Diagnosis
Osteosarcoma.
Discussion
Osteosarcoma is the most common primary malignant bone tumor in the adolescent and young adult population, and can have a heterogeneous clinical and radiographic picture at presentation. One of the most common presenting scenarios is a painful mass in the region of the tumor. Approximately half of all osteosarcomas occur around the knee joint [1]. Presentation with a pathologic fracture is rare, but well described, and occurs in approximately 10% of osteosarcomas [1]. Magnetic resonance imaging, computed tomography, and nuclear imaging are commonly used to augment the plain radiograph for diagnostic and staging purposes, as well as surgical planning [1]. Most osteosarcomas are diagnosed after cortical breakthrough with obvious clinical and radiographic findings.
Typically, osteosarcoma presents as a destructive, mixed lytic and sclerotic lesion with cortical destruction and soft tissue extension. There have been reports of purely lytic osteosarcomas as well as other radiographic variants. Early clinical detection of osteosarcoma can lead to subtle plain film findings and cause delay in diagnosis or the incorrect interpretation of the lesion as benign. Several cases of early osteosarcoma with subtle plain radiographic findings have been described previously [2]. Some of these were missed initially, but on retrospective review after the diagnosis was made, radiographic abnormalities were noted. In this case, the initial radiographs were read as normal and showed only a subtle periosteal reaction better seen on the sagittal CT reconstruction. MRI showed a stress fracture through a well-defined lesion. Biopsy of the lesion revealed osteosarcoma.
Both stress fractures and bone tumors present with similar clinical features. Stress fractures in children are relatively common in the athletic population and are usually the result of overuse injury [3]. The most common site of juvenile stress fractures is the tibia. The most common presenting symptom is pain with activities. Roughly 50% of pediatric stress fractures occur in athletes [3]. Tenderness is usually present over the involved area. They can be unapparent early on plain radiographs. Later, they are characterized by periosteal new bone formation, cortical osteolysis, and osteosclerosis along the fracture plaine [3]. MRI and CT have been shown to be valuable in the imaging of stress fractures when the history, physical examination, and plain radiograph are insufficient to make the diagnosis. In this case the plain radiographic changes are very subtle. Features typical for stress fracture were seen on CT and MRI.
An adolescent with knee pain and suspected stress fracture or tumor should be evaluated with plain radiographs. Absence of radiographic abnormality is distinctly rare in osteosarcoma; however, subtle radiographic changes only seen retrospectively have been described. As in this case if no diagnosis can be made by plain films, then further imaging is necessary. As the use of MRI increases, discovery of subtle or unapparent lesions on plain radiography may continue to increase. Aggressive imaging and follow up for suspicious musculoskeletal complaints is warranted in the pediatric age group. Biopsy is indicated for indeterminate lesions. An aggressive approach can lead to early diagnosis and treatment, and potentially a more favorable prognosis.
References
1. Sweet DE, Madewell JE, Ragsdale BD. Radiologic and pathologic analysis of solitary bone lesions. Part I: Matrix patterns. Radiol Clin North Am 1981; 19(4): 785-814.
2. Walker RN, Green NE, Spindler KP. Stress fractures in skeletally immature patients. J Pediatr Orthop 1996; 16: 578-584.
3. de Santos LA, Edeiken BD. Subtle early osteosarcoma. Skeletal Radiol 1985; 13(1): 44-48.
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