CASE 62

By: Dr. Ahmad M. Aljefri M.B.B.S

HISTORY: Knee pain.

What are your findings?

What is the differential diagnosis?

What are the causes?

FINDINGS:

AP and lateral knee radiographs in a skeletally immature patient demonstrate an aggressive proximal tibial metaphyseal mass. There’s a cloudy osseous matrix.

DIAGNOSIS:

Osteosarcoma.

PEARLS AND DISCUSSION:

Osteosarcoma is a malignant bone forming neoplasm. The WHO classifies the malignant bone forming tumors into eight categories: conventional osteosarcoma, surface osteosarcoma, such as pareosteal and periosteal, and secondary osteosarcoma arising within a pre-existing bone abnormality such as Paget’s disease, radiation-induced and chronic bone infarct. Other categories include telangectatic osteosarcoma, small cell, low-grade central, high-grade surface.

Osteosarcoma is the most common primary bone tumor in children. The peak age is 10-25 years with male predominance. Another age group of peak incidences in the 60s and it is usually secondary osteosarcoma. The commonest location is in long bones distal femur, proximal tibia and humerus. 91% of cases occur in the metaphysis. On radiography, they appear as an aggressive lesion with sclerotic osseous matrix in 90% of cases. There might be soft tissue mass with cortical destruction containing tumor osteoid matrix. Codman’s triangle is aggressive form of interrupted periosteal reaction. Other forms of aggressive periosteal reaction when tumor osteoid is deposited in the Haversian canals giving arise to the characteristic sunburst periosteal reaction where linear calcifications are noted perpendicular to the bone cortex. Skip metastasis can occur in the same or adjacent bone. Bone scan is usually sensitive to detect skip metastasis if the lesions are more than 1 cm. Nodal metastasis might occur and usually contains the osseous matrix. Pulmonary osteosarcoma metastasis is usually calcified and might be complicated by pneumothorax either spontaneous or after chemotherapy.

CT scan can better delineate the radiographic features and it is usually reserved for preoperative planning if limb sparing surgery is being contemplated. Lung CT scan is sufficient for systemic staging along with bone scan.

MRI is reserved for local staging and biopsy planning. On MRI, the osteoid matrix is low signal on all sequences. The soft tissue mass is heterogeneous and shows enhancement on post contrast image. The necrotic components of the soft tissue mass are non-enhancing and must be avoided in percutaneous biopsies. In our experience, the best diagnostic yield is obtained from the enhancing soft tissue component. The mature osseous matrix and necrotic components are better to be avoided.

Treatment usually involves a preoperative adjuvant chemotherapy and surgical resection. After chemotherapy, the tumor might demonstrate maturation of the osseous matrix and the soft tissue component might increase in size. The tumor becomes more sclerotic as the soft tissue component decreases in size.

FURTHER READING:

1. Greenspan A. Orthopedic imaging, a practical approach. Lippincott Williams & Wilkins. (2004) ISBN:0781750067.

2. Yarmish G, Klein MJ, Landa J et-al. Imaging characteristics of primary osteosarcoma: nonconventional subtypes. Radiographics. 2010;30 (6): 1653-72.

3. Greenspan A. Orthopedic imaging, a practical approach. Lippincott Williams & Wilkins. (2004) ISBN:0781750067.