Radiology Resident Case of the Week
Etiology/Pathophysiology
The tumor develops from the periosteum and grows outward, tending to suround the bone.
Pathology:
The cellular background consists of a fibrous stroma containing osseous spicules and trabeculae. Foci of high grade conventional osteosarcoma may be present, particularly in areas of recurrence.
Miscellaneous
Parosteal Osteosarcoma occurs most frequently in the second to fifth decades of life in long tubular bones. The most common location is the posterior surface of the distal metaphyseal portion of the femur, followed by proximal tibia, fibula and humerus.
Prognosis: Best prognosis of all of the osteosarcoma with 80-90 % succussfully treated. Once the medullary space is involved, however, a more aggressive course similar to the central osteosarcoma can be expected.
Imaging
Typically, a sessile, broad based, radiodense mass with smooth lobulated or irregular margins is present. Early on, a fine radiolucent line (cleavage plane) may be seen separating the tumor from underlying bone. Ossification occurs from the base peripherally, as opposed to myositis ossificans, which ossifies at the periphery initially.
CT does not demonstrate the continuation of the medullary space into tumor, differentiating parosteal osteosarcoma from a sessile osteochondroma. MR is used to demonstrate medullary involvement, which infrequently occurs. In addition, radiolucent regions within the tumor may be present representing fibrous or cartilaginous tissue, fat, entrapped soft tissues or dedifferentiated area of high grade parosteal osteosarcoma.
DDX
myositis ossificans, sessile osteochondroma, periosteal osteochondroma (usually smaller in size, has a distinctive spiculated periosteal reaction, and is predominantly chondroblastic histiologically.
Key references
1. Bone and Joint Imaging; 2nd Edition; Resnick, D.; W.B Saunders Company, 1996; Pgs. 1006-1007.
2. Roentgen Diagnosis of Diseases of Bone; 3rd Edition; Edeiken, J.; Williams & Wilkins; 1981; Volume 1 Pgs. 209-223.
3. Radiology Review Manual, 2nd Edition; Dahnert, W; Williams & Wilkins; 1991; Pgs. 69-70.
ACR Code
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