Sarcomas are a malignant type of cancer that starts in tissues like bone or muscle. Bone and soft tissue sarcomas are the two main types of sarcoma. Soft tissue sarcomas can develop in soft tissues such as fat, fibrous or deep skin tissues, blood vessels, muscle or nerves. They can appear in any part of the body, however, most start in the arms or legs. Sarcomas that start in the bones, such as osteosarcomas, are most common in children.