S3426 A Rare Case of Pancreatic Metastatic Synovial Sarcoma

Introduction: Synovial sarcoma is a rare and aggressive soft tissue malignancy. Its pathogenesis is defined by a specific chromosomal translocation, t(X;18)(p11;q11), resulting in the SS18-SSX fusion protein, which drives tumor progression. This sarcoma typically arises in deep soft tissues near large joints, especially in the extremities, and most commonly affects adolescents and young adults (ages 15–40). Metastasis to the pancreas is exceptionally uncommon, with only a handful of cases reported in the literature. Case Description/Methods: A 40-year-old man with a history of synovial sarcoma of the left lower extremity—initially misdiagnosed as Ewing sarcoma 8 years prior and later reclassified—status post left lower extremity amputation and chemotherapy, presented with cough, hemoptysis, and unintentional weight loss. He had recently undergone rigid bronchoscopy with ablation of a vascular endobronchial tumor suspected to be metastatic sarcoma. Vital signs were stable, and laboratory studies were largely unremarkable. Computed tomography of the chest revealed a new left lower lobe consolidation. He underwent left thoracotomy with lower lobectomy and intercostal nerve cryoablation. Histology demonstrated small round blue cells, with immunohistochemistry (IHC) staining positive for SS18-SSX/SSX C-term, confirming poorly differentiated metastatic synovial sarcoma. A surveillance computed tomography scan 1 year later showed no evidence of recurrence. However, 6 months after that, a follow-up computed tomography abdomen and pelvis revealed a new 6 cm non-enhancing lesion in the pancreatic tail. Magnetic resonance imaging of the abdomen and pelvis further characterized the lesion as a solid mass measuring 4.7 × 5.3 × 4.6 cm. Computed tomography-guided biopsy confirmed metastatic synovial sarcoma to the pancreas. Positron emission tomography (PET) scan revealed no evidence of additional metastatic disease beyond the pancreatic mass. Discussion: Synovial sarcoma is an aggressive malignancy that can mimic other high-grade sarcomas histologically. Immunohistochemistry for the SS18-SSX fusion protein remains the most sensitive and specific marker for diagnosis. Most common sites of metastasis for synovial sarcoma are the lungs, followed by lymph nodes and bone. Pancreatic metastasis is exceedingly rare and makes overall management difficult. Surgical resection is the mainstay of treatment with isolated metastasis to the pancreas.