Extraskeletal myxoid chondrosarcoma: A study of 17 cases focusing on the diagnostic utility of INSM1 expression and presenting rare morphological variants associated with non-EWSR1::NR4A3 fusions.

Extraskeletal myxoid chondrosarcoma (EMC) is a rare sarcoma of uncertain lineage. Insulinoma-associated protein 1 (INSM1) has recently been described as a highly specific and sensitive immunohistochemical marker for EMC. The goal of this study was to evaluate the diagnostic significance of INSM1 immunohistochemistry in EMC. Furthermore, correlations between molecular and morphological findings were performed. Sixteen of 17 EMC cases were stained with the INSM1 antibody. Tumors with at least 5% INSM1-positive cells and any staining intensity were considered positive. Molecular testing was successfully performed in 12/17 cases. The immunohistochemical analysis detected 13 INSM1-positive (81%) and 3 INSM1-negative tumors (19%). The extent of the staining was classified as 1+ in 7 cases (44%), 2+ in 2 cases (13%), 3+ in 2 cases (13%) and 4+ in 2 cases (13%). Intensity of immunostaining was weak in 5 cases (31%), moderate in 2 cases (13%) and strong in 6 cases (38%). Molecular assays revealed 8 EWSR1::NR4A3 positive tumors (67%), 2 TAF15::NR4A3 positive tumors (17%), 1 TCF12::NR4A3 positive tumor (8%) and 1 NR4A3 positive tumor (8%) in which no other gene alteration was identified. Two of them, namely TCF12 positive and one TAF15 positive tumors, were highly cellular and partially associated with pseudopapillary architecture. Our study found that moderate/strong expression of INSM1 in more than 25% of tumor cells was present in only 31% of cases. Thus, the diagnostic utility of INSM1 is rather low. Two morphologically unique cases of non-EWSR1 rearranged EMC with an extremely rare pseudopapillary growth pattern are also reported.