Embryonal 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. This image shows an embryonal rhabdomyosarcoma which is composed of primitive-appearing round, spindle, or stellate cells with hyperchromatic nuclei.