What would a PET (Positron Emission Tomography) scan show in a patient with TRAPS (Tumor Necrosis Factor Receptor-Associated Periodic Syndrome)?

PET Scan Findings in TRAPS (TNF Receptor-Associated Periodic Syndrome)

In patients with TRAPS, a PET scan would typically show increased FDG uptake in areas of active inflammation, particularly in affected joints, muscles, and serosal surfaces during disease flares, reflecting the underlying inflammatory process characteristic of this autoinflammatory condition.

Key PET Findings in TRAPS

Inflammatory Pattern

• Musculoskeletal system: Increased FDG uptake in affected muscles and joints, corresponding to the myalgia and arthralgia that are hallmark symptoms of TRAPS
• Abdominal region: Focal or diffuse uptake in the peritoneum and serosal surfaces during episodes of abdominal pain
• Skin: Increased uptake in areas corresponding to migratory rashes
• Systemic inflammation: Diffuse uptake in multiple organ systems during acute flares

Differential Features

• Unlike lymphoma, which shows nodal involvement with high SUV values in a distribution pattern corresponding to lymph node chains 1
• Unlike Takayasu syndrome, which typically shows more prominent and consistent vascular wall uptake, especially in the aorta and its major branches 1
• Unlike prosthetic valve endocarditis, which shows focal uptake around prosthetic heart valves with specific SUV ratio patterns 1

Clinical Utility of PET in TRAPS

PET scanning can be valuable in TRAPS management for several reasons:

1. Disease activity assessment: PET can help determine if symptoms are due to active inflammation during suspected flares
2. Treatment monitoring: Can assess response to biological therapies such as IL-1 inhibitors or anti-TNF agents
3. Complication detection: May help identify early signs of amyloidosis, a serious long-term complication of TRAPS

Interpretation Guidelines

When interpreting PET scans in TRAPS patients:

• SUV values: Increased standardized uptake values in affected tissues compared to background
• Distribution pattern: Typically multifocal, corresponding to clinical symptoms
• Timing considerations: Most valuable during active flares rather than during remission periods
• Correlation: Findings should be correlated with clinical symptoms and inflammatory markers (CRP, ESR)

Important Caveats

• PET findings are not specific for TRAPS and must be interpreted in the clinical context of periodic fever, characteristic symptoms, and genetic confirmation 1
• False positives can occur in other inflammatory conditions or infections
• Recent administration of cytokines can cause increased splenic uptake that may persist for up to 10 days 1
• PET scanning is not included in the standard diagnostic criteria for TRAPS but can be useful as a supplementary investigation

Recommended Protocol

For optimal PET imaging in suspected TRAPS:

• Whole-body 18F-FDG PET/CT during active disease flare
• Patient preparation with fasting for 4-6 hours before the scan
• Imaging acquisition 60 minutes after FDG injection
• Correlation with clinical symptoms and laboratory markers of inflammation

PET scanning should be considered as part of a comprehensive evaluation in patients with suspected or confirmed TRAPS, particularly when assessing disease activity or monitoring treatment response.