Parosteal osteosarcoma with string sign

Case contributed by Matt A. Morgan


Right knee mass

Patient Data

Age: 30 years
Gender: Female

Right knee radiographs


There is a lobulated juxtacortical mass with dense calcification and smooth borders arising from the lateral cortex of the right femoral metaphysis measuring approximately 5.8 x 3.3 x 9.3 cm. There is no aggressive periosteal reaction. Superiorly, a thin radiolucent line separates the tumor from the cortex. 

Annotated images

Annotated image

A curvilinear lucency between the tumor and cortex has been called the "string sign."

Case Discussion

A parosteal osteosarcoma typically appears as a "lobulated and exophytic mass with central dense ossification adjacent to the bone" 1.

A lucency between the tumor and adjacent cortex has been called the "string sign" and "corresponds histologically to the periosteum interposed between the cortex and the tumor mass"  1.

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