Reference Organ for FDG PET with Diffuse Liver Metastasis
When diffuse liver metastases preclude using the liver as a reference organ, the spleen should be used as the primary alternative reference site for FDG PET imaging, with the mediastinal blood pool serving as a secondary option when the spleen is also unsuitable. 1
Primary Alternative: Spleen
- The spleen provides the most reliable alternative reference organ when hepatic parenchyma is diffusely involved with metastatic disease, maintaining relatively stable FDG uptake in most clinical scenarios and remaining unaffected by metastatic disease from most primary tumors 1
- The spleen offers consistent physiologic FDG uptake suitable for background normalization and lesion-to-background ratio calculations when the liver is compromised 1
- Place a spherical volume of interest (VOI) with 3 cm diameter in the splenic parenchyma, carefully avoiding organ boundaries, vascular structures, and any focal lesions 2, 3
Secondary Alternative: Mediastinal Blood Pool
- When both liver and spleen are unsuitable, use the descending thoracic aorta or mediastinal blood pool as the reference region, though this approach has higher variability due to cardiac output fluctuations 1, 2
- Position the VOI within the descending thoracic aorta, meticulously avoiding the vessel wall where uptake may be elevated from vascular inflammation 2, 3
- Blood pool SUV typically measures around 1.6 (SUL around 1.2) in FDG studies, providing a consistent baseline for comparison 2, 3
Rationale for Alternative Reference Sites
- The liver normally serves as the standard reference because it demonstrates relatively homogeneous physiologic FDG activity with expected SUL values between 1.0-2.2 (SUV 1.3-3.0) 2, 1
- Diffuse hepatic metastases create heterogeneous FDG activity throughout the liver parenchyma, making it unsuitable as a reference standard for SUV calculations or visual interpretation 1, 2
- While the liver maintains 92-100% accuracy for detecting metastases when parenchyma is normal, this reliability is lost when diffuse disease is present 2, 1
Critical Implementation Details
- Document which reference region is used in the imaging report, as this directly affects SUV calculations and serial comparison of disease burden 1
- Ensure the VOI does not include lesions, metastases, or areas of abnormal uptake, as this artificially elevates baseline measurements and reduces tumor-to-background ratios 3
- Visually verify all semiautomatically generated VOIs, as automated segmentation may fail with anatomical variants or pathologic involvement 2, 3
- Maintain consistency in reference region selection across serial scans for the same patient to enable accurate assessment of treatment response 1
Important Caveats
- Be aware that false-positive FDG uptake can occur in the spleen due to infectious or inflammatory causes, similar to lymph nodes 1, 2
- The reference organ SUV may be affected by tumor burden, with studies showing negative correlation between volumetric index of disease and both liver and blood pool SUV (r = -0.502 to -0.547) 4
- Avoid including the splenic hilum or perisplenic vessels in the VOI to prevent measurement artifacts from vascular structures 2, 3