Pathology: Ewing sarcoma

Sheets of spindle-round cells

Scant amphophilic cytoplasm

Ovoid-round nuclei with small nucleoli and moderate pleomorphism; mitotic activity

Ectatic vessels with perivascular hyalinization

IHC: CD99

RNA-seq: EWSR1::FLI1 gene fusion

Sheets and cords of round cells

Scant amphophilic cytoplasm

Round-ovoid hyperchromatic nuclei with moderate pleomorphism; occasional mitotic activity

Collagenous stroma, with ostoid matrix

IHC: CD99, desmin, p63

RNA-seq: EWSR1::FLI1 gene fusion

Sheets of small round cells

Scant eosinophilic cytoplasm

Round-ovoid nuclei with small nucleoli and mild pleomorphism; mitotic activitiy 

IHC: CD99

RNA-seq: EWSR1::FLI1 gene fusion

Sheets-nests of small round cells

Scant eosinophilic cytoplasm

Round-ovoid nuclei with small nucleoli and mild pleomorphism; mitotic activity

Occasional thin fibrovascular septa

IHC: S100, CD99

RNA-seq: EWSR1::FLI1 gene fusion

Sheets-nests of small round-polygonal cells

Scant eosinophilic cytoplasm

Round-ovoid nuclei with vescicular chromatin, small nucleoli and mild pleomorphism; mitotic activity

Delicate thin-walled vasculature

RNA-seq: EWSR1::FLI1 gene fusion

Sheets of small round cells with occasional rosettes and pseudorosettes

Scant eosinophilic cytoplasm

Round-ovoid nuclei with vescicular chromatin, small nucleoli and mild pleomorphism; mitotic activity

Small thin-walled vasculature; necrosis

IHC: CD99

RNA-seq: EWSR1::FLI1 gene fusion

Sheets-nests of small round cells

Scant amphophilic cytoplasm

Monomorphic round-ovoid nuclei with small nucleoli; mitotic activity

Delicate branching vasculature

IHC: CD99

RNA-seq: EWSR1::FLI1 gene fusion

PMID: 21649689

Sheets and nests of epithelioid cells

Scant amphophilic cytoplasm

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

Areas of desmoplastic fibrous stroma

IHC: CD99

RNA-seq: EWSR1::FLI1 gene fusion

Sheets of small round cells with pseudorosettes and subtle rosettes

Scant eosinophilic cytoplasm

Round-ovoid nuclei with small nucleoli and mild pleomorphism; mitotic activity

Small thin-walled vasculature; necrosis

IHC: CD99

RNA-seq: EWSR1::FLI1 gene fusion

Sheets of epithelioid cells

Minimal amphophilic cytoplasm

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

Prominent thin-walled vessels with hyalinization

IHC: CD99

RNA-seq: EWSR1::FLI1 gene fusion

Small round cells in sheets-nests with occasional pseudoacini

Scant amphophilic cytoplasm

Round nuclei with small nucleoli and mild pleomorphism; inconspicuous mitotic activity

Delicate fibrovascular septa

IHC: ERG

RNA-seq: FUS::ERG gene fusion

PMID: 34747554

Nests of epithelioid cells

Scant amphophilic cytoplasm

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

Prominent hyaline stroma; ectatic thin-walled vasculature

IHC: CD99, ERG

RNA-seq: EWSR1::ERG gene fusion

Sheets and anastomosing cords of epithelioid cells

Scant amphophilic cytoplasm

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

Prominent hyaline stroma; ectatic thin-walled vasculature

RNA-seq: EWSR1::ERG gene fusion

Sheets-nests of small round-rhabdoid cells

Scant eosinophilic cytoplasm

Eccentric round-reniform nuclei with mild pleomorphism; mitotic activity

Delicate branching vasculature

IHC: synaptophysin, CD99

RNA-seq: EWSR1::FEV gene fusion