Light micrograph of a section of human soft tissue showing paediatric rhabdomyosarcoma or childhood rhabdomyosarcoma. This is a disease in which malignant (cancerous) cells form in the muscle tissue. Sarcoma is cancer of soft tissue, (such as muscle), connective tissue, (such as tendon or cartilage), or bone. The disease generally begins in muscles that are attached to bones that help the body move, but it may begin in many places in the body. Usually, the first sign of paediatric or childhood rhabdomyosarcoma is a lump or swelling that gradually increases in size. It is known that certain genetic conditions increase the risk of childhood rhabdomyosarcoma.