Case Author(s): Thomas H. Vreeland , 4/7/94 . Rating: #D3, #Q4

Diagnosis: Osteosarcoma

Brief history:

11 year-old boy with a one month history of right knee pain.

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Full history/Diagnosis is available below

Diagnosis: Osteosarcoma

Full history:

While playing basketball, this eleven year-old boy developed knee pain which became progressively worse over the month prior to this examination. There is no history of significant trauma or infection, although the patient has a history of intermittent fevers and mild weight loss. On physical examination there is a palpable mass at the medial aspect of the right proximal tibia. The patient was referred by an orthopedic surgeon to rule out a malignant tumor.

Findings:

Plain films of the right knee and tibia:

A mixed sclerotic/lytic lesion is seen that is felt to be most consistent with osteomyelitis or, less likely, osteosarcoma. A marker has been placed over the palpable mass.

Bone Scan:

There is markedly increased activity in the proximal 1/3 of the right tibia, with a second focus of intense activity slightly more distally. These findings are felt to be most consistent with an osteosarcoma with infectious a less likely possibility. No other abnormal foci are noted throughout the skeleton.

MRI:

A large soft tissue mass is seen within the proximal tibial metaphysis extending proximally into the epiphysis but not involving the knee joint. There is a "skip lesion" more distally in the tibial diaphysis. A break is seen in the medial cortex of the tibia with extension of the soft tissue mass which abuts but does not appear to evade the neurovascular bundle. All portions of the mass enhance with gadolinium.

CT:

There is a destructive lesion in the proximal right tibia with cortical breakthrough anteriorly and cortical tunneling medially. A small anterior soft tissue or fluid collection is seen subperiosteally. The appearance was felt to be atypical for osteosarcoma and more likely indicated osteomyelitis.

Discussion:

Initially, the referring physician felt this palpable mass represented an osteosarcoma, leading to a staging workup including CT of the chest, abdomen, and pelvis as well as an MRI and bone scan, before a tissue diagnosis was established. Difficulties arose when the unusual appearance of this lesion on CT scan with cortical tunneling and the appearance of the plain films of the knees raised the possiblility of osteomyelitis. The patient's history of fever and relatively sudden onset of this mass gave mild support to the possibility of osteomyelitis.

Followup:

Biopsy of this lesion demonstrated findings classic for osteosarcoma. The pathologic specimens suggested osteoclastic-rich osteosarcoma.

Major teaching point(s):

1. The primary use of bone scintigraphy was to assess for other metastatic lesions, not to define the type of tumor.

2. Bone scintigraphy is not the best way to distinguish osteomyelitis from osteosarcoma. However, in this example, the skip lesion (noted on both bone scintigraphy and MRI) strongly favored malignancy, especially osteosarcoma, over osteomyelitis which would otherwise have a similar appearance.

Differential Diagnosis List

Common:

1. Osteosarcoma

2. Ewing's Sarcoma

3. Osteomyelitis

ACR Codes and Keywords:

References and General Discussion of Bone Scintigraphy (Anatomic field:Skeletal System, Category:Neoplasm, Neoplastic-like condition)

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Case number: bs001

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