A 14-year-old male presents to your office with a chief complaint of swelling about the left knee and decreased range of motion. His parents have read about pediatric orthopaedic malignancy and are extremely anxious about this possibly being due to a malignant process.

Which of the following is the most common malignant bone tumor in children?

1. Ewing sarcoma

2. Chondroblastoma

3. Osteosarcoma

4. Metastatic neuroblastoma

Discussion

The correct answer is (C). Osteosarcoma is the most common malignant bone tumor in children, with 1,000 to 1,500 new cases per year in the United States. It is most common in the second decade of life, with a bimodal age distribution and a second peak in the sixth decade of life. While Ewing sarcoma and metastatic neuroblastoma are also seen frequently in children, they are not as common as osteosarcoma. Chondroblastoma is another lesion of childhood, however, it is not malignant.

The imaging finding in Figure 8–8 represents:

1. Aggressive periosteal reaction

2. Normal bone growth within the tumor

3. Calcification of soft-tissue mass associated with parosteal osteosarcoma

4. Healing callus at previous site of pathologic fracture

Figure 8–8

Discussion

The correct answer is (A). This characteristic imaging finding is termed “hair on end” periosteal reaction, which represents the periosteum’s reaction to an aggressive, cortically destructive lesion. The imaging finding is seen in many malignant bone tumors and is not diagnostic of any one particular entity but is usually associated with a highly malignant tumor. The malignant bone production seen in parosteal osteosarcoma is more “cloudlike” in appearance and does not demonstrate the “hair on end” or “sunburst” pattern seen here, which can be seen in more aggressive osteosarcomas. An “onion-skinning” pattern can also be seen in which wisps of bone parallel to the cortex are seen in layers. This usually represents an intermediate-aggressive process which can be thought of as the bone making attempts to wall off the process repeatedly. Ewing sarcoma can often display this appearance. Healing callus at a fracture site has the appearance of mature normal bone.

Which of the following is true regarding the imaging findings in osteosarcoma?

1. Classic osteosarcomas are cortically based and extend radially

2. In skeletally immature individuals, most osteosarcomas do not extend past the epiphyseal plate

3. When they extend into the soft tissues, this mass is not visible on x-ray

4. Osteosarcomas are characterized by direct extension, and skip lesions are not seen

Discussion

The correct answer is (B). While osteosarcomas may approach the physis in skeletally immature patients, they rarely violate it and extend into the epiphysis, although this is possible. Classic osteosarcomas originate in the medullary canal and are not cortically based. There is also a category of juxtacortical osteosarcomas, which includes both periosteal and parosteal subtypes. Osteoid matrix appears as “fluffy” or “cloud-like” opacification within the tumor’s soft-tissue mass. It is important to image the entire bone involved in osteosarcoma because skip lesions often exist which can change management significantly.

Objectives: Did you learn...?

To recognize the appearance and significance of different types of radiographing findings on malignant bone imaging?

To recognize and differentiate the subtypes of osteosarcoma?