Primary Localized Peritoneal Malignant Mesothelioma Involving the Stomach and Transverse Colon: A Case Report.

We report a rare case of primary localized peritoneal malignant mesothelioma (PMM) involving both the stomach and the transverse colon. A 55-year-old male presented with right upper quadrant pain and a palpable abdominal mass. Imaging revealed a large tumor invading the stomach and colon, with suspected early lymph node involvement. Preoperative biopsy results were inconclusive, and surgery was performed under the suspicion of a liposarcoma or gastrointestinal stromal tumor. The patient underwent laparoscopic subtotal gastrectomy and right hemicolectomy. Histopathological examination confirmed epithelioid-type malignant mesothelioma with no lymph node metastasis. Immunohistochemical staining was positive for calretinin, Wilms tumor 1, and cytokeratin 5/6. There was no history of asbestos exposure. The patient was discharged on postoperative day 25 and remained recurrence-free at a 9-month follow-up. Localized PMM is extremely rare and challenging to diagnose preoperatively due to nonspecific symptoms and overlapping radiological features. While diffuse PMM carries a poor prognosis and limited treatment options, localized forms may benefit from complete surgical resection. In select cases, cytoreductive surgery combined with hyperthermic intraperitoneal chemotherapy has shown promise. This case highlights the importance of considering PMM in the differential diagnosis of abdominal tumors and pursuing complete resection whenever feasible.