A Study of Dostarlimab in Untreated dMMR/MSI-H Locally Advanced Rectal Cancer

Inclusion Criteria: * Participant has histologically confirmed Stage II to III (T3-T4, N0, or T any, N+), locally advanced rectal cancer * Participant has radiologically and endoscopically evaluable disease. * Participant has a tumor which can be categorized as dMMR or MSI-H by local or central assessment Exclusion Criteria: * Participant has distant metastatic disease. * Participant has received prior radiation therapy, systemic therapy, or surgery for management of rectal cancer. * Participant has any history of interstitial lung disease or pneumonitis * Participant has experienced any of the following with prior immunotherapy: any imAE of Grade ≥3, immune-related severe neurologic events of any grade (e.g., myasthenic syndrome/myasthenia gravis, encephalitis, Guillain Barré Syndrome, or transverse myelitis), exfoliative dermatitis of any grade (Stevens-Johnson Syndrome, toxic epidermal necrolysis, or DRESS syndrome), or myocarditis of any grade. Non clinically significant laboratory abnormalities are not exclusionary. * Participant has a known additional malignancy that progressed or required active treatment within the past 2 years. Exceptions include adequately treated superficial skin cancers, superficial bladder cancers, and other in situ cancers. * Participant has an active autoimmune disease that has required systemic treatment in the past 2 years (i.e., with use of disease-modifying agents, corticosteroids, or immunosuppressive drugs). Replacement therapy (e.g., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic treatment. * Participant has a history of severe allergic and/or anaphylactic reactions to chimeric, human or humanized antibodies, fusion proteins, or has known allergies to dostarlimab or its excipients. * Has received or plans to receive an organ or stem cell transplant that uses donor stem cells (allogeneic stem cell transplant).