A Case of Successful Combination of Immune Checkpoint Inhibitors and Radiotherapy for MLH1 Gene-Deficient Colon Cancer

A 77-year-old woman was diagnosed with advanced transverse colon cancer(poorly differentiated adenocarcinoma)cT3N3H0P0, cStage Ⅲc and underwent extended left hemicolectomy in April 2020. The tumor tissue revealed RAS: wild type, BRAF: mutant type, dMMR(MLH1 deficiency)by immunochemical staining, and MSI-H by CDx. She received CAPOX as adjuvant chemotherapy after surgery. At the end of the 3 courses, she developed abdominal aortic lymph nodes recurrence( diameter 32×18 mm)and began to receive pembrolizumab therapy in August 2020. But the tumor re-grew 6 months after the start of this regimen. However the recurrent lesion was localized, radiation therapy(IMRT: 54 Gy/27 Fr) was added in combination with pembrolizumab. Two months later the tumor had shrunk significantly. As of June 2024, the patient has remained recurrence-free, suggesting that the abscopal effect may have been involved by immuno-radiotherapy in this case.