Extraskeletal osteosarcoma

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

Epidemiology

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.

Clinical presentation

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

Pathology

Location

• lower extremity (thigh): 50-60%

• upper extremity: 10-20%

• retroperitoneum: 10-15%

• trunk: 10-15%

Macroscopic appearance

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

Microscopic appearance

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 recognised, similar to conventional osteosarcoma: 

• osteoblastic

• chondroblastic 

• fibroblastic

• telangiectatic

• small cell

Radiographic features

Plain radiograph

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.

CT

• tumour appears to have a pseudocapsule

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

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

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

MRI

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

• T1: isointense to muscles 

• T2: hyperintense  

Hyperintense foci on both T1 and T2 sequences (due to methaemoglobin) or hypointense foci on T2 (due to haemosiderin). In extreme cases of haemorrhage, it can mimic haematoma.

Nuclear medicine

PET-CT

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

Treatment and prognosis

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 favours wide local excision of a primary tumour, 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 favours polychemotherapeutic regimens in addition to wide resection, and found 5-year overall survival rates of 77% and 66%.