Pathology: myoepithelial tumour

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