Pathology: alveolar rhabdomyosarcoma

Sheets and nests of round cells

Scant eosinophilic cytoplasm

Round hyperchromatic nuclei with mild pleomorphism; conspicuous mitotic activity

Thin fibrovascular septa separate nests of cells; centrally there is necrosis yielding an "alveolar-like" pattern

IHC: myogenin

RNA-seq: PAX3::FOXO1

Sheets of discoherent round cells

Scant eosinophilic cytoplasm

Eccentric round-reniform nuclei with mild pleomorphism; conspicuous mitotic activity

IHC: desmin, myogenin

RNA-seq: PAX3::FOXO1