Dostarlimab for Locally Advanced or Metastatic Cancer (Non-colorectal/Non-endometrial) With Tumor dMMR/MSI

This study is testing dostarlimab — an immunotherapy drug that helps the immune system recognize and attack cancer — as a first-line treatment for people with certain rare, advanced cancers that have a specific genetic feature called dMMR/MSI-H (meaning the cancer cells have a defect in their DNA repair system). This type of cancer tends to respond well to immunotherapy. The study includes several rare cancer types not usually covered in other dostarlimab trials. **You may be eligible if:** - You are 18 or older - You have a confirmed advanced or metastatic cancer with dMMR/MSI-H (deficient mismatch repair / microsatellite instability-high) that is NOT colorectal or endometrial cancer - Eligible cancer types include: small bowel adenocarcinoma, gastric/esophagogastric junction cancer (with low immune marker), pancreatic cancer, ampullary cancer, adrenocortical carcinoma, cancer of unknown primary, high-grade neuroendocrine carcinoma, or soft tissue sarcoma (not GIST) - You have not yet received systemic treatment for your advanced cancer - Tumor tissue is available for testing to confirm the dMMR/MSI-H status **You may NOT be eligible if:** - Your cancer is colorectal or endometrial cancer - You have already received chemotherapy or immunotherapy for your advanced disease - You do not have dMMR/MSI-H confirmed on testing - Adequate tumor tissue is not available Talk to your doctor to see if this trial is right for you.