Renal Ultrasound Detection of Retroperitoneal Lymph Nodes and Masses
Renal ultrasound has limited accuracy for detecting retroperitoneal lymph nodes or masses and should not be relied upon for this purpose; CT or MRI are the appropriate imaging modalities for evaluating the retroperitoneum. 1
Limitations of Ultrasound for Retroperitoneal Evaluation
Ultrasound of the abdomen and retroperitoneum to detect retroperitoneal lymph node metastases is less accurate and reproducible than CT, MRI, or PET/CT. 1 The ACR Appropriateness Criteria explicitly state this limitation across multiple guidelines.
Technical Challenges with Ultrasound
- Anatomical evaluation of retroperitoneal structures on ultrasound is difficult due to patient factors including limited positioning and lack of sufficient acoustic windows to evaluate the entire retroperitoneum 1
- The retroperitoneal space is deep and often obscured by bowel gas, making consistent visualization challenging 1
- Small volumes of retroperitoneal masses or lymph nodes are particularly limited in their detection by ultrasound 1
When Retroperitoneal Findings May Be Visible on Renal Ultrasound
While ultrasound is not the appropriate modality for retroperitoneal evaluation, certain scenarios may allow incidental detection:
- Large retroperitoneal masses that directly invade or engulf the kidney may be visible, particularly when they extend into the renal hilum or subcapsular space 2, 3, 4
- Very large lymph node masses in the para-aortic or paracaval regions adjacent to the kidneys might be detected incidentally 3
- However, even when visible, ultrasound cannot reliably characterize these lesions or assess their full extent 1
Appropriate Imaging for Retroperitoneal Assessment
CT is the reference standard imaging test for assessing the retroperitoneum for lymphadenopathy or masses. 1 CT is rapid, reproducible, and provides excellent imaging assessment of the para-aortic and paracaval regions with accuracy ranging from 73% to 97% for detecting retroperitoneal lymph nodes 1
- MRI is an appropriate alternative with comparable accuracy to CT for detecting retroperitoneal lymphadenopathy, particularly when iodinated contrast is contraindicated 1
- MRI studies show comparable efficacy to CT for detection of retroperitoneal lymphadenopathy, even without gadolinium-based contrast agents when diffusion-weighted imaging is included 1
Clinical Implications
If there is clinical suspicion for retroperitoneal lymphadenopathy or masses, do not order renal ultrasound for this indication. Order CT abdomen and pelvis with IV contrast or MRI abdomen and pelvis as the initial imaging study 1.