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Skeletal Case of the Day

A 39-year-old woman had an abnormality noted on a bone scan obtained to assess back pain. The radiologic and clinical features of this case nicely illustrate the differences between myositis ossificans, osteochondroma, and surface osteosarcomas. Often, patients with myositis ossificans give a history of trauma, although in many cases the trauma may have been forgotten. Radiologically, myositis ossificans has a more matune (ossified) periphery with a nadiolucent center [1 , 2]. Osteochondromas are benign neoplasms typically arising from the metaphyses of long bones. The penosteum, cortex, and marrow are continuous with that of the underlying bone. These lesions stop growing after skeletal maturity. Any growth after this time should suggest possible malignant degeneration [1 , 2]. This patient had a biopsy-proved parosteal osteosarcoma, one of three types of surface osteosancomas. In addition to parosteal osteosarcoma, surface osteosarcomas also include peniosteal osteosarcoma and high-grade conventional osteosarcoma. Surface high-grade conventional osteosarcomas are identical to conventional osteosancomas histologically and in terms of prognosis; the only distinction between the two is location. Peniosteal osteosarcomas are rare lesions that most often involve the diaphysis of long bones, especially the tibia. These tumors arise juxtacortically and typically have a “hair on end” appearance with spicules of bone perpendicular to the cortex. The medulla.ry cavity is typically uninvolved. Clinically, patients tend to be somewhat older than those with conventional osteosarcoma, and the prognosis is better [1, 2]. Parosteal osteosarcomas also occur in older patients and have a better prognosis than do conventional osteosancomas. They are more common than peniosteal osteosarcomas but still represent only 3-4% of all osteosarcomas. The location of the tumor in this patient was typical; approximately two thirds of these osteosancomas involve the posterior distal femoral metaphysis (Figs. 1A-i C). These lesions are typically large, lobulated, and contain a densely sclerotic matrix with the more mature elements located centrally (Fig. 1 D). Parosteal osteosarcomas are often extremely large when detected and can grow to encircle the underlying bone. Finally, as our case demonstrates, the tumor may be relatively asymptomatic [1,2].