Based on the history, physical examination, imaging studies, and histologic picture, what is the diagnosis and how should the lesion be treated?
See page 276 for diagnosis and treatment.
Continuation of ORP Conference from page 275.
The resection specimen consisted of the posterior portion of the distal femur. A 7.0 × 4.0 × 4.0 cm mass arose from the posterior cortex of the femur. The mass was circumscribed sharply from the adjacent skeletal muscle, but extended into the medullary cavity. After sectioning the specimen, the intramedullary extent of the tumor is apparent and correlates well with preresection imaging (Fig 8). The cut surface of the mass appeared heavily ossified and contained several gray chondroid nodules centrally. Microscopically, the tumor consisted of an anastomosing network of relatively mature bone, which in some areas had a pagetic appearance (Fig 6). Osteoblastic rimming was present on the surface of some trabeculae. The intervening fibrous stroma were moderately cellular and contained bland spindled cells with rare mitotic activity (Fig 7). Scattered islands of bland hyaline cartilage were present within the central portion of the tumor.
Parosteal osteosarcoma, low-grade, Musculoskeletal Tumor Society Stage IB9
DISCUSSION AND TREATMENT
Parosteal osteosarcoma, or low-grade juxtacortical osteosarcoma, is a rare cancer and accounts for only 3% of all osteosarcomas.5 These tumors have been recognized for more than 50 years and are well described in the literature,2-4,6,8,11,12,19 as are the difficulties in diagnosing them. Parosteal osteosarcoma is a surface lesion composed of low-grade fibroblasts that deposit woven or lamellar bone and is, therefore, a less aggressive form of osteosarcoma.16 On average, patients presenting with parosteal osteosarcoma are older than patients with conventional osteosarcoma. This lesion is diagnosed most often in patients in their third and fourth decades of life. The female-to-male ratio of patients with this lesion is 2:1. The most common location is the distal posterior femur (70-80% of cases); the second most common site is the tibia. No clear etiology has been determined, but a few case reports have been linked to radiation exposure.5 The clinical presentation is usually a chronic, painless mass or swelling which prevents the patient from flexing the involved knee.4,15,17 Uncommonly, pain is a symptom. The duration of symptoms before diagnosis is considerably longer than for conventional osteosarcoma.
Radiographically, the lesion grows on the cortex of bone outward in an exophytic fashion. There may be a plane that separates the neoplasm from the cortex. Parosteal osteosarcomas do not elevate the periosteum because they grow on its surface.7,13-15,17,20,23 The mass typically is denser in the center than at the periphery. The grade of the neoplasm correlates to the radiodensity of the lesion. Radiolucent lesions are frequently of a higher grade and tend to recur more than the lower-grade, radiodense lesions.3 There is no concurrent radiographic evidence of periosteal new bone formation.5 These features can be seen more clearly in cross-sectional CT and MR images than in plain radiographs. Computed tomography and MRI are also useful in assessing extension into the medullary cavity. Previous studies have shown intramedullary extension in 8 to 59% of cases, with higher-grade lesions having more frequent intramedullary involvement.17,21 The presence of satellite nodules in the soft tissue may indicate a recurrent lesion. A notable finding in imaging studies is lucent areas at the periphery of the lesion, which may be a sign of the development of large cartilaginous areas that can be indicative of a less differentiated lesion, such as a high-grade lesion.3,5
Imaging characteristics alone usually are diagnostic for bony surface lesions. Biopsy of these lesions is controversial, but when done, should be directed at the areas of radiolucency. Either an incisional or CT-guided core biopsy can be used to rule out a high-grade lesion.
On gross examination, the lesion is a firm, exophytic, polypoid, bony mass attached to the cortex of a long bone by a broad base. Histologically, it contains spindle-shaped fibroblastic cells with well-differentiated trabeculae of bone.5,22 The fibroblasts look deceptively benign because they show minimal atypia. With effort, mitoses also may be found microscopically. As stated earlier, there may be a cartilage cap associated with the neoplasm. The diagnosis of parosteal osteosarcoma is difficult because of the well-differentiated appearing fibroblasts and mature tumor bone. Consequently, the neoplasm can be labeled as a benign reactive lesion5 and misdiagnosed histologically as fibrous dysplasia.
