Gastrocutaneous Fistula in a Patient with Locally Recurrent MSI-High Colorectal Cancer: Local Complications Arising from Therapeutic Response to Immune Checkpoint Blockade.

Colorectal cancers with high microsatellite instability (MSI-H) have distinct clinical features in terms of their prognosis, recurrence patterns, and sensitivity to immunotherapeutic agents. We present the case of a woman with a left-sided MSI-H colon cancer who had repeated recurrences concentrated exclusively in the left upper quadrant of the abdomen, including gastric involvement. Despite multiple surgical resections, radiation, and several lines of chemotherapy, her disease eventually eroded through the chest wall. Treatment with an immune checkpoint inhibitor produced a rapid clinical response with significant tumor necrosis; however, this necessitated surgical debridement that ultimately led to a large gastrocutaneous fistula. This case highlights the importance of recognizing locoregional tumor-associated complications that may result from robust therapeutic responses to immuno-oncology drugs, which are increasingly being used in clinical practice today.