Peritoneal Recurrence of Cecal Cancer with Specific Imaging Findings and Shrinkage after Treatment with Pembrolizumab

Pembrolizumab is one of the treatment options for treatment-refractory unresectable advanced or metastatic colorectal cancer with microsatellite instability-high (MSI-H) or deficiencies in DNA mismatch repair (dMMR). Herein, we report a case in which a recurrent cecal cancer lesion showed specific imaging findings and local inflammatory findings during treatment with pembrolizumab, followed by marked shrinkage. The patient was an 80-year-old woman. Postoperative peritoneal recurrence of cecal cancer of approximately 7 cm in size was observed. The patient had MSI-H and was treated with pembrolizumab. After five courses of treatment, the patient presented to our hospital with a chief complaint of abdominal pain. A blood test showed a strong inflammatory reaction, and computed tomography (CT) showed diffuse low-density area in the tumor. Under the suspicion of an abscess, conservative treatment was initiated and the patient quickly recovered. A CT at 1 month showed a marked reduction in size at the same site, and a CT at 3 months showed that the recurrent foci had almost disappeared. The inflammatory reaction before shrinkage in this case may have been caused by tumor immune response to pembrolizumab.