The Phoenix Trial: Phase II Trial of Cemiplimab for the Non-operative Management of Localized dMMR Colon Cancer

Inclusion Criteria: Participants are eligible to be included in the study only if all of the following criteria apply: * Age ≥18 years * Histological confirmation of colon adenocarcinoma, as determined by pathology review (inferior colon margin defined as \>10 cm from anal verge). * Colon cancer that is deficient in mismatch repair (dMMR) or microsatellite Instability high (MSI-H) as determined by one of three methods: * Immunohistochemistry-determined dMMR by complete tumor nuclear loss of MLH1, PMS2, MSH2 or MSH6 * PCR-determined MSI at \>30% of tested microsatellites * Next-generation-determined MSI-H based upon instability at multiple microsatellites as determined by the specific next generation sequencing panel * Localized colon cancer with (1) radiological staging of T3 or T4 or lymph node positive (stage II or III) OR (2) locally recurrent with luminal component OR (3) stage I with a surgical mortality defined as \>5% by American College of Surgeons (ACS) National Surgery Quality Improvement Program * Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1. * Primary tumor that is deemed to be accessible by endoscopic intervention and willingness to undergo repeated endoscopic evaluations * Measurable or non-measurable disease by cross-sectional imaging per RECIST v1.1 criteria * Laboratory values (obtained within 7 days prior to registration) meeting the following criteria: * Absolute neutrophil count (ANC) ≥1000/mme * Platelet cou…