Incidental Disappearing Proficient Mis-Match Repair Colon Cancer in a Patient with Advanced Metastatic Melanoma and the Rare Complications of Immunotherapy.

Complete pathological response (cPR) to immunotherapy of proficient in mismatch repair (pMMR) colorectal cancer is a rare event. Here we present and discuss a case of an incidental pMMR colon cancer which had a cPR to combination immunotherapy given for the treatment of metastatic melanoma. Whilst the patient had a cPR to immunotherapy, his clinical course was complicated by secondary adrenal insufficiency from immunotherapy-related hypophysitis. This case highlights a number of emerging topics in immunotherapy and colorectal cancer, specifically the treatment and selection of pMMR tumours with immunotherapy and the balance of risk when managing oncological outcomes, side effects or immunotherapy and surgical complications.