Practice Essentials
Parosteal osteosarcoma arises from periosseous tissues adjacent to the cortex. [1] It is usually considered as an osteosarcoma variant, which must be differentiated from the intracortical and periosteal variants of osteosarcoma.
Parosteal osteosarcoma is slow-growing and late to metastasize; accordingly, it carries a better prognosis than other variants of osteosarcoma. It is generally a low-grade highly differentiated tumor but has the potential to turn into a high-grade poorly differentiated tumor, even though the incidence of such conversion is very low. Often, the tumor is palpable because of the large amount of homogeneous lobulated new bone outside the bone shell extending into the soft tissue. It is juxtacortical in position and densely ossified.
More than half of the tumors arise from the posterior aspect of the distal femur in the metaphysis. The other common sites are the proximal tibia and the humerus. Rare examples of involvement of acral skeleton and craniofacial bones are also described in the literature.
Chemotherapy and radiotherapy generally are not very effective in treating parosteal osteosarcomas. Wide resection with limb salvage is the surgical treatment of choice. Gene therapy is the future for treating such cancers in the orthopedic setting; however, considerable research and development will be required before such therapy can be widely implemented. Survival rates have not significantly increased over the past several decades. [2]
Pathophysiology
Parosteal osteosarcoma occurs in the periosseous tissues. It has not been described to arise from sites other than the major tubular bones. The most commonly affected sites, in descending order, are the distal femur (typically the posterior surface), the proximal humerus, the proximal femur, the proximal tibia, and the proximal ulna. By contrast, the proximal tibia is the most common site for periosteal osteosarcoma.
Parosteal osteosarcoma has a slow rate of development; however, it eventually destroys the cortex and involves the medulla at later stages. It is a well-differentiated and late-metastasizing tumor and thus has a better prognosis.
Etiology
The exact etiology of osteosarcoma is still unknown. However, numerous chemical agents, including methylcholanthrene, beryllium oxide, and zinc beryllium silicate, have been implicated in animal models. Other causes include radiation, viruses and heredity.
Epidemiology
Parosteal osteosarcomas make up 4% of all osteosarcomas. There is no sex predilection, though some studies suggest female preponderance.
Prognosis
Because parosteal osteosarcoma is typically a low-grade tumor, it carries the best prognosis of all the osteosarcomas. For low-grade lesions, complete resection ensures cure. If high-grade areas are present in the lesion, the prognosis then approaches that of conventional high-grade osteosarcomas, and the patient requires chemotherapy in addition to surgical wide excision. [3] A study by Viola et al did not find age, sex, or tumor size to be directly related to dedifferentiation from parosteal osteosarcoma to high-grade osteosarcoma. [4]
Routine follow-up is needed to rule out local recurrence or the appearance of metastasis, even though these are rare in parosteal osteosarcoma.
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Plain radiograph showing increased opacity in lesion (parosteal osteosarcoma) in posterior aspect of distal femur (classic site).
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CT sagittal reconstruction of same lesion as in plain radiograph shown above.
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CT axial section of same lesion as in image above.
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T1-weighted MRI scan of same lesion as in plain radiograph shown above.
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T2-weighted MRI axial section of same lesion shown in image above.
