Neoplasms, Rectal

Neoadjuvant therapy has been widely applied to locally advanced rectal cancer. However, about 50% of patients receiving this therapy do not respond well as evidenced by the fact that their T or N stages are not effectively decreased judged by postoperative pathological examination. The purpose of this trail is to identify the biomarkers (from within patients' tumor mass before neoadjuvant therapy) to predict resistance to neoadjuvant therapy. These biomarkers can help stratify neoadjuvant-resistant patients towards surgery while avoiding unnecessary chemoradio-based neoadjuvant therapy.

This study is conducted in adults with rectal cancer that has a certain genetic type. The main goal is to find out whether a longer treatment period with dostarlimab can help more participants have a complete response to treatment and avoid standard treatments like chemotherapy, radiation treatment, or surgery. Scans and endoscopy tests will be conducted to see if the tumor disappears and how long the cancer stays under control. The study will also collect information on side effects and health outcomes after dostarlimab.