(P 338) POORLY DIFFERENTIATED SYNOVIAL SARCOMA: A HISTOLOGIC CHALLENGE OF AN AGGRESSIVE VARIANT

Objective: Poorly differentiated synovial sarcoma (PDSS) is a more aggressive variant of synovial sarcoma (SS). Histologic features differ from that of conventional SS; leading to incorrect diagnosis. We aimed to study histologic and molecular features of PDSS and highlight their differences from conventional SS.

Methods: 47 patients diagnosed with PDSS were identified amongst a total of 300 synovial sarcomas from a period of 8 years (2018-2025). Retrospective analysis of histologic features was performed and compared to a subset of conventional SS. Immunohistochemistry and molecular profiles were documented.

Results: Age range of 47 patients was 12 to 64 years, median being 39 years. Male: Female ratio was 1.2: 1. Commonest site of involvement was soft tissue of lower extremity (52%). Morphologically, 11% tumours contained exclusive round cell component. Other 15% showed partial areas of malignant round cells in combination of conventional spindle cell pattern. Only 2% tumours showed biphasic morphology. Necrosis was present in 60% of tumours. Brisk mitoses (> 10/ 2mm2) were noted in 50%. Poor differentiation was seen in recurrent SS in 5 cases. 22% patients experienced distal metastasis. On immunohistochemistry (IHC), all PDSS were positive for TLE1, AE1/AE3 and EMA. Immunohistochemistry for SS18 was performed in 30 % of cases, all of which were positive. Molecular translocation for SS18-SXX was done in 10 % cases, all of which showed positive results

Conclusion: Poor differentiation is a poor prognostic parameter in SS. PDSS has morphologic deviation from the conventional variant, leading to diagnostic challenges. We could demonstrate round cell morphology in a significant proportion of tumours mimicking Ewing sarcoma, lymphoma and rhabdomyosarcoma. Presence of necrosis and brisk mitoses were not found in conventional SS, thus being important diagnostic parameters for PDSS. TLE1 and epithelial markers aided the diagnosis; however IHC for SS18 was far more specific and sensitive, negating the need for molecular testing in most cases.
Thus recognition of PDSS is relatively easier if conventional morphology co-exists; otherwise high index of suspicion with use of SS18 IHC preferably with molecular testing is required.