A malignant tumor that begins in fibrous connective tissue at the ends of the arm or leg bones and may spread to surrounding soft tissue. Fibrosarcoma is the most common soft tissue sarcoma found in children under one year of age. It presents as a rapidly growing mass at birth or early infancy. Fibrosarcoma can also occur in older children and adults. The symptoms may include a lump, soreness, pain, or a limp (if the tumor is in the leg).