Multiple imagings to diagnose the chondrosseous metaplasia within a lipoma near the knee

A 61-year-old man with a slow-growing, painless mass in the area of the right knee underwent radiographic, computed tomographic (CT), arthrographic, arteriographic, and bone scintigraphic imaging studies. Scintigraphy showed an area of intense uptake in the anterolateral part of the knee; the uptake of the knee was much higher than that of the knee joints, but the area was not connected to the joint. Radiographic findings suggested an osteocartilaginous mass which was seen to contain low-density fatty tissue on the CT exam. Arthrography revealed that there was no connection of the mass to the knee joint. Arteriography showed a mildly vascularized tumor mass. Upon removal, the mass was well encapsulated, measuring 10 x 7 x 7 cm, and consisted of integrated nodules of bone, cartilage, and fat tissue. Microscopic examination confirmed lipoma with osteochondromatous metaplasia. The intense uptake in the lipoma near the bone or joint on the bone scan and multiple osteochondromatous nodules shown on CT may serve as characteristic features of the rare chondrosseous metaplasia within a lipoma.