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

Abstract

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. Falsenegative 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.

Diagnostic Immunohistochemistry of Soft Tissue and Bone Tumors: An Update on Biomarkers That Correlate with Molecular Alterations

“…In the same study, a second antibody against a conserved C-terminus region of the proteins SSX1, SSX2 and SSX4 (clone E5A2C), which represent the 3 fusion partners, showed 100% sensitivity. These results have since been corroborated by other groups and it now seems that the combination of SS18-SSX and SSX C-terminus immunohistochemistry even has the potential to replace molecular testing [38,39].…”

DNA damage‐inducible transcript 3 immunohistochemistry is highly sensitive for the diagnosis of myxoid liposarcoma but care is required in interpreting the significance of focal expression

“…The fact that a subsequent recurrence that was not decalcified showed diffuse strong DDIT3 expression makes it very likely that the decalcification was the cause of false‐negative staining. Given the experience with false‐negative staining with other transcription factors in decalcified specimens, 21 we are cautious about the reliability of DDIT3 IHC in decalcified specimens. Having carefully considered the data from our study and the study of Scapa et al ., we conclude that completely negative staining almost completely excludes the diagnosis of MLPS.…”

“…The second negative control cohort comprised sections from a tissue microarray (TMA) of unselected sarcomas. The details of this TMA have previously been reported; it comprised two 1‐mm duplicate cores from 543 individual samples from 485 patients, including a range of tumours, including leiomyosarcoma ( n = 119), atypical lipomatous tumour (ALT)/well‐differentiated liposarcoma (WD‐LPS) ( n = 97), myxofibrosarcoma (MFS) ( n = 66), undifferentiated pleomorphic sarcoma (UPS) ( n = 66), dedifferentiated liposarcoma (DD‐LPS) ( n = 40), rhabdomyosarcoma (several types, n = 23), angiosarcoma ( n = 17), synovial sarcoma ( n = 17), Ewing sarcoma ( n = 14), pleomorphic liposarcoma (Pleo‐LPS) ( n = 11), extraskeletal myxoid chondrosarcoma (EMCS) ( n = 10), and others 21,22 . This TMA cohort also included core samples from many of the cases that were also available for whole section IHC.…”

Usefulness of SS18-SSX antibody as a diagnostic marker for pulmonary metastatic synovial sarcoma

“…The standard test for the diagnosis of SS has been genomic tests to detect the SS18-SSX gene fusion by FISH or RT-PCR [13], which have reported to be highly speci c tests. However, several studies suggested the relatively low sensitivity (83-94%) of these techniques [16, 17] and their technical and cost-related issues [13]. Alternatively, IHC of transducing-like enhancer split 1 (TLE1) has been recognized to distinguish SS from other soft tissue malignancies [3,18].…”

“…Therefore, making a de nitive diagnosis of SS based only on histological ndings is di cult, and genetic con rmation of the SS18-SSX fusion by uorescence in situ hybridization (FISH) or reverse transcriptase-polymerase chain reaction (RT-PCR) has been the gold standard for the diagnosis of SS [3,4]. However, these tests are not widely available because of their high cost and time-consuming process [13].…”