A 62-year-old man presented with shunt failure manifesting as consciousness disturbance 4 years after placement of a ventriculoperitoneal shunt for subarachnoid hemorrhage. Physical examination found subcutaneous pneumocele around the peritoneal catheter extending from the abdomen to the neck. He had undergone pelvic radiation therapy for bladder cancer 2 years before. The peritoneal catheter was removed from the cervical region, and external ventricular drainage and a descending colon stoma for ileus release were positioned. The cerebrospinal fluid was clear and yielded no cultures. No inflammatory changes were seen. He developed carcinomatous peritonitis and died 4 months later. Retrograde colon gas reflux due to catheter perforation into the colon occluded by metastatic sigmoid cancer was probably the cause. Fragility of the wall of colon associated with the prior abdominal radiation therapy might have been a contributing factor. Subcutaneous pneumocele around the peritoneal catheter, i.e. pneumocele within the fibrous sheath surrounding the catheter, is a differential diagnosis to cerebrospinal fluid collection in patients with subcutaneous swelling around the catheter.