A Thrombosed Popliteal Artery Aneurysm Masquerading as a Benign Soft-Tissue Mass: A Case Report Emphasizing the Role of Multimodal Imaging.

Popliteal artery aneurysms (PAA) lose their pulsatility once thrombosed and may therefore be mistaken for a Baker cyst or a soft-tissue tumor. A 58-year-old male presented with a 6-month history of progressive right popliteal swelling and pain, accompanied by right foot paresthesia and diminished distal pulses for two months. The presence of accompanying neurovascular deficits shifted the diagnostic consideration from a benign soft-tissue lesion to a vascular disorder. In this patient, non-contrast imaging protocol integrating grayscale ultrasound, power Doppler, and magnetic resonance imaging (MRI) successfully identified a chronically thrombosed popliteal artery aneurysm, supporting a vascular rather than neoplastic etiology. While ultrasound cannot replace contrast-enhanced MRI for definitive tissue characterization, its real-time capability established a vascular etiology and guided subsequent diagnostic digital subtraction angiography with urgent endovascular stent-graft reconstruction. Furthermore, serial ultrasonographic surveillance at three and 18 months confirmed long-term stent-graft patency and favorable aneurysmal sac remodeling. Neurovascular deficits accompanying a popliteal mass should redirect the workup toward vascular imaging. This case underscores a critical diagnostic pitfall, demonstrating that a stepwise strategy combining real-time ultrasound and non-contrast MRI ensures rapid definitive management for complex popliteal masses while preserving the unique role of sonography in non-invasive follow-up.