Anatomical Location and Ultrasonographic Features of Retroperitoneal Extra-Adrenal Paragangliomas: A Report of Three Cases.

Retroperitoneal extra-adrenal paragangliomas are rare neuroendocrine tumors that frequently arise in a confined anatomical space bordered by the right side of the abdominal aorta, the left side of the inferior vena cava, and the posterior aspect of the horizontal part of the duodenum (hereafter referred to as the duodenum-adjacent aortocaval region). Although ultrasonography (US) is a noninvasive and useful modality for initial diagnosis, detailed imaging characteristics of such tumors have not been sufficiently reported. This report aimed to describe the anatomical location and ultrasonographic features of three cases of retroperitoneal extra-adrenal paraganglioma, with a focus on blood flow evaluation using superb microvascular imaging (SMI) and contrast-enhanced ultrasound (CEUS). We retrospectively analyzed B-mode US, SMI, CEUS, computed tomography (CT), magnetic resonance imaging (MRI), and 123I-MIBG scintigraphy findings in all three cases. In every case, tumors were located in the duodenum-adjacent aortocaval region. US revealed well-defined, hypoechoic, oval-shaped masses with heterogeneous internal structures. SMI showed abundant intratumoral blood flow, while CEUS demonstrated contrast enhancement. SMI under contrast enhancement revealed blood flow that was not visible with conventional SMI. Our findings indicate that retroperitoneal extra-adrenal paragangliomas exhibit characteristic morphology and vascularity on US. The combined use of SMI and CEUS may further enhance diagnostic accuracy and contribute to US as an important noninvasive diagnostic tool. Moreover, when a mass is identified in the duodenum-adjacent aortocaval region, paraganglioma should be considered in the differential diagnosis.