Eight year old girl with back of the right knee mass.
Heterogeneous uptake in a mass located in the deep soft tissue behind the right proximal tibia (SUVmax 3.6).No bone involvement.
Focal uptake in the distal diaphyso-metaphyseal junction of the left femur representing probable non-ossifying fibroma.
No other suspicious lesions.
Malignant nerve sheath tumor
Synovial sarcoma is a rare neoplasm but the most common non-rhabdomyosarcomatous malignant soft tissue sarcoma in children and teenagers. This tumor is thought to arise from primitive mesenchymal cells rather than from the synovial membrane. It does not arise in an intra-articular location but usually occurs near joints. The extremities are the common primary site with the popliteal fossa of the knee being the most frequent.
Metastatic disease mostly occurs in the lung, followed by bone and lymph nodes. Liver metastases are rare with tumors located in extremities.
On CT, common presentation is a heterogeneous soft tissue mass with attenuation similar to or slightly lower than muscle. Lower attenuation areas may represent hemorrhage or necrosis. Sometimes those lower attenuation areas predominate and the lesion appears cystic. Smaller lesions may look more homogeneous. Bone involvement, either as erosion or marrow invasion, can be seen. Calcifications may also be present.
In children, synovial sarcoma can show a nonaggressive appearance and may mimic a benign process, making it challenging to arrive at the correct preoperative diagnosis.
Prognostic role of PET/CT FDG
Various prognostic factors have been reported in synovial sarcoma. Two studies suggested that SUVmax on pre-treatment FDG PET/CT was found to be a prognostic factor of overall survival with cut-off SUVmax of 4,35 and 6,1 respectively. Chang and al. also reported poorer overall survival with higher metabolic tumor volume and higher total lesion glycolysis.
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Bixby SD et al: Synovial sarcoma in children: imaging features and common benign mimics. AJR Am J Roentgenol. 195(4):1026-32, 2010
Lisle JW et al: Risk assessment based on FDG-PET imaging in patients with synovial sarcoma. Clin Orthop Relat Res. 467(6):1605-11, 2009
O'Sullivan PJ et al: Radiological features of synovial cell sarcoma. Br J Radiol. 81(964):346-56, 2008
Murphey MD et al: From the archives of the AFIP: Imaging of synovial sarcoma with radiologic-pathologic correlation. Radiographics. 26(5):1543-65, 2006
Chang KJ et al : The Role of (18)F-FDG PET/CT as a Prognostic Factor in Patients with Synovial Sarcoma. Nucl Med Mol Imaging. 2015 Mar;49(1):33-41