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.