SS18-SSX Expression and Clinicopathologic Profiles in a Contemporary Cohort of Primary Paratesticular Synovial Sarcoma

Synovial sarcoma (SS) is a rare genitourinary malignancy with a specific SS18::SSX 1/2 gene fusion in majority of the instances. The paratesticular location of this neoplasm is extremely rare and only 4 cases are reported in the literature. Herein, we describe the clinicopathologic features and molecular profile of paratesticular SS in the largest case series to date and to the best of our knowledge, and the only series to use novel SS18-SSX antibody for immunohistochemistry. Clinicopathologic, immunohistochemical (IHC), molecular, treatment, and follow-up data of the patients were analyzed. There were 14 patients, ranging from 15 to 47 years (mean: 30 y). The tumor size ranged from 4​​​​​​ to 15 cm. The tumors were unilateral, solid, and homogeneous tan-white with monomorphic spindle cell histology. All 14 tumors expressed SS18-SSX and TLE1 IHC and harbored SS18 rearrangement. In addition, the tumor with multifocal SS18-SSX expression had lower break-apart signals in the FISH assay (38% of the tumor cells; range: 29% to 85%). Radical orchiectomy was performed in all 14 patients and adjuvant chemotherapy was administered in 9 patients. Follow-up was available in 9 patients. The follow-up duration ranged from 5 to 24 months (median=10 mo). Four patients died of metastatic disease (range: 5 to 16 mo) and 2 patients who are alive had metastatic disease at the last follow-up. Based on our experience with the largest series to date and aggregate of the published data, paratesticular SS has a poor prognosis despite aggressive therapy. Owing to its rarity, the differential diagnosis is wide and requires a systematic approach for ruling out key morphologic mimics aided with SS18-SSX IHC and molecular confirmation because this distinction carries important therapeutic and prognostic implications. Due to the excellent concordance of SS18-SSX IHC results with FISH results as observed in our study, we would like to suggest inclusion of SS18-SSX in the diagnostic immunohistochemistry panel of all spindle cell sarcomas where synovial sarcoma is considered as a morphologic differential. SS18-SSX-positive staining may be used as a surrogate for FISH assay in a resource-limited setting where molecular assay is not available. Furthermore, IHC has a fairly shorter turn-around-time, is less complex, and of low cost.