Synchronous Malignant Peritoneal Mesothelioma and Sigmoid Adenocarcinoma: a Challenging Clinical Entity.

Background: Malignant peritoneal mesothelioma (MPM) represents a rare clinical entity. The synchronous existence of MPM with other malignancies as colonic adenocarcinoma have been rarely reported. Its diagnosis and management are challenging given its complexity and rarity. Objective: Herein, we report a case of epithelioid subtype of MPM occurring synchronously with sigmoid colonic adenocarcinoma, along with review of the literature. Case presentation: An elderly female patient was referred as case of rectosigmoid mass. She reported history of abdominal pain, per-rectal bleeding, anorexia, and significant weight loss. Her computed-tomography scan of the abdomen revealed a fistulizing sigmoid mass and multiple enlarged lymphnodes with omental nodulation. The colonoscopy revealed a large fungating mass and the endoscopic biopsies were reported as colonic adenocarcinoma. The patient was scheduled laparoscopic low anterior resection. However, the diagnostic laparoscopy revealed several nodules disseminated all over the peritoneum, suggestive of peritoneal mesothelioma. Therefore, the decision was changed to create transverse colostomy after examination obtaining multiple biopsies from the omental and peritoneal nodules. The histopathological revealed MPM and the final diagnosis was sigmoid adenocarcinoma with synchronous MPM. The patient was started on palliative chemotherapy (capecitabine) without active management of MPM because of her general condition. She was followed up with a good clinical course. Conclusion: MPM is an overlooked entity with vague clinical presentation. Synchronous MPM with colorectal cancer is rare with only few published case reports. Its diagnosis is challenging, and its management should be tailored according to the patient. This case is the first reported case in Saudi Arabia and the Middle East.