Extraskeletal osteosarcoma (ESO) is a rare cancer originating in the body’s soft tissues, such as muscle and fat, completely separate from the skeleton. This malignancy is defined by cancer cells that produce bone or a bone-like substance called osteoid. Accounting for less than 1% of all soft tissue sarcomas, ESO is distinct from conventional osteosarcoma, which arises from within bone.
Causes and Associated Risk Factors
The specific trigger for extraskeletal osteosarcoma is often unknown, with most cases developing spontaneously. A well-documented risk factor is prior exposure to therapeutic radiation. These tumors can develop in the soft tissue within a previously irradiated field, often appearing many years after the initial cancer treatment was completed.
Other potential but less definitive links include significant physical trauma at the site where an ESO later develops. In these instances, the tumor’s symptoms may be mistaken for an injury, delaying diagnosis. Certain rare genetic syndromes that predispose individuals to cancer may also play a role, but this accounts for a very small fraction of cases.
Common Locations and Symptoms
The most common initial sign of extraskeletal osteosarcoma is a soft-tissue mass that grows over time. This lump may or may not be painful, especially in its early stages. As the tumor enlarges, it can cause noticeable swelling and tenderness in the affected area. Diagnosis is sometimes delayed because the symptoms can be vague or initially absent.
These tumors most frequently develop in deep soft tissues, with the thigh, upper arms, and shoulders being common locations. Another site is the retroperitoneum, the space behind the abdominal organs. The symptoms experienced depend on the tumor’s location. For example, a retroperitoneal mass might cause abdominal pain, while a tumor in a limb could decrease the range of motion.
The Diagnostic Process
Diagnosis begins with a physical examination of the mass and a review of the patient’s medical history. Medical imaging is then used to get a detailed view of the tumor. An X-ray may be performed first and can sometimes reveal cloud-like calcifications within the soft tissue, a sign that bone is being produced.
More advanced imaging, such as a computed tomography (CT) scan or magnetic resonance imaging (MRI), provides a clearer picture of the mass. These scans help determine the tumor’s precise size, location, and relationship to surrounding structures like blood vessels and nerves. This information is used to plan the next steps in treatment.
While imaging provides strong evidence, a definitive diagnosis requires a biopsy. During this procedure, a small sample of tumor tissue is removed and examined by a pathologist. The pathologist looks for malignant cells producing osteoid or bone, which confirms an ESO diagnosis. This biopsy should be performed by a team experienced with sarcomas to ensure accuracy and prevent the spread of cancer cells.
Standard Treatment Modalities
The primary treatment for extraskeletal osteosarcoma is surgery. The standard approach is a wide local excision, which removes the tumor along with a margin of surrounding healthy tissue. This ensures no microscopic cancer cells are left behind, reducing the risk of recurrence. Achieving these clean margins can be challenging with very large tumors or those near blood vessels and nerves.
Chemotherapy is often integrated into the treatment plan, using protocols modeled on those for conventional osteosarcoma. It may be given before surgery (neoadjuvant) to shrink the tumor, making it easier to remove. It can also be given after surgery (adjuvant) to target any cancer cells that may have spread elsewhere in the body.
Radiation therapy is also used with surgery to improve local control of the disease. It can be delivered before surgery to reduce the tumor’s size or, more commonly, after the operation. Post-surgical radiation is used when the tumor could not be removed with a wide margin of healthy tissue, as it helps eliminate residual cancer cells.
Factors Influencing Prognosis
Several factors influence the long-term outlook for a patient with extraskeletal osteosarcoma. The size of the primary tumor at diagnosis is a significant factor, as smaller tumors are associated with better outcomes. The depth of the tumor within the soft tissue also plays a part in the prognosis.
The success of the surgery is another determinant. Achieving clear surgical margins, where the removed tissue border is free of cancer cells, is linked to a lower chance of local recurrence. A patient’s age and overall health also influence their ability to tolerate aggressive treatments.
Whether the cancer has spread to distant sites is a primary prognostic indicator. The presence of metastasis, most commonly to the lungs, significantly affects the long-term outlook. The five-year overall survival rate for ESO is estimated to be between 37% and 52%.