Case of the Month: April 2008

Answer: B. Giant Cell Tumor of Bone

• Conventional radiographs of the knee demonstrate a lucent lesion in the medial femoral condyle with minimal reactive bone and a narrow zone of transition.
• MRI of the knee demonstrates T1 hypointensity and T2 isointensity to muscle with multiple round areas of T2 hyperintensity predominantly at the margins.

Differential Diagnosis:

• Giant Cell Tumor
• Chondroblastoma
• Clear Cell Chondrosarcoma

Diagnosis: Giant Cell Tumor of Bone

Discussion: Giant cell tumor of bone (osteoclastoma) is a rare lesion though to arise from osteoclasts. These tumors most commonly present in young adults and almost invariably involve the epiphysis, with extension to the subchondral cortex and into the metaphysis. Approximately 50% of tumors are found in the knee, with other long bones and the sacrum also commonly involved.

Radiology: Typical radiographic appearance is a lytic tumor of long bone which is well defined with a nonsclerotic margin and a narrow zone of transition. Additionally, these tumors are commonly eccentrically located and abut an articular surface. Epiphyses are invariably closed. These tumors are often expansile and may have cystic blood-filled regions similar to aneurysmal bone cysts. Rarely, lesions erode from the epiphysis into the joint cavity and provoke synovitis. Approximately 10-30% of patients present with pathologic fracture due to cortical thinning. CT or MRI may be required to show the extent of tumor and the relationship to the adjacent joint. While this is a benign lesions, rare instances of pulmonary and cutaneous metastases have been described. Malignant transformation is exceedingly rare.

Treatment: While these are benign tumors with minimal malignant potential, they do abut articular surfaces and erode adjacent cortex. Cortical thinning predisposes to pathologic fracture. Typical treatment includes currettage and packing with methylmethacrylate. The reported rate of recurrence is 10-25%.

Key Points

• Eccentrically located, juxtaarticular lytic lesion with nonsclerotic margin and narrow zone of transition which presents in patients with closed epiphyses.
• Predominantly involves the knee, although other long bones and the sacrum are also commonly affected.
• Predisposes to pathologic fracture in 10-30% of cases.
• Treatment includes currettage and packing.

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