Muscle cell cancer. Light micrograph of a section through a rhabdomyosarcoma, a fast-growing highly malignant (cancerous) tumour that arises from primitive muscle cells (rhabdomyoblasts). This cancer is most common in children under 10, where it accounts for over half of the soft tissue sarcomas. It is more common in boys than girls. The tumour causes a noticeable lump under the skin. In the majority of cases the tumour is in the head or neck, but can occur anywhere in the body. Treatment is with surgical removal of the tumour, often combined with radiation therapy and chemotherapy.