Is FDG (Fluorodeoxyglucose) PET (Positron Emission Tomography) scan as effective as temporal artery biopsy (TAB) for diagnosing giant cell arteritis (GCA)?

From the Guidelines

FDG PET scan is not as effective as temporal artery biopsy (TAB) for diagnosing giant cell arteritis (GCA), but it can be a useful complementary diagnostic tool, particularly for detecting large-vessel involvement. According to the most recent guidelines, ultrasound of temporal and axillary arteries should be considered as the first imaging modality to investigate mural inflammatory changes in patients with suspected GCA 1. TAB remains the gold standard diagnostic test for GCA with high specificity, though its sensitivity is limited (around 70-90%) 1. FDG PET scanning shows promise as a complementary diagnostic tool, particularly for detecting large-vessel involvement in GCA that might be missed by TAB.

Some key points to consider when using FDG PET scans for GCA diagnosis include:

• The ability to visualize the entire vascular system and detect inflammation before structural changes occur 1
• The potential to guide biopsy sites and improve diagnostic accuracy
• Limitations such as lower specificity than TAB, limited resolution for smaller cranial vessels, high cost, limited availability, and radiation exposure
• The importance of combining clinical assessment, laboratory markers (ESR, CRP), TAB, and imaging techniques including ultrasound, MRI, and PET when appropriate 1

In clinical practice, TAB is typically performed first, with FDG PET used in cases where TAB is negative but clinical suspicion remains high, or when large-vessel involvement is suspected. The diagnostic approach often combines multiple modalities to ensure accurate diagnosis and effective management of GCA. As recommended by the 2023 EULAR update, ultrasound, MRI, or FDG-PET can be used as alternative imaging techniques, particularly in situations where rapid conduction of ultrasound is not feasible or when specialists diagnosing GCA have insufficient training in ultrasound techniques 1.

From the Research

Comparison of FDG PET Scan and TAB for Diagnosing GCA

• The diagnostic performance of FDG PET scan and temporal artery biopsy (TAB) for giant cell arteritis (GCA) has been compared in several studies 2, 3, 4, 5, 6.
• A study published in 2016 found that FDG PET scan had a sensitivity of 66.7% and a specificity of 100%, while TAB had a sensitivity of 77% and a specificity of 100% 2, 3.
• Another study published in 2019 found that FDG PET-CT had a sensitivity of 61% and a specificity of 80% for diagnosing large vessel involvement in patients with suspected GCA and negative TAB 6.
• The use of FDG PET scan as a non-invasive technique for diagnosing GCA has been suggested, especially in patients with atypical manifestations of the disease and negative or unavailable biopsy of the temporal artery 4, 5.

Diagnostic Yield of FDG PET Scan and TAB

• A study published in 2016 found that both FDG PET scan and CTA had a strong diagnostic yield for the diagnosis of GCA, with FDG PET scan appearing to have a higher positive predictive value (PPV) compared to CTA 2.
• Another study published in 2023 found that TAB had a false-negative rate ranging from 9% to 61%, highlighting the need for alternative diagnostic methods 3.
• The combination of ultrasonography and FDG PET imaging has been suggested as a helpful approach for diagnosing GCA of large arteries, particularly in patients with atypical manifestations of the disease and negative or unavailable biopsy of the temporal artery 5.

Limitations and Future Directions

• The diagnostic performance of FDG PET scan and TAB may be influenced by various factors, such as the timing of biopsy, the length of specimen, and the existence of "skip lesions" 3.
• Further studies are needed to establish the optimal diagnostic approach for GCA, including the use of FDG PET scan and other non-invasive imaging techniques 2, 6.