Extraskeletal osteosarcoma

Last revised by Joshua Yap on 7 May 2024

Extraskeletal osteosarcoma (ESOS) is a rare mesenchymal malignant tumor that occurs in the retroperitoneum and soft tissue of extremities without any attachment to bone.

Extraskeletal osteosarcoma, in contrast to other subtypes of osteosarcoma, occurs infrequently in individuals <40 years of age, most commonly occurring in individuals 40-80 years of age with a median age of presentation of 61 years. Males are more commonly affected than females. Radiation exposure is a well-documented risk factor for the development of extraskeletal osteosarcoma. The role of trauma in development of extraskeletal osteosarcoma is unclear.

Patients most commonly present with an enlarging soft-tissue mass either with or without pain.

  • lower extremity (thigh): 50-60%

  • upper extremity: 10-20%

  • retroperitoneum: 10-15%

  • trunk: 10-15%

Grossly, it is a well defined lesion with hemorrhagic and necrotic areas within.

Microscopically, extraskeletal osteosarcoma is typically a high grade spindle cell malignancy with osteoid and chondroid matrix. Its histologic appearance resembles malignant fibrous histiocytoma, osteoblastic osteosarcoma and chondroblastic osteosarcoma.

Five pathologic subtypes of extraskeletal osteosarcoma are recognized, similar to conventional osteosarcoma: 

  • osteoblastic

  • chondroblastic 

  • fibroblastic

  • telangiectatic

  • small cell

Extraskeletal osteosarcoma appears as soft tissue density with a variable amount of calcification which represents osteoid matrix formation, and is seen in ~50% of cases.

  • tumor appears to have a pseudocapsule

  • calcification is best appreciated on CT rather than plain film or MRI

  • extensive central mineralization and lack of peripheral well-defined ossification distinguishes ESOS from myositis ossificans 

  • heterogeneous post contrast enhancement (due to hemorrhagic and necrotic areas within)

Well-circumscribed heterogeneous mass lesion with presence of hemorrhage and necrotic areas

  • T1: isointense to muscles 

  • T2: hyperintense  

Hyperintense foci on both T1 and T2 sequences (due to methemoglobin) or hypointense foci on T2 (due to hemosiderin). In extreme cases of hemorrhage, it can mimic hematoma.

Extraskeletal osteosarcoma shows intense peripheral FDG uptake with central photopenia due to central necrosis and hemorrhage.

Metastasis and local recurrence are quite common with extraskeletal osteosarcoma. Up to 90% patients develop metastases at the time of presentation which makes the overall prognosis poor. Most common site for distant metastasis is lungs followed by bone, lymph nodes, liver, peritoneum, adrenals, and rarely brain.

The initial treatment strategy favors wide local excision of a primary tumor, including limb salvage surgery for extraskeletal osteosarcoma of the extremity.

Extraskeletal osteosarcoma is insensitive to chemotherapy and radiotherapy. A retrospective evaluation of patients with extraskeletal osteosarcoma favors polychemotherapeutic regimens in addition to wide resection, and found 5-year overall survival rates of 77% and 66%.

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