Rhabdomyosarcomas in adults: classification and differential diagnosis

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. In contrast, RMS is rare in adults, accounting for <1% of all malignancies. It is subdivided into three main subtypes, namely embryonal, alveolar and pleomorphic RMS. Embryonal and alveolar RMS are nearly exclusive to the paediatric population, whereas pleomorphic RMS generally affects older adults. In recent years, spindle cell and sclerosing variants of RMS have also been recognized in adults. The diagnosis of RMS depends on the confirmatory detection of skeletal muscle markers such as desmin and Myf-4 in neoplastic cells. The differential diagnosis includes small round blue cell tumours and pleomorphic sarcomas, among other entities. RMS may also be present as a heterologous component of other tumour types, such as malignant peripheral nerve sheath tumour and dedifferentiated liposarcoma. Because of the scarcity of data in the adult population, staging and treatment of adult RMS is usually based on modified paediatric protocols. However, prognosis is worse in adults when compared to children, with an overall 5-year survival of less than 30%. This review will discuss the clinicopathological characteristics of RMS in adults, the different histological variants, heterologous rhabdomyoblastic differentiation in other tumour types and differential diagnosis.