Pathology: spindle cell / sclerosing rhabdomyosarcoma

Spindle cells in storiform-fascicular pattern

Pale eosinophilic cytoplasm

Ovoid nuclei with vesicular chromatin, and mild pleomorphism; inconspicuous mitotic activity

IHC: desmin, myogenin

RNA-seq: CITED2-rearrangement 

Spindle-epithelioid cells in fascicles

Abundant eosinophilic cytoplasm 

Round-ovoid nuclei with prominent nucleoli and moderate pleomorphism; conspicuous mitotic activity

Interspersed rhabdomyoblasts

IHC: desmin, myogenin, keratin (AE1/AE3) 

Spindle-epithelioid cells in fascicles

Abundant eosinophilic cytoplasm 

Round-ovoid nuclei with prominent nucleoli and moderate pleomorphism; conspicuous mitotic activity

IHC: desmin, myogenin, keratin (AE1/AE3), p63, ALK

RNA-seq: FUS::TFCP2 gene fusion

PMID: 29431183, 36374445

Spindle-epithelioid cells in fascicles

Abundant eosinophilic cytoplasm 

Round-ovoid nuclei with prominent nucleoli and moderate pleomorphism; conspicuous mitotic activity

IHC: desmin, myogenin, keratin (AE1/AE3) 

RNA-seq: FUS::TFCP2 gene fusion

PMID: 29431183, 36374445

Spindle cell fascicles with variable cellularity

Abundant eosinophilic cytoplasm with bipolar processes

Ovoid monomorphic nuclei; inconspicuous mitotic activity

Collagenous stroma

IHC: desmin, SMA, CD34, S100

RNA-seq: ZFP64::NCOA3 gene fusion

PMID: 35521817

Spindle cell fascicles with variable cellularity

Abundant eosinophilic cytoplasm with bipolar processes

Ovoid monomorphic nuclei; conspicuous mitotic activity

Areas of myxoid stroma, particularly around vessels

IHC: desmin 

RNA-seq: ZFP64::NCOA3 gene fusion

PMID: 35521817

Spindle cell fascicles with variable cellularity

Abundant eosinophilic cytoplasm with bipolar processes

Ovoid monomorphic nuclei; inconspicuous mitotic activity

IHC: desmin, MyoD1; (myogenin negative)

RNA-seq: ZFP64::NCOA3 gene fusion

PMID: 35521817

Hypocellular spindle cell fascicles

Abundant eosinophilic cytoplasm with bipolar processes

Ovoid monomorphic nuclei; inconspicuous mitotic activity

Areas of hyalinization, particularly around vessels

IHC: desmin, MyoD1; (myogenin negative)

RNA-seq: ZFP64::NCOA3 gene fusion

PMID: 35521817

Short spindle cell fascicles; infiltrating native skeletal muscle

Scant eosinophilic cytoplasm with bipolar processes

Ovoid monomorphic nuclei; inconspicuous mitotic activity

IHC: desmin, myogenin

RNA-seq: PPARGC1A::VGLL3 gene fusion

PMID: 35766997

Short spindle cell fascicles; infiltrating native skeletal muscle

Scant eosinophilic cytoplasm with bipolar processes

Ovoid monomorphic nuclei; inconspicuous mitotic activity

IHC: desmin, myogenin

RNA-seq: EP300::VGLL3 gene fusion

PMID: 35766997

Infiltrative

Polygonal-spindle cells with vague fascicular-storiform pattern

Eosinophilic cytoplasm

Ovoid nuclei with occasional multinucleation and moderate pleomorphism; mitotic activity

IHC: desmin, myogenin

RNA-seq: EP300::VGLL3 gene fusion

PMID: 35766997

Spindle cell fascicles

Eosinophilic cytoplasm 

Ovoid-elongated nuclei with prominent nucleoli and mild pleomorphism; occasional mitotic activity

Interspersed rhabdomyoblasts with abundant dense eosinophilic cytoplasm and multiple nuclei; occasional thin-walled and ectatic vessels

IHC: desmin, myogenin

Spindle cell fascicles

Pale amphophilic cytoplasm

Hyperchromatic ovoid-elongated nuclei with prominent nucleoli and mild pleomorphism; occasional mitotic activity

Interspersed collagenous stroma

IHC: desmin, myogenin

Spindle cell fascicles

Pale amphophilic cytoplasm with indistinct cell borders

Hyperchromatic ovoid-elongated nuclei with mild pleomorphism; occasional mitotic activity

Interspersed rhabdomyoblasts with abundant dense eosinophilic cytoplasm; multiple peripheral nuclei; myxoid-collagenous stroma; ectatic thin-walled vasculature

IHC: desmin, myogenin