When used together SS18–SSX fusion-specific and SSX C-terminus immunohistochemistry are highly specific and sensitive for the diagnosis of synovial sarcoma and can replace FISH or molecular testing in most cases

Aims: Synovial sarcoma is defined by recurrent t(X;18)(p11;q11) translocations creating SS18–SSX1, SS18–SSX2 or SS18–SSX4 fusions. Recently, a novel rabbit monoclonal antibody designed to identify these fusions (SS18–SSX, clone E9X9V) was proposed to be highly specific (100%), but not completely sensitive (95%) for this diagnosis. Another antibody designed to identify the C-terminal end of SSX (SSX_CT, clone E5A2C) was proposed to be highly sensitive (100%), but not completely specific (96%). We sought to validate these antibodies in an independent cohort. Methods and results: We performed immunohistochemistry for SS18–SSX and SSX_CT on 39 synovial sarcoma samples from 25 patients with confirmed gene rearrangements. Thirty-four (87%) and 36 (92%) were positive for SS18–SSX and SSX_CT, respectively. False-negative staining was associated with suboptimally handled small biopsies and decalcified specimens, even when staining was diffuse and strong in subsequent optimally processed excisions and non-decalcified areas. None of 580 non-synovial sarcoma tumours (76 whole sections, 504 TMA samples) were positive for SS18–SSX (100% specificity), whereas 39 (93% specificity) were positive for SSX_CT. Conclusions: SS18–SSX fusion-specific IHC is 87–95% sensitive for the diagnosis of synovial sarcoma and highly (perhaps perfectly) specific. Therefore, positive SS18–SSX staining definitively confirms the diagnosis of synovial sarcoma. SSX_CT is less specific (93–96%) but highly sensitive (92%, but approaching 100% when suboptimally processed biopsies and decalcified specimens are excluded). Negative SSX_CT staining may therefore have an ancillary role as a rule-out test for synovial sarcoma. We caution that both antibodies are prone to false-negative staining in decalcified specimens.