(P 348) SYNOVIAL SARCOMA INVOLVING NERVES: HIGH RATE OF MISIDENTIFICATION AS BENIGN TUMORS

Objective: Synovial sarcoma can present as a slow-growing mass in the extremities, frequently mimicking benign or less aggressive conditions. Its nonspecific clinical and radiographic features can lead to diagnostic confusion, particularly with peripheral nerve sheath tumors. Misidentification may result in delayed diagnosis and potential compromise of oncologic outcomes. This study aims to characterize cases of primary synovial sarcoma involving peripheral or central nerves, evaluate the clinical and radiologic features contributing to misdiagnosis, and assess the impact on treatment and outcomes.

Methods: A retrospective review was conducted of all patients diagnosed with synovial sarcoma of the extremities, pelvis, or trunk at a single institution between 1994 and 2022. Patients with synovial sarcomas involving or initially thought to arise from nerves were identified for subgroup analysis. For these patients, data regarding initial diagnosis, misdiagnosis, initial treatment approach, and time to definitive oncologic management were recorded.

Results: A total of 173 patients were diagnosed with synovial sarcoma of the extremities, pelvis, or trunk. Of these, 41 tumors (24%) involved a peripheral nerve (n=38) or a nerve root (n=3). The mean age of patients was 40 years at diagnosis. Most commonly, the tumor arose in the lower extremity (n=20, 49%). At the time of presentation, 15 (37%) were misdiagnosed as a benign tumor, most commonly a schwannoma (n=6). Thirteen of 15 patients were presumed to have a benign diagnosis based on imaging, and two patients had an initial needle biopsy diagnosis of a schwannoma and a desmoid tumor (Table 1). Among patients with nerve involvement, misdiagnosis was associated with a higher rate of non-oncologic excision (OR 13.3, 95% CI [2.80, 63.4], p< 0.001) but not disease-specific survival (DSS) (p=0.99). Thirteen of the 15 patients (87%) required repeat wide local excision or amputation for oncologic margins. None of the misdiagnosed patients had a local recurrence. Among patients with nerve involvement, tumors that were misdiagnosed were more likely to be smaller than 5 cm (OR 0.13, 95% CI [0.03–0.58]) and were less frequently treated with chemotherapy (OR 0.19, 95% CI [0.04–0.86]).

Conclusion: A high proportion of patients with synovial sarcoma involving nerves were misdiagnosed as benign conditions, with a majority receiving subsequent non-oncologic surgical interventions that may compromise surgical margins and local control.