Localized biphasic malignant peritoneal mesothelioma presenting as a rectal tumor

An 84-year-old male presented with bloody stool. On digital rectal examination, a large and firm tumor was palpated in the anterior wall of the rectum at 2 cm from the anal verge. The colonoscopy revealed an ulcerated mass with smooth margins in the anterior wall of the rectum. Enhanced computed tomography showed a huge tumor in the pelvis, invading rectum, urinary bladder and the prostate, with signs of splenic and peritoneal metastases. Findings from an endoscopic biopsy and endoscopic ultrasound-guided fine needle aspiration suggested spindle cell carcinoma. We tentatively diagnosed as spindle cell carcinoma of the rectum and administered panitumumab as palliative chemotherapy. He eventually died at 4 months after the first visit to our institution. The autopsy findings resulted in the confirmed diagnosis as biphasic malignant peritoneal mesothelioma. There are only four previous reports on malignant peritoneal mesothelioma presenting as a colorectal tumor. Although rare, malignant peritoneal mesothelioma should be considered in differential diagnosis of colorectal tumors.