The Many Faces of Osteosarcoma

Abstract Introduction Primary Intramedullary Osteosarcoma

Surface Osteosarcoma Extraskeletal Osteosarcoma Secondary Osteosarcoma

Conclusions Source Information References

Surface Osteosarcoma

Surface lesions account for 4%-10% of all osteosarcomas (1,2,3,4,66,67,68,69,70,71). Some authors prefer to group all these osteosarcomas into a single surface or "juxtacortical" category. However, we prefer to distinguish intracortical, parosteal, periosteal, and high-grade surface lesions because of differences in radiologic and pathologic appearances as well as in treatment and prognosis.

Intracortical Osteosarcoma

Intracortical osteosarcoma is the rarest form of osteosarcoma, and the term applies to those cases in which the lesion arises within the cortex. It was originally described by Jaffee (72) in 1960 in a report of two cases, and through 1991 only nine cases had been published (72,73,74,75,76). The lesion is histologically characterized as a sclerosing variant of osteosarcoma, which may contain small foci of chondrosarcoma or fibrosarcoma (73,74,75).

Although it is difficult to make generalizations on the basis of such a small number of cases, the typical lesion shows geographic bone lysis (with small areas of mineralized osteoid), is intracortical; occurs in the femur or tibia; and measures less than 4 cm in diameter (Figure 12) (73,76). The tumor margin may be remarkably well defined with thickening of the surrounding cortex, and medullary invasion is only rarely reported (73,76).

Although long-term data are lacking, Mirra and coworkers (73) reported that in a review of seven patients, five were disease-free after 3-19 years of follow-up. One patient died of metastatic disease and another had inoperable metastases. This high rate of metastasis (29%) may be sufficient to justify adjuvant chemotherapy.

Parosteal Osteosarcoma

Parosteal osteosarcoma accounts for 65% of all juxtacortical osteosarcomas and is thought to originate from the outer layer of periosteum (66,67,68,69,70,71,77,78,79,80,81,82,83). These lesions usually affect patients in the 3rd and 4th decades of life and show a slight female predilection. Clinical symptoms frequently include a palpable mass. Parosteal osteosarcomas affect the metaphyseal region of long bones (80%-90% of cases), most frequently the posterior distal femur (50%-65%) (Figure 13) (77,82). Other commonly involved regions are the proximal humerus, tibia, and fibula.

Parosteal osteosarcomas are frequently low-grade lesions (Figure 14), as suggested in the original description in 1951 by Geschickter and Copeland (78), who used the term parosteal osteoma. However, these large, lobulated parosteal lesions contain higher-grade regions in 22%-64% of cases and may demonstrate invasion (back growth) into the medullary canal (8%-59%) (66,67,70,77,80,82). Fibrous stroma and extensive osteoid are the predominant histologic characteristics, although smaller foci of cartilage are also frequent.

The radiologic appearance of parosteal osteosarcoma is often characterized by a large, lobulated (cauliflower-like), ossific (denser centrally), juxtacortical mass (Figure 13, Figure 14). Initially, only a narrow zone (stalk) of attachment to the cortex may be apparent, creating a partial radiolucent cleavage plane (Figure 13, Figure 14) between the lobulated ossific mass and the remaining bone (68,79). However, continued tumor growth often obliterates the cleavage plane. Cortical thickening without aggressive periosteal reaction may be seen. CT and MR imaging can demonstrate both the soft-tissue extent and evidence of medullary involvement (Figure 13b). The ossified regions show high attenuation on CT scans and low signal intensity on all MR images, regardless of pulse sequence. In addition, Jelinek and coworkers (79) have recently shown that nonmineralized soft-tissue components larger than 1 cm³ identified at CT or MR imaging correspond to high-grade foci (Figure 13). Parosteal osteosarcomas may be confused both pathologically and radiologically with myositis ossificans (84). However, in contradistinction to parosteal osteosarcoma, myositis ossificans is denser peripherally and is usually not attached to the cortex.

