Prepared by William Yaakob, MD, Department of Radiology; John Eady, MD, Department of Orthopedics; and Leigh B. Thorne, MD, Department of Pathology, Medical University of South Carolina, Charleston, SC.

CASE SUMMARY:

A 13-year-old female presented with right knee pain which had developed while she was playing basketball. Radiographs of the knee were taken at an outpatient clinic (figure 1). The patient's past medical history was noncontributory, and all relevant blood work was within normal limits. Physical examination revealed a healthy young woman with a normal gait and pattern. She was 5'5" tall and weighed approximately 95 pounds. Mild fullness of the bony contours of the anteromedial proximal tibia was found without evidence of vascular or neurologic abnormality to the right lower extremity. Subsequent imaging of the lesion consisted of a radionuclide bone scan (figure 2) and an MR exam (figure 3). What is the most likely diagnosis?

DIAGNOSIS:

Well differentiated osteosarcoma

DISCUSSION:

Well differentiated osteosarcoma is a relatively rare form of osteosarcoma, representative of approximately 1% of all osteosarcomas. These lesions are low grade neoplasms which share many prognostic similarities with parosteal osteosarcoma, though the survival rate is significantly higher than in conventional osteosarcomas. ¹

Well differentiated osteosarcomas typically affect young or middle-aged adults and are located mainly in the tibia or femur; the femur is involved in approximately 50% of cases. ²

Radiographically, these lesions may be extremely difficult to diagnose. They typically are identified as a large metaphyseal lesion. These lesions may be purely osteolytic or osteosclerotic without periosteal reaction or relatively well defined borders, which at times has led to misdiagnosis as a benign entity. Subtle radiologic features, such as areas of cortical destruction, aid in recognition of this lesion as a sarcoma. Occasionally there is a small amount of osteosclerosis in the lytic lesions, which may suggest the correct diagnosis. In advanced disease, obvious extraosseous extension of the tumor may be present.

Histologically, this lesion may be misinterpreted as a benign fibrous dysplasia. Often, this type of osteosarcoma lacks the nuclear atypia and pleomorphism of more aggressive lesions, and has a fibrous stroma. The amount of osteoid produced by the tumor is highly variable, likely leading to its variable radiographic appearance. Minimal nuclear irregularities and the capability of this tumor to penetrate among bony trabeculate indicate its malignant quality. Production of osetoid or cartilaginous loci by the neoplasm aids in differentiating it from the so-called desmoplastic fibroma.

The 5-year survival rate for well differentiated osteosarcoma is 80 to 90% when removal of the tumor is complete. ³ However, as with most sarcomas, inadequate resection at the time of initial surgery often leads to local recurrence. ^1,2 Approximately 10% of recurrent sarcomas are highly malignant, emphasizing the importance of appropriate diagnosis and accurate characterization of the extent of the disease.