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Findings: Plain films confirm the huge soft tissue mass without evidence of calcification or involvement of the bone. MRI scans show on the T1 weighted images a heterogeneous pattern with areas of increased and decreased attenuation. The mass appears to be very well circumscribed without surrounding inflammatory changes.

Diagnosis: Synovial Carcinoma.

Discussion: Surgical biopsy demonstrated a synovial sarcoma. This is a rare soft tissue tumor and even rarer to occur in a child. Described as occurring in approximately 8 to 10% of all soft tissue sarcomas in adults, approximately 30% will show calcification by plain film. Typically found close to joints, these tumors do not arise from the joint. The exact origin of these tumors is still debatable. They are assumed to be derived from primitive mesenchymal cells that have some but not all of the electron microscopic features of synovial cells. Histologically they can be either monophasic or biphasic. Glandular-like areas may also be demonstrated which have an appearance similar to epithelium. This glandular-like picture can confuse the pathologist to suggest an adenocarcinoma.

Metastases occur to the lungs, regional lymph nodes and bone. Metastatic disease may occur years after treatment of the primary tumor.

The area of increased attenuation on the T1 weighted image is thought to represent myxoid or gelatinous material that was found on excision of the lesion and can be seen very typically with synovial sarcomas. Areas of hemorrhage and necrosis are also commonly seen within the tumor. To confirm how rare these lesions are in children at Toronto Hospital for Sick Children, only seven cases were seen between 1919 and 1983.

References:
Israels SJ, et al. Synovial Sarcoma in Childhood. AJR 142:803-806, April 1984.

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