Abstract
A 61-year-old man presented with abdominal distension without any symptoms. On colonoscopy and computed tomography findings, it was clinically diagnosed as peritoneal metastasis of sigmoid colon cancer, and diagnostic laparoscopy was performed. Only the peritoneum was partially resected, and the pathology was signet ring cell carcinoma with predominantly local mucinous carcinoma component. However, the patient complained of persistent symptoms and, despite the progress of chemotherapy, the peritoneal dissemination worsened, and additional cytoreductive surgery with hyperthermic intraperitoneal chemotherapy (HIPEC) was performed. Mixed adenoneuroendocrine carcinomas (MANECs) were reported in the appendix with perforated visceral peritoneum. After additional chemotherapy, the patient was discharged. Patients with advanced MANEC with peritoneal spreading may benefit from aggressive treatment by cytoreduction surgery with HIPEC, followed by intravenous chemotherapy.