A rapidly growing malignant mesenchymal neoplasm. It is characterized by the presence of round cells with myoblastic differentiation and a fibrovascular stroma resembling an alveolar growth pattern. The tumor usually presents in the extremities. The disease is Monarch Disease Ontology term MONDO_0009994 (alveolar rhabdomyosarcoma). Also known as: ARMS, alveolar childhood rhabdomyosarcoma, alveolar rhabdomyosarcoma (disease), alveolar rhabdomyosarcoma (morphologic abnormality), arms, monomorphous round cell rhabdomyosarcoma, paediatric alveolar rhabdomyosarcoma, pediatric alveolar rhabdomyosarcoma.