S5265 Beyond the Pancreas: Retroperitoneal Sclerosing Epithelioid Fibrosarcoma Presenting as a Pancreatic Mass With Osseous Metastases

Introduction: Fibrosarcoma is a rare malignant tumor of spindle-shaped fibroblasts, typically found in deep soft tissues of the extremities and trunk. Retroperitoneal Fibrosarcoma with osseous metastases is extremely rare and can be a challenging diagnosis. This report discusses a rare case of retroperitoneal metastatic fibrosarcoma mimicking a pancreas mass and identified via endoscopic ultrasound (EUS) guided biopsy. Case Description: A 48-year-old man with no significant medical history presented with 1 year history of chronic right shoulder pain, mild abdominal discomfort and unexplained weight loss. Magnetic resonance imaging (MRI) of his shoulder showed questionable malignancy due to bone marrow lesions. A subsequent positron emission tomography (PET) scan revealed a FDG-avid retroperitoneal peripancreatic mass and a suspicious lesion in the right humeral head, concerning for metastasis. Further imaging via computed tomography (CT) of the abdomen confirmed an ill-defined 5.7 × 8.6 × 6 cm heterogeneous mass in the retroperitoneal area, suspicious for pancreatic adenocarcinoma, affecting surrounding organs and vasculature. EUS revealing a hypoechoic mass greater than 5 cm in the peri-pancreas region. An EUS-guided biopsy of this mass showed spindle cell neoplasm and a CT-guided bone biopsy of the humeral lesion showed spindle cells with sclerosis indicated sclerosing epithelioid fibrosarcoma through specific immunohistochemical staining. The case was reviewed in a tumor board and confirmed retroperitoneal fibrosarcoma, leading to the initiation of treatment with pazopanib at 800 mg. After 2 months of treatment, a follow-up PET scan revealed metabolic improvement in the retroperitoneal mass, but new metastatic lung nodules had developed. Discussion: This case highlights the diagnostic utility of EUS guided biopsy in evaluating retroperitoneal masses, which may often mimic pancreatic adenocarcinoma. Clinicians should consider rare tumors like sclerosing epithelioid fibrosarcoma in the differential diagnosis of retroperitoneal masses especially when associated with atypical metastatic pattern.