any prior cancer treatment - You do not have an immune system disorder, significant organ dysfunction, or active infection **You may NOT be eligible if...** - You have previously had any anti-cancer treatment (surgery, chemotherapy, radiation, or immunotherapy) in the past 5 years - You have another active cancer - You have conditions affecting how your body absorbs oral medications - You are on immunosuppressive drugs - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.