Prognosis for patients with parosteal osteosarcoma is excellent, with 5- and 10-year survival rates of 80%-90% (77,82). Detection of higher-grade foci may alter therapy, and presurgical biopsy should be directed toward these sites. Higher-grade parosteal osteosarcomas may warrant neoadjuvant chemotherapy. Lesions of higher grade have been called dedifferentiated parosteal osteosarcoma, although we reserve this term for those lesions that contain a second tumor cell line (often fibrosarcoma or malignant fibrous histiocytoma) (83). The presence of intramedullary back growth has previously been reported to imply a worse prognosis. More recent studies suggest that this finding does not change the overall excellent prognosis of patients with parosteal osteosarcoma (79,80,83). However, it remains important to identify medullary extension to ensure adequate surgical resection during limb-salvage operations (Figure 13).

Periosteal Osteosarcoma

Periosteal osteosarcoma accounts for 25% of all juxtacortical osteosarcomas and was originally described in 1976 by Unni and coworkers (66,69,85). The age group affected is similar to that for conventional osteosarcoma (patients in the 2nd and 3rd decades of life). However, unlike conventional osteosarcoma, these lesions show a strong propensity to arise in the diaphysis (Figure 15) or metadiaphysis of bone. The most commonly affected sites are the femur and tibia (85%-95% of cases), followed by the ulna and humerus (5%-10%) (85,86,87).

Periosteal osteosarcomas are intermediate-grade lesions that arise from the deep layer of periosteum and cause cortical scalloping typically without intramedullary invasion (85). Pathologic assessment demonstrates a highly chondroblastic lesion with smaller areas of osteoid formation (Figure 15).

The radiologic appearance of periosteal osteosarcomas is usually characteristic and distinctive from that of parosteal lesions. The surface of the thickened diaphyseal cortex is scalloped, with perpendicular periosteal reaction extending into a broad-based soft-tissue mass (Figure 15a) (86,87). Solid (cortical thickening) or aggressive (Codman triangle) periosteal reaction may also be apparent at the upper and lower margins of the lesion. CT and MR imaging show similar findings, with the highly chondroblastic areas corresponding to relatively low-attenuation regions on CT scans, low-signal-intensity areas on T1-weighted images, and very high-signal-intensity areas on T2-weighted MR images (Figure 15b, Figure 15e, Figure 15f) (86,87). In our recent study, we demonstrated that these lesions usually involve approximately 50% of the osseous circumference and that the perpendicular periosteal reaction is seen as rays of low signal intensity with all MR pulse sequences (87). Marrow invasion is rare and, when seen, is directly continuous with the surface mass. Marrow invasion should be distinguished from reactive marrow changes (Figure 15b), which appear as foci of marrow replacement (low signal intensity on T1-weighted MR images and high signal intensity on T2-weighted or inversion recovery images) adjacent to but noncontiguous with the surface mass, as described in more than 50% of cases by Murphey and coworkers (87).

Prognosis for patients with periosteal osteosarcoma is improved compared with that for patients with conventional osteosarcoma, but it is not as good as that for patients with parosteal lesions. Metastatic disease leads to patient death in 8%-16% of cases (85,88). Surgical treatment is usually wide local excision with an associated limb-salvage procedure.

High-Grade Surface Osteosarcoma

High-grade surface osteosarcoma is rare and accounts for 10% of all juxtacortical osteosarcomas (69,89). These lesions have a high predilection to involve the diaphysis of bone and most commonly affect the femur, humerus, and fibula. Pathologically and prognostically, high-grade surface osteosarcomas are identical to conventional intramedullary lesions (69). Radiologically, these lesions are similar in appearance to periosteal osteosarcoma (Figure 16) (69,89). However, in our experience, high-grade surface osteosarcomas often involve the entire circumference of bone and frequently invade the medullary canal.

Abstract Introduction Primary Intramedullary Osteosarcoma

Surface Osteosarcoma Extraskeletal Osteosarcoma Secondary Osteosarcoma

Conclusions Source Information References