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