What happened, in summary

A 76-year-old woman was found to have a stenosing cancer at the hepatic flexure after a positive fecal occult-blood test. Biopsy showed poorly differentiated colon adenocarcinoma with deficient mismatch repair and MSI-high disease. Imaging did not show distant metastases, but surgery revealed extensive local involvement of nearby organs, so only a bypass was performed initially. She then started pembrolizumab in June 2021. After 4 cycles, imaging showed a striking reduction in the primary tumor and its local extension, and she received a fifth cycle before definitive surgery. In September 2021 she underwent an extended right hemicolectomy. Pathology found no residual tumor and no involved lymph nodes among 58 examined, confirming a pathological complete response. She did not receive further anticancer treatment after surgery and continued follow-up without evidence of relapse. Genetic testing did not identify a pathogenic germline finding, while tumor testing supported dMMR/MSI-H disease with MLH1 promoter hypermethylation.