Epithelioid cells in nests and cords
Eosinophilic cytoplasm
Ovoid nuclei with visible nucleoli; moderate pleomorphism; conspicuous mitotic activity
Collagenous-myxoid stroma
RNA-seq: EWSR1::KLF15 gene fusion
Fascicles of spindle-epithelioid cells
Pale amphophilic cytoplasm
Ovoid-angulated nuclei with small nucleoli and mild atypia; inconspicuous mitotic activity
Areas of hyalinized stroma
IHC: smooth muscle actin, S100, keratin (AE1/AE3)
RNA-seq: EWSR1::PBX3 gene fusion
Fascicles of spindle-epithelioid cells
Pale amphophilic cytoplasm
Ovoid nuclei with small nucleoli and mild atypia; inconspicuous mitotic activity
Focal cystic degeneration
IHC: S100, smooth muscle actin, epithelial membrane antigen
RNA-seq: EWSR1::PBX3 gene fusion
Infiltrative; cellular spindle cell neoplasm with vague storiform-fascicular pattern
Eosinophilic cytoplasm
Round-ovoid nuclei with prominent nucleoli and mild atypia; brisk mitotic activity
Occasional thin and thick-walled vessels, osteoclast-type giant cells, haemosiderin-laden macrophages, and lymphocytes
RNA-seq: EWSR1::PBX3 gene fusion
Spindle-epithelioid cells in fascicles and cords
Pale amphophilic cytoplasm
Round-ovoid-angulated nuclei with small nucleoli and mild atypia; inconspicuous mitotic activity
Intervening bands of hyalinized stroma; occasional thick-walled ectatic vessels with hyalinized walls
IHC: calponin, SATB2
RNA-seq: EWSR1::PBX3 gene fusion
Infiltrative; spindle cell fascicles
Pale amphophilic cytoplasm with delicate bipolar processes
Ovoid-angulated nuclei with small nucleoli and mild atypia; inconspicuous mitotic activity
Intervening bundles of thin collagen fibres; focal myxoid stroma; vessels with hyalinized walls; occasional plugs of lymphocytes in lymphovascular spaces
RNA-seq: EWSR1::POU5F1 gene fusion
Cords and nests of epithelioid cells
Abundant eosinophilic cytoplasm; areas with clear cytoplasmic vacuolation
Round-ovoid nuclei with mild atypia; inconspicuous mitotic activity
Intervening broad bands of hyalinized stroma
RNA-seq: FUS::KLF17 gene fusion
Circumscribed; spindle-stellate cells with fascicular-storiform pattern
Pale amphophilic cytoplasm with delicate tapering processes
Ovoid-angulated nuclei with small nucleoli and mild atypia; occasional mitotic activity
Intervening myxoid-collagenous stroma; long wavy thin-walled vessels
IHC: S100, SOX10, keratin (AE1/AE3)
RNA-seq: IRF2BP2::CDX1 gene fusion
Circumscribed; short spindle cell fascicles
Pale amphophilic cytoplasm
Ovoid-angulated nuclei with small nucleoli and mild atypia; inconspicuous mitotic activity
Interspersed thick collagen bands; prominent hyalinized vessels
RNA-seq: EWSR1::POU5F1 gene fusion
Infiltrative; short fascicles of spindle-ovoid cells
Pale amphophilic cytoplasm
Ovoid nuclei with frequent clefts, small nucleoli and mild atypia; mitotic activity
Background of plump spindle cells with more abundant eosinophilic cytoplasm; microcystic degeneration
RNA-seq: EWSR1::ZNF444 gene fusion