Small cell osteosarcoma (SCOS), a variant of conventional high-grade osteosarcoma (COS), may mimic fusion-driven round cell sarcomas (FDRCS) by overlapping clinico-radiological and... Show moreSmall cell osteosarcoma (SCOS), a variant of conventional high-grade osteosarcoma (COS), may mimic fusion-driven round cell sarcomas (FDRCS) by overlapping clinico-radiological and histomorphological/immunohistochemical characteristics, hampering accurate diagnosis and consequently proper therapy. We retrospectively analyzed decalcifed formalin-fxed parafn-embedded (FFPE) samples of 18 bone tumors primarily diagnosed as SCOS by methylation profling, fusion gene analysis, and immunohistochemistry. In eight cases, the diagnosis of SCOS was maintained, and in 10 cases it was changed into FDRCS, including three Ewing sarcomas (EWSR1::FLI1 in two cases and no identifed fusion gene in the third case), two sarcomas with BCOR alterations (KMT2D::BCOR, CCNB3::BCOR, respectively), three mesenchymal chondrosarcomas (HEY1::NCOA2 in two cases and one case with insufcient RNA quality), and two sclerosing epithelioid fbrosarcomas (FUS::CREBL3 and EWSR1 rearrangement, respectively). Histologically, SCOS usually possessed more pleomorphic cells in contrast to the FDRCS showing mainly monomorphic cellular features. However, osteoid was seen in the latter tumors as well, often associated with slight pleomorphism. Also, the immunohistochemical profle (CD99, SATB2, and BCOR) overlapped. Clinically and radiologically, similarities between SCOS and FDRCS were observed, with by imaging only minimal presence or lack of (mineralized) osteoid in most of the SCOSs. In conclusion, discrimination of SCOS, epigenetically related to COS, versus FDRCS of bone can be challenging but is important due to diferent biology and therefore therapeutic strategies. Methylation profling is a reliable and robust diagnostic test especially on decalcifed FFPE material. Subsequent fusion gene analysis and/or use of specifc immunohistochemical surrogate markers can be used to substantiate the diagnosis. Show less
