Low grade central osteosarcoma

Last revised by Joachim Feger on 6 May 2023

Low-grade central osteosarcomas (LGCOS) or well-differentiated intramedullary osteosarcomas are low-grade malignant bone-forming tumors typically arising within the intramedullary cavity of bone.

Low-grade central osteosarcomas are uncommon and account for 1-2% of all osteosarcomas 1-4. Most tumors are seen in young adults with a peak in the third decade of life 1-3. Women are slightly more commonly affected 1.

The diagnosis of low-grade central osteosarcomas is based on a combination of typical radiographic features, location, histology and immunohistochemistry. Due to similar features to fibrous dysplasia, the diagnosis is often difficult, even in the setting of an experienced multidisciplinary meeting 2.

Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5th edition) 1:

  • imaging features of a bone tumor

  • location within the medullary cavity

  • fibroblastic bone-forming tumor with mild nuclear atypia and well-formed neoplastic trabeculae

MDM2 amplification is a desirable criterion.

Typical symptoms include swelling and/or pain 1 lasting from several weeks to several years at the time of presentation 1-4.

Low-grade central osteosarcomas consist of fibroblastic tumor cells with mild nuclear atypia and well-formed neoplastic osseous trabeculae and originate from the medullary cavity 1,2.

The etiology of low-grade central osteosarcomas is unknown 1.

Low-grade central osteosarcomas are often found in the metaphyses or metadiaphysis of long bones 1 particularly the femur and tibia 1-3. They frequently extend to the end of bones 7. Rare locations are the jaws, small tubular bones of the hand and feet and the axial skeleton 6.

Grossly tumors are characterized by the following features 1-4:

  • fairly well-circumscribed margins

  • firm rubbery fibrous consistency

  • endosteal erosion of the cortex

Microscopically, low-grade central osteosarcomas display the following histological features 1-4:

  • interlacing fascicles of spindle cells with mild to moderate cellularity

  • embedded in a fibrosclerotic background

  • mild nuclear atypia

  • woven or lamellar typically long and thick bony trabeculae usually in a parallel arrangement

  • possible presence of pagetoid bone

  • possible cartilage formation

  • focal lack of osseous matrix in some cases

In cases of differentiation, features resemble the histology of conventional osteosarcoma, undifferentiated pleomorphic sarcoma or fibrosarcoma and might lack osteoid formation 1.

On immunohistochemistry stains low-grade central osteosarcomas often express MDM2 and/or CDK4 as do high-grade sarcomas that result from their dedifferentiation 1, which can be used in the differentiation to its benign mimics 8.

Because fibrous dysplasia and central low-grade osteosarcoma are so similar histologically, the radiographic features are an extremely important part of the diagnosis. Radiographic features of low-grade osteosarcomas are variable.

  • the most common pattern is as a large intracompartmental expansile lytic fibro-osseous lesion with coarsely thick or thin incomplete trabeculations 1,7

  • a less common pattern is a dense sclerotic lesion 7,9

  • cortical erosion/destruction and soft tissue extension are also common features 1,7

The radiological report should include a description of the following:

  • form, location and size

  • tumor margins and transition zone

  • periosteal reaction

  • cortical erosion, cortical breakthrough, soft tissue extension

  • bimorphic appearance

  • skip metastases

  • distant metastases

Treatment consist in wide resection and then low-grade central osteosarcoma has a good prognosis with overall 5-year and 10-year survival rates of >90% and >80% and a chance of metastases <5%. Recurrences happen after curettage and incomplete excision. Dedifferentiation carries a worse prognosis and such tumors are also treated with chemotherapy 1.

Low-grade central osteosarcomas were first described as ‘intraosseous well-differntiated osteosacroma’ in 1977 by the American pathologist Krishnan K Unni and his colleages David C Dahlin, Richard A McLeod and Douglas J Pritchard from the Mayo Clinic 11,12.

Differential radiologic diagnoses include benign fibro-osseous lesions such as 4,8,13:

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