Alveolar rhabdomyosarcoma, light micrograph. Rhabdomyosarcoma (RMS) is a malignant soft tissue tumour that shows skeletal muscle differentiation. There is no evidence it arises from skeletal muscle cells. Many cases occur at sites with scant or no skeletal muscle tissue. RMS can occur anywhere in the body, but affects three regions more commonly - the head and neck, genitourinary tract and retroperitoneum, and upper and lower extremities. The World Health Organization recognizes four major types of rhabdomyosarcomas - embryonal, alveolar, pleomorphic and spindle cell/sclerosing type. In the alveolar subtype (shown here), the tumour cells are arranged in irregular aggregates surrounded by fibrous septa. Loss of cellular cohesion within the aggregates results in formation of alveolar spaces, hence the name.