Unplanned Excision of Synovial Sarcoma: Factors Associated with Recurrence and Survival.

: Synovial sarcoma is rare and may present as a small, slow-growing mass. These tumors are often mistaken as benign and are therefore prone to unplanned and/or non-oncologic excision. We sought to identify the rate of unplanned excision of synovial sarcoma and risk factors for recurrence and survival among this cohort. : The medical records of 246 patients evaluated at a single institution for synovial sarcoma between 1997 and 2022 were retrospectively reviewed. Of these, 87 (35%) underwent unplanned, non-oncologic excision. The mean age of the cohort was 49 years. Primary tumors were located in the extremity (n = 63), abdomen (n = 6), thorax (n = 7), head/neck (n = 8), and paraspinal region (n = 3). The median maximum pre-treatment dimension of the primary tumor was 4.8 cm (IQR 7-2.4). Seventy-seven (86%) patients underwent re-excision of the tumor bed, 39 (45%) received chemotherapy, and 63 (72%) received radiation therapy. : Among patients who underwent unplanned excision, local recurrence-free survival (LRFS) was 98% at 1 year and 82% at 5 years. Metastasis-free survival (MFS) was 91% at 1 year and 72% at 5 years. Disease-specific survival (DSS) was 98% at 1 year and 72% at 5 years. When adjusting for tumor size, tumors which underwent unplanned excision did not have worse recurrence or survival compared to those which had planned excision ( > 0.10). Size > 5 cm, monophasic subtype, and axial location were associated with increased risk of disease recurrence. Forty-six patients had residual tumor following re-excision, which was associated with worse MFS (HR 8.17, 95% CI [1.89, 35.2], < 0.01) and DSS (HR 7.66, 95% CI [1.76, 33.4], < 0.01). Patients who received radiotherapy had improved MFS (HR 6.4, 95% CI [1.42, 29.0], = 0.02) and DSS (HR 5.86, 95% CI [1.27, 26.9], = 0.02). : One-third of patients presenting with synovial sarcoma were diagnosed after unplanned, non-oncologic excision. Patients with large, axial tumors had worse survival. Approximately half of patients who underwent unplanned excision had no residual tumor after pre-operative radiation. The use of radiation was associated with decreased rates of recurrence and improved disease-specific survival. Our results suggest that margin-negative re-resection and radiotherapy should be considered when feasible following unplanned excision of synovial sarcoma.