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<strong>Background:</strong> Synovial sarcoma is an aggressive soft tissue sarcoma. It has a wide spectrum of histopathologic patterns and uncertain immunohistochemistry, rendering it a diagnostic challenge. The objective of this study was to investigate the diagnostic utility and impact of SS18-SSX rearrangement evaluation in an Iranian population previously diagnosed with synovial sarcoma without such molecular tests.<br /> <strong> Method:</strong> We conducted this cross-sectional study on 44 formalin-fixed paraffin-embedded tissue blocks obtained from 23 synovial sarcoma patients (males, 69%; mean age, 36.4 years) and 11 cases with other neoplasms as negative controls. We assessed these specimens for SSX-SS18 gene rearrangement by break-apart fluorescent in situ hybridization (FISH) probes.<br /> <strong>Results:</strong> FISH study showed SS18-SSX fusion gene in 17 (73.91%) cases while six (26.09%) cases and negative controls did not show SS18-SSX fusion. Histopathologic type of tumor was significantly related to the presence of rearrangement (P=0.002) (rearranged: 11 biphasic and six monophasic tumors; non-rearranged: three monophasic and three poorly-differentiated tumors).<br /> <strong>Conclusion:</strong> This study supports the idea that molecular studies contribute to the confirmation of synovial sarcoma diagnosis, particularly in monophasic and poorly-differentiated subtypes, which show vague immunohistochemical results.