Small poorly differentiated synovial sarcoma with concentric periarterial growth causing popliteal arterial stenosis: a case report

Soft-tissue sarcomas arising in the popliteal fossa are uncommon but clinically important because of their close relationship to major neurovascular structures. Poorly differentiated synovial sarcoma is an aggressive histologic subtype of synovial sarcoma. We report a case of a small poorly differentiated synovial sarcoma located in the perivascular region of the popliteal fossa that showed concentric periarterial growth with popliteal arterial stenosis. A 68-year-old woman reported a several-month history of heaviness in the right lower extremity. Ultrasonography revealed a 2-cm irregular hypoechoic solid mass circumferentially surrounding the popliteal artery, with no definite internal Doppler flow. The lesion showed intermediate signal intensity on T1-weighted images and heterogeneous intermediate-to-high signal intensity on fat-suppressed T2-weighted images. MRI showed circumferential arterial encasement with stenosis. Non-contrast-enhanced quiescent-interval slice-selective (QISS) MRA helped define the longitudinal extent of popliteal arterial narrowing. Surgical resection required vascular resection and reconstruction bypass of the popliteal artery and vein. Histopathological examination showed nested proliferation of tumor cells with ovoid-to-round nuclei in a hyalinized fibrous stroma. Immunohistochemistry showed positivity for SS18-SSX and SSX, supporting the diagnosis of synovial sarcoma. This case illustrates that even a small synovial sarcoma in the popliteal fossa can circumferentially encase the popliteal artery and cause stenosis.