ABSTRACT
A lveolar rhabdomyosarcoma (ARMS) is one subtype o f rhabdomyosarcoma (RMS), a heterogeneous family o f soft tissue tumors that generally occur in pediat ric patients and are related to the skeletal muscle lineage.1 Recognition o f this subtype (that has no relationship to the lung despite the descriptor “alveolar”) is primarily based on histopathologic criteria that permit ARM S to be distinguished from the other major child hood RMS subtype, embryonal rhabdomyo sarcoma (ERMS). Though ARM S and ERM S were initially established as histopathologic entities, these tumor categories are associated with distinct clinical behaviors. ARM S presents mainly in adolescents and young adults, often occurs in the extremities and trunk, and is asso ciated with an unfavorable prognosis.2 The unfavorable prognosis o f the ARM S subtype is related to the propensity for early and wide dissemination, often involving bone marrow, and to the poor response to chemotherapy. In contrast to the typical clinical picture o f ARM S, ERM S mainly presents in children less than 10 years old; predominantly occurs in the head and neck, genitourinary tract and retroperitoneum; and is associated with a favorable prognosis.1,2
Diagnosis o f ARM S is often complicated by a paucity o f features o f striated muscle dif ferentiation and the relatively subtle histologic
criteria for distinguishing ERM S and ARM S.3 The problem is compounded by the fact that a variety o f pediatric solid tumors including RM S, neuroblastoma, Ewings sarcoma and non-Hodgkins lymphoma can present as col lections o f poorly differentiated cells. The presence o f subtle evidence o f myogenic dif ferentiation can be detected by staining these tumors with immunohistochemical reagents specific for muscle proteins (such as M yoD, desmin, myoglobin, or muscle-specific actin) as well as electron microscopic examination for myofilaments. However, there is no well-estab lished immunohistochemical or ultrastructural marker that will distinguish ARM S and ERMS.
