What happened, in summary

A 62-year-old man was evaluated for abdominal pain and found to have a mass at the hepatic flexure. Biopsy confirmed colon adenocarcinoma with deficient mismatch repair and MSI-high disease, including MLH1 and PMS2 loss and a BRAF V600E mutation. CT showed no distant metastases, but the cancer was too locally extensive for upfront surgery. He started pembrolizumab in June 2022. After 4 cycles, imaging showed a marked reduction in the primary tumor and regional nodal disease. He received a fifth cycle and proceeded to surgery in September 2022. The surgical specimen showed no residual tumor and no cancer in any of 43 lymph nodes, confirming a pathological complete response. He then entered follow-up rather than receiving additional therapy, with no evidence of relapse. Molecular testing confirmed the BRAF V600E alteration and MLH1 promoter hypermethylation.