Benign sessile osteocartilaginous exostoses, or osteochondroma, are among the lesions most frequently mistaken for parosteal osteosarcoma on plain radiographs. Computed tomography and MRI can be used to distinguish the two lesions. Cross-sectional imaging studies typically will show an intact cortex beneath a parosteal osteosarcoma. Parosteal osteosarcoma lacks the continuity of the cortex between the native bone and the lesion and the direct connection between the intramedullary cavities of the lesion and the underlying bone. These findings are pathognomonic for an osteochondroma. Unlike osteochondromas, the cartilage found in parosteal osteosarcoma has atypical chondrocytes and does not exhibit the orderly enchondral ossification pattern seen in the osteochondromas.5
Myositis ossificans may be precipitated by direct trauma to soft tissue, or the patient may present without previous injury (such as pseudomalignant myositis ossificans). Myositis ossificans is seen most commonly in the thigh musculature. The resulting hemorrhagic fibrotic mass tends to occur within the muscle belly. This may evolve into a florid and diffuse ossification approximately 6 months later. After 12 months, a mature bony zone appears at the periphery of the lesion with a central lucency. When the lesion is deeper in the thigh, it may involve the periosteum of the femur. This lesion, called parosteal myositis ossificans, may mimic parosteal osteosarcoma. The pathology of the tissue will yield peripheral mature bone and central fibrosis not seen in a parosteal osteosarcoma.16
Periosteal and high-grade surface osteosarcomas differ radiographically and histologically from parosteal osteosarcoma. Periosteal osteosarcomas look more aggressive and cause an elevation of the periosteum forming a Codman’s triangle or sunburst pattern on plain radiographs. They also are more delicate and feathery in appearance and tend to contain large areas of cartilage which are radiolucent. Parosteal osteosarcomas differ histologically from high-grade surface osteosarcomas by grade. However, if parosteal osteosarcomas dedifferentiate, then such high-grade areas may be found within the low-grade cellular morphology.5
Extraosseous osteosarcoma is a malignant mesenchymal neoplasm located in soft tissues. It does not have a direct attachment to bone, but characteristically produces osteoid, bone, or chondroid material.1 Extraosseous osteosarcoma has clinical and pathologic features distinct from myositis ossificans. It has been suggested that extraosseous osteosarcoma might be similar in clinical behavior to osteosarcoma of bone origin and should be treated the same.1,10
Treatment for parosteal osteosarcoma is wide surgical resection, with an overall survival rate of 80% to 90%.8,15-17,19,21 No adjuvant therapy is necessary with low-grade lesions. In patients with metastases, the tumor usually has dedifferentiated to a high-grade lesion and adjuvant therapy may be added to the treatment regimen. Okada et al17 reported their institution’s experience with 226 patients (67 patients treated at their institution and 159 patients treated through consultations) with parosteal osteosarcoma. Eighty-two percent of these patients were classified as low grade (Grade 1 of 3), 18% were classified as intermediate grade (Grade 2 of 3), and 16% of their patients had associated dedifferentiated tumors and a poorer prognosis. Medullary involvement was reported in 28% of the patients. Medullary involvement was found more frequently with higher tumor grades (18% in patients classified as Grade 1; 25% in patients classified as Grade 2; and 43% in patients with dedifferentiation). However, medullary involvement alone was not associated with a poorer prognosis. Eleven of the 67 patients died from their disease; 10 of these patients had a dedifferentiated tumor.17 Local recurrence occurred more frequently with incomplete resection, and an increased risk of developing metastatic disease was associated with dedifferentiated tumors. The increased frequency of medullary invasion in the dedifferentiated form of parosteal osteosarcoma is likely the reason why previous studies have reported medullary involvement as a poor prognostic factor.
A more recent study by Temple et al21 had similar results. Their study reviewed the clinical outcome of 38 patients with previously untreated, low-grade parosteal osteosarcoma. Twelve lesions were considered dedifferentiated because of minor areas of high-grade tumor. All patients had enbloc resection and were alive after an average followup of 6.75 years. The data suggested that negative margins were sufficient in controlling the local disease and preventing metastasis.
The patient in the current presentation was treated by wide surgical resection of the tumor and preservation of the anterior femur (Fig 9). The limb was reconstructed with a hemicortical osteoarticular allograft fashioned to fit the resected area. Standard plate-and-screw fixation was used for stabilizing the allograft (Fig 10). The posterior cruciate and lateral collateral ligaments were reconstructed. No adjuvant therapy was given because of the low-grade features of parosteal osteosarcoma.
The authors thank Joanne Clarke for her editorial assistance with this manuscript.
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