Solitary Malignant Mesothelioma of the Greater Omentum Mimicking Small Bowel GIST: A Rare Case Report

Solitary malignant mesothelioma arising from the greater omentum is an exceptionally rare tumor and presents significant diagnostic challenges due to its clinical and radiological similarity to gastrointestinal stromal tumors (GIST) and other intra-abdominal malignancies. We present the case of a 72-year-old man with a history of treated pulmonary tuberculosis who was incidentally found to have a pelvic mass on computed tomography (CT), initially reported as an ileal GIST. He presented without abdominal pain, weight loss, or loss of appetite. Exploratory laparotomy revealed a solitary omental tumor attached to the greater omentum near the transverse colon, with no involvement of the small or large intestines or mesentery. Histopathology initially suggested moderately differentiated papillary adenocarcinoma, but second-opinion immunohistochemistry (IHC) confirmed a well-differentiated papillary mesothelial tumor (WDPMT), with CK7 and calretinin strong positivity, and negativity for GIST markers. The patient underwent complete surgical excision with an uneventful postoperative recovery, and the patient was discharged on the sixth postoperative day. Follow-up colonoscopy and endoscopy revealed no intraluminal lesion. He remains under oncological supervision for adjuvant chemotherapy. This case underscores the importance of considering localized omental mesothelioma in the differential diagnosis of intra-abdominal masses and highlights the pivotal role of histopathology with immunohistochemistry in distinguishing rare mesothelial tumors from more common abdominal neoplasms such as GIST