Extraskeletal Osteosarcoma

Extraskeletal osteosarcomas are rare, malignant soft tissue tumors that produce osteoid or bone matrix. They can be primary, or secondary to radiation, occurring 2-40 years following exposure.

Unlike osteosarcomas of bone, which tend to affect children and adolescents, extraskeletal osteosarcomas tend to affect adults, with a mean age at presentation of about 50 years. Extraskeletal osteosarcoma is rare in children, but recent data suggests that children may have a more favorable response to treatment than adults.

Like intraosseous osteosarcomas, extraskeletal osteosarcomas can be classified histologically as osteoblastic, chondroblastic, fibroblastic, telangiectatic, or small cell.

Extraskeletal osteosarcomas have a predilection for the lower extremities, followed by the upper extremity, and retroperitoneum. They are usually located deep and well-attached to surrounding tissue, but superficial or subcutaneous lesions have also been described.

Radiographs may reveal variable mineralization and a dense fluffy area of increased density, but up to half of cases have no calcification on radiographs. The mineralization pattern is distinguished from that of the much more common myositis ossificans by, which is ossified, well-defined, and peripherally sclerotic.

The adjacent bone is usually not involved in extraskeletal osteosarcoma.

CT can better demonstrate mineralization and necrosis within the lesion. MRI is nonspecific, and may reveal a poorly defined, heterogeneous lesion on T1- and T2-weighted sequences. Dynamic sequences reveal intense, early enhancement. The MRI appearance may be similar to that of giant cell tumour of the tendon sheath.

The masses are hypervascular, as revealed on angiography and nuclear medicine bone scans.

References

• Beall DP, Ly J, Bell JP, Parker EE, John CM, Babb JC, Stapp AM. Pediatric extraskeletal osteosarcoma. Pediatr Radiol. 2008 May;38(5):579-82.
• Kind M, Stock N, Coindre JM. Histology and imaging of soft tissue sarcomas. Eur J Radiol. 2009 Oct;72(1):6-15.
• Kransdorf MJ, Meis JM. From the archives of the AFIP. Extraskeletal osseous and cartilaginous tumors of the extremities. Radiographics. 1993 Jul;13(4):853-84.