Immunohistochemical validation of INI1/SMARCB1 in a spectrum of musculoskeletal tumors: an experience at a Tertiary Cancer Referral Centre.

The purpose of this study was to evaluate and validate immunohistochemical (IHC) expression of INI1/SMARCB1 in various musculoskeletal tumors in the light of the established literature. Twenty-seven cases of epithelioid sarcoma (ES); 4 of extrarenal rhabdoid tumor (ERRT) of soft tissue and 97 other tumors, including 16 cases of synovial sarcoma (SS), were evaluated for IHC expression of INI1 on formalin-fixed, paraffin-embedded tissue sections of various biopsies. Out of 128 tumors, INI1/SMARCB1 staining was completely lacking in cases of ES (23/27) (85.1%), ERRTs (4/4) (100%), myoepithelial tumors (4/14) (28.5%) and in (1/16) (6.2%) cases of SS. Fourteen out of 15 SSs displayed a reduced staining pattern. Other 67 studied tumors were INI1-positive. Sensitivity for complete INI1 negativity in ES was 85.1%, and specificity with respect to its differentials, excluding ERRTs, was 94.8%. Complete lack of INI1 immunostaining in most ESs indicates its value as a diagnostic marker for ESs, including those occurring at rare sites; in ERRTs and in some myoepithelial tumors, within an appropriate clinicopathological context, in all kinds of biopsies. ES, at least in some cases, is immunohistochemically the most closely related tumor to an ERRT. A unique pattern of reduced INI1 expression in a SS is useful during triage of some cases for molecular testing. Its expression should be interpreted in the tumor cells, rather than intermixed stromal cells and or inflammatory cells that retain INI1 expression.