Tumor growth. Osteosarcoma arises in bone in teenagers and young adults. Most of the time it extends outside of the bone. That outgrowth is often called the soft tissue mass. Treatment is with chemotherapy and complete removal of the cancer. A rare subtype of osteosarcoma is extra skeletal, meaning it occurs outside of bone in the soft tissues.Treatment there is also surgery and chemotherapy.
Osteosarcoma is. Usually a bony tumor, occurs in adolescence, most commonly above the knee, but any long or flat bone can be the primary site. I've not heard of soft tissue os, like there is in myeloma and ewings. The malignant timor can extend to adjacent soft tissues. These kill by metastases, more to lung than elsewhere, but local extension can influence resectability.