Extraskeletal Osteosarcoma

A deep, ossifying soft-tissue mass in the thigh of an older adult raises two very different possibilities, and distinguishing them is critical. Extraskeletal osteosarcoma is a rare, high-grade soft-tissue sarcoma that produces osteoid or bone but arises in soft tissue with no attachment to bone or periosteum, typically in older adults, with a poor prognosis and a high rate of lung metastasis. Myositis ossificans is a benign reactive heterotopic ossification, often post-traumatic. The key discriminator is the zoning of the ossification: myositis ossificans shows zonal maturation, with mature, well-organised bone at the periphery, the rim, and an immature, cellular centre, and it matures and stabilises over time; extraskeletal osteosarcoma shows disorganised ossification that is denser centrally - the opposite pattern - and progresses. I would assess this with MRI of the primary and radiograph or CT for the matrix and zoning, and correlate with the clinical context. Because a needle biopsy can be misleading in these lesions - for example reported as a giant cell tumour of soft tissue, with the true diagnosis only confirmed on the resection specimen - I would obtain a properly planned biopsy at a sarcoma centre with specialist pathological review, and stage the chest given the metastatic risk. If it is an extraskeletal osteosarcoma, management is wide surgical resection with clear margins as the mainstay; unlike conventional osteosarcoma, the benefit of chemotherapy is uncertain, so it is often managed like a high-grade soft-tissue sarcoma with selective chemotherapy or radiotherapy, and careful long-term follow-up is needed because of the poor prognosis. The error to avoid is in either direction - over-treating myositis ossificans as a sarcoma, or under-treating an extraskeletal osteosarcoma as benign.