A case of dMMR/MSI-H/TMB-H colon cancer with brain metastasis treated with PD-1 monoclonal antibody

A 70-year-old man had radical surgery for colon cancer one year before he presented with symptoms of memory loss and decreasing cognitive function. Subsequent magnetic resonance imaging revealed a brain mass, which was surgically resected and confirmed to be metastatic intestinal adenocarcinoma. Immunohistochemistry of the primary tumor and brain metastasis showed mismatch repair deficiency (dMMR). The patient received adjuvant chemotherapy after surgery, however, brain metastasis relapsed one month after last chemotherapy. The genetic testing on the resected colon tumor samples confirmed MSI-H with a high tumor mutation burden (TMB-H) by 77.7 muts/Mb. The patient subsequently treated with PD-1 monoclonal antibody pembrolizumab (keytruda), the brain metastatic lesions were completely shrunk and clinical complete response (cCR) was achieved.