Utility of characteristic ‘Weak to Absent’ INI1/SMARCB1/BAF47 expression in diagnosis of synovial sarcomas

Recently, very few studies have shown value of immunohistochemical (IHC) expression of INI1/SMARCB1 in diagnosis of synovial sarcomas (SSs). This study was aimed at testing reproducibility and utility of this finding. Sixty‐eight SSs and 147 other tumours, in the form of various biopsies, were tested for IHC expression of INI1. Twenty‐six SSs were further confirmed with positive SS18 rearrangement. Forty monophasic spindle cell type (58.8%), 13 biphasic (19.1%), 12 poorly differentiated (17.6%) and three calcifying SSs (4.4%) were positive for epithelial membrane antigen (EMA) (46/62) (74.1%), pan cytokeratin (AE1/AE3) (31/47) (65.9%), cytokeratin (CK7) (20/31) (64.5%), BCL2 (62/66) (93.9%), MIC2 (61/63) (96.8%), transducin‐like enhancer of split 1 (TLE1) (29/31) (93.5%) and CK19 (14/24) (58.3%). INI1 expression was ‘weak to absent’ in 60/68 (88.2%) SSs; in 1/3 atypical ossifying fibromyxoid tumours (AOFMTs) and in 3/10 (30%) malignant peripheral nerve sheath tumours (MPNSTs) of various types. INI1 was completely absent in 10/10 (100%) epithelioid sarcomas (ESs), 4/4 (100%) malignant rhabdoid tumours, single paediatric undifferentiated sarcoma, 5/19 (26.3%) myoepithelial carcinomas and in 2/4 (50%) epithelioid‐subtype of MPNSTs. Remaining 100 tumours, including 12 Ewing sarcomas, 15 carcinomas, eight solitary fibrous tumours (SFT), seven extraskeletal myxoid chondrosarcomas, three fibrosarcomas and other tumours retained INI1 expression. A unique ‘weak to absent’ IHC expression of INI1 is highly sensitive (88.2%) and specific (97.3%) for a SS, irrespective of its subtypes and types of biopsies. This can be considered useful in diagnosing SSs, especially in settings lacking molecular and/or cytogenetic analysis. A similar INI1 expression is shared by certain AOFMTs and MPNSTs.