Extraskeletal osteosarcoma (ESOS) is a rare mesenchymal malignant tumour that occurs in the retroperitoneum and soft tissue of extremities without any attachment to bone.
ESOS in contrast to other subtypes of osteosarcoma occurs infrequently in individuals under 40 years of age, most commonly occurring in individuals 40 to 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 ESOS. The role of trauma in development of ESOS is unclear.
Patients most commonly present with an enlarging soft-tissue mass either with or without pain
Grossly, it is a well defined lesion with haemorrhagic and necrotic areas within. Microscopically ESOS is typically a high grade spindle cell malignancy with osteoid and chondroid matrix. The histologic appearance of ESOS resembles malignant fibrous histiocytoma, osteoblastic osteosarcoma and chondroblastic osteosarcoma.
Five pathologic subtypes of ESOS are recognized, similar to conventional osteosarcoma:
- small cell
- lower extremity (thigh): 50-60%
- upper extremity: 10-20%
- retroperitoneum: 10-15%
- trunk: 10-15%
ESOS appears as soft tissue density with variable amount of calcification which represents osteoid matrix formation, and is seen in ~ 50% of cases.
- tumour 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 haemorrhagic and necrotic areas within)
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.
ESOS 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 ESOS. 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 the primary tumour, including limb salvage surgery for extraskeletal osteosarcoma of the extremity.
ESOS is insensitive to chemotherapy and radiotherapy. Recent retrospective evaluation of patients with ESOS favour more polychemotherapeutic regimens in addition to wide resection, and found 5-year overall survival rates of 77% and 66%.
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