Parosteal osteosarcoma

Case contributed by Yasser Asiri


Mass in the distal right thigh

Patient Data

Age: 35 years
Gender: Female

Exophytic and cauliflower-like mass with dense osteoid matrix arising from the posterior lateral aspect of the distal femoral metaphysis. On the lateral view, a thin lucent line is noted separating the bulky mass from the adjacent cortex. The radiographic findings are highly suggestive of juxtacortical osteosarcoma; parosteal type. No pathological fracture seen.


The CT study demonstrates the morphology of the tumor and the epicenter of the mass at the posterolateral distal femoral metaphysis with no invasion to the medulla. There are areas of low attenuation seen peripherally within the mass which may represent chondroid tissue. 


On MRI, the mass demonstrate heterogeneous low to intermediate signal intensity on T1, T2 fat-suppressed and STIR sequences with a heterogeneous pattern of enhancement on post-contrast images.These features are likely explained by internal dense calcifications. Perilesional edema is noted. No intramedullary involvement or skip lesions. 

Case Discussion

Parosteal osteosarcoma is typically visualized as a bulky exophytic osseous mass arising from the outer periosteum. The common location is usually the posterior aspect of the distal tibia metaphysis. The string sign is commonly seen in the parosteal type which describes a lucent line between the bulky exophytic tumor and the adjacent cortex. The prognosis is usually good.

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