Unfortunately, this sarcoma has usually spread by the time of diagnosis. Lymphangiosarcoma Stewart-Treves Syndrome Lymphangiosarcoma, or Stewart-Treves Syndrome, is a malignant tumor originating from the endothelial cells of lymphatic vessels. It is the most common type of soft tissue sarcoma that develops in children. Cancerous tumors in the skeletal muscles are one of the most common types of soft tissue sarcoma found in children. It is one of the more common types of soft tissue sarcoma. When there is a tumor seen at the surgical margin, radiotherapy is often added. Often, the cell of origin cannot be determined with certainty; hence, the more Multiple malignant tumors in skeletal muscles adult term angiosarcoma is used.