Combination Chemotherapy With or Without Atezolizumab in Treating Patients With Stage III Colon Cancer and Deficient DNA Mismatch Repair

Inclusion Criteria: * Histologically proven stage III colon adenocarcinoma (any T \[Tx, T1, T2, T3, or T4\], N1-2M0; includes N1C); tumors must be deemed to originate in the colon including tumors that extend into/involve the small bowel (e.g. those at the ileocecal valve) * Presence of deficient (d) DNA mismatch repair (dMMR); MMR status must be assessed by immunohistochemistry (IHC) for MMR protein expression (MLH1, MSH2, MSH6, PMS2) where loss of one or more proteins indicates dMMR; dMMR may be determined either locally or by site-selected reference lab; Note: loss of MLH1 and PMS2 commonly occur together; formalin-fixed paraffin-embedded (FFPE) tumor tissue is required for subsequent retrospective central confirmation of dMMR status * Patients with testing that did not show dMMR (loss of MMR protein) are not eligible to participate; patients whose tumors show MSI-H by PCR-based assay are not eligible to participate unless they also have MMR testing by IHC and are found to have dMMR (i.e. loss of one or more MMR proteins) * Patients who are known to have Lynch syndrome, have been found to carry a specific germline mutation in an MMR gene (MLH1, MSH2, MSH6, PMS2), and have been shown to be dMMR by IHC are eligible to participate * Tumors must have been completely resected; in patients with tumor adherent to adjacent structures, en bloc R0 resection must be documented in the operative report or otherwise confirmed by the surgeon; near or positive radial margins are acceptab…