Abstract
This case report highlights the diagnostic challenges in distinguishing between metastatic peritoneal mesothelioma with duodenal involvement and synovial sarcoma of the duodenum, two rare and complex entities. A 59-year-old woman presented with nonspecific abdominal symptoms, and imaging revealed a heterogeneous lesion between the right hepatic lobe and duodenum. Endoscopic ultrasound-guided biopsy and subsequent histopathological analysis initially suggested synovial sarcoma, but further examination, including a FISH assay, confirmed the diagnosis of malignant peritoneal mesothelioma. This case underscores the importance of integrating detailed medical history, imaging, and advanced diagnostic techniques to achieve an accurate diagnosis in rare conditions. Early and precise identification of such diseases is crucial for appropriate therapeutic management and has significant implications for patient prognosis.