Aneurysm of a Supra-Articular Branch of the Popliteal Artery Mimicking a Soft-Tissue Sarcoma: A Case Report

We report an unusual case of a partially thrombosed aneurysm arising from the descending genicular artery, a supra-articular collateral branch of the popliteal artery, in an 80-year-old man, whose clinical and radiological features initially mimicked a soft-tissue sarcoma. The patient was referred for image-guided biopsy of a left popliteal mass that had been progressively enlarging over nine months and had become bothersome during walking. Magnetic resonance imaging (MRI) revealed a well-circumscribed, heterogeneous lesion with hypointense areas on T1- and PD-weighted images, initially interpreted as a fibrous component suggestive of a sarcoma. A pre-biopsy Doppler ultrasound subsequently demonstrated intralesional arterial flow, prompting computed tomography angiography (CTA), which confirmed a partially thrombosed aneurysm of the descending genicular artery, arising from the supra-articular segment of the left popliteal trunk. The main popliteal artery itself was of normal caliber. The most likely etiology was degenerative, given the patient’s age, the diffuse atherosclerotic changes seen on imaging, and the absence of any traumatic, iatrogenic, or systemic predisposing factor. The patient subsequently underwent surgical exclusion of both the aneurysm and its feeding artery, with an uneventful postoperative course. This case illustrates the misleading appearance that aneurysms of popliteal artery branches may have on MRI, and emphasizes that any popliteal mass should be evaluated with Doppler ultrasound before any biopsy is considered, to avoid potentially life-threatening hemorrhage.