Expanding the Clinicopathologic Spectrum of EWSR1::SSX-Rearranged Sarcomas: Series of 11 Cases Including Osteosarcomas and a Novel EWSR1::SSX4 Fusion.

Gene rearrangements involving EWSR1 or SSX genes are known to play a role in sarcomagenesis; however, sarcomas harboring EWSR1::SSX fusions are rare. To better understand tumors associated with this distinctive genetic event, we studied 11 additional EWSR1::SSX sarcomas, affecting 10 women and 1 man with an average age of 46 years (range, 22-72 years). Eight tumors arose in the bone (rib, femur, pelvis, or vertebra with multifocal bone involvement at presentation in 3 cases); 2 tumors arose in soft tissue (deep thigh or groin); and 1 patient presented with a visceral (lung) mass. The tumors were bulky, averaging 12.1 cm (range, 4-20 cm). Histologically, in keeping with a translocation-driven sarcoma, all tumors were cytologically monotonous. Seven tumors were osteosarcomas, 6 of which were classified as sclerosing osteosarcomas with extensive bone matrix production. Three tumors were composed of uniform fascicles of spindle cells, punctuated in 2 cases by a biphasic glandular or nested epithelioid component, reminiscent of synovial sarcoma. One tumor was an undifferentiated sarcoma with round to spindle cell cytomorphology and focal osteogenic differentiation. By immunohistochemistry, 5 of 5 cases tested were positive for SSX C-terminus; 1 of 5 showed patchy weak staining with SS18::SSX fusion-specific antibody. All tested tumors were positive for CD99 (4/4) and TLE1 (3/3). Next-generation sequencing identified EWSR1::SSX fusions in all cases, involving SSX1 (n = 7), SSX2 (n = 2), and SSX3 (n = 1), along with a novel EWSR1::SSX4 fusion. Follow-up was available for 9 patients; 5 patients died of disease 1.5 to 14 years (average, 6 years) after diagnosis, 2 patients were alive with metastatic disease, 1 patient was alive without disease at 25 months, and 1 patient presented recently. Sarcomas with EWSR1::SSX fusions are rare and clinically aggressive; the histologic patterns in this series and in prior reports are heterogeneous, consisting of essentially 3 phenotypes: (1) primitive round or epithelioid cells, (2) osteoblasts that produce bone, or (3) uniform small spindle cells arranged in tight fascicles, sometimes with a biphasic epithelioid component, as seen in synovial sarcoma. By studying 11 additional examples of these rare sarcomas, we provide an expanded view of their clinicopathologic and molecular genetic spectrum.