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Presented by Edward McCarthy, M.D. and prepared by Orin Buetens, M.D.
Case 6: A 27 year-old woman had a painless swelling on the posterior aspect of her knee that had been growing for over a year
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Week 21: Case 6
A 27 year-old woman had a painless swelling on the posterior aspect of her knee that had been growing for over a year. Radiographically, there was a radiodensity on the posterior aspect of the distal femur with a pattern of bone formation. The lesion was irregular in contour and fixed to the surface of the bone./images/osteosarc1.jpg
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Answer: Parosteal osteosarcoma
Histology: The radiographic feature is a bony mass arising from the surface of the bone with irregular contours and irregular patterns of radiodensity. Histologically, parosteal osteosarcoma consists of anastomosing trabecular bone spicules separated by a dense fibrotic matrix that contains atypical fibroblast-like cells. These are the neoplastic cells. Atypia is mild, and it is often difficult to reconcile these histologic features with the lesion being a malignant tumor.
Discussion: A parosteal osteosarcoma is a low-grade fibroblastic osteosarcoma that arises from the surface of the bone and projects into the soft tissues. Occasionally, a small portion of the medullary canal may be involved. By contrast an osteochondroma, while being primarily composed of bone, has a continuation of its medullary canal with the medullary canal of the bone. The presence of cartilage is very abundant and distinct on the surface of an osteochondroma. Myositis ossificans may also show a soft tissue osseous mass. However, in the early stages there is a zonal pattern of radiodensity with more dense areas being on the periphery of the lesion. In the later stages of maturation, a lesion of myositis ossificans shows very mature trabecular bone. A lesion of myositis ossificans is, in its early stages, a very cellular neoplasm with many spindle cells in a tissue culture-like background. Mitotic figures are numerous. Thin spicules of osteoid are present as the lesion matures. In this stage, the lesion may be confused with a soft tissue osteosarcoma, but nuclear pleomorphism is not present. An osteochondroma consists of a hyaline cartilage cap overlying mature bony trabeculae. Occasionally islands of cartilage may be entrapped in the substance of the osteochondroma. Finally, conventional osteosarcoma with extraosseous spread demonstrates intramedullary involvement on a CT scan. Moreover, a conventional osteosarcoma shows extensive nuclear pleomorphism and abnormal mitotic figures.
Incorrect
Answer: Parosteal osteosarcoma
Histology: The radiographic feature is a bony mass arising from the surface of the bone with irregular contours and irregular patterns of radiodensity. Histologically, parosteal osteosarcoma consists of anastomosing trabecular bone spicules separated by a dense fibrotic matrix that contains atypical fibroblast-like cells. These are the neoplastic cells. Atypia is mild, and it is often difficult to reconcile these histologic features with the lesion being a malignant tumor.
Discussion: A parosteal osteosarcoma is a low-grade fibroblastic osteosarcoma that arises from the surface of the bone and projects into the soft tissues. Occasionally, a small portion of the medullary canal may be involved. By contrast an osteochondroma, while being primarily composed of bone, has a continuation of its medullary canal with the medullary canal of the bone. The presence of cartilage is very abundant and distinct on the surface of an osteochondroma. Myositis ossificans may also show a soft tissue osseous mass. However, in the early stages there is a zonal pattern of radiodensity with more dense areas being on the periphery of the lesion. In the later stages of maturation, a lesion of myositis ossificans shows very mature trabecular bone. A lesion of myositis ossificans is, in its early stages, a very cellular neoplasm with many spindle cells in a tissue culture-like background. Mitotic figures are numerous. Thin spicules of osteoid are present as the lesion matures. In this stage, the lesion may be confused with a soft tissue osteosarcoma, but nuclear pleomorphism is not present. An osteochondroma consists of a hyaline cartilage cap overlying mature bony trabeculae. Occasionally islands of cartilage may be entrapped in the substance of the osteochondroma. Finally, conventional osteosarcoma with extraosseous spread demonstrates intramedullary involvement on a CT scan. Moreover, a conventional osteosarcoma shows extensive nuclear pleomorphism and abnormal mitotic figures.
