E4509. Where’s the (Nuclear) Source? The Role of FDG-PET in Fever of Unknown Origin Work-Up
  1. Kyle Atcheson; Wake Forest University School of Medicine
  2. Turgot Cengiz; Wake Forest University School of Medicine
  3. Yong Bradley; Wake Forest University School of Medicine
Fever of unknown origin (FUO) is defined as a fever higher than 38.3<sup>o</sup>C (101<sup>o</sup>F) on several occasions over at least 3 weeks with uncertain diagnosis. Up to a third of patients with FUO die from FUO-related causes, making early diagnosis and treatment essential. Though not the primary modality for evaluation, FDG-PET is accurate for identifying underlying etiologies and provides a reliable option for patients who require prompt and accurate diagnosis.

Educational Goals / Teaching Points
We will review the normal biodistribution of F18-FDG-PET and pathophysiology related to FUO etiologies. We will then utilize a case-based approach to demonstrate the imaging appearance of the etiologies related to FUO. There will be discussion of pertinent differential diagnoses and the next-step recommendations for radiologists to consider and convey to clinicians in each case. Finally, we will provide appropriate CT and MRI correlation (as applicable) to further demonstrate these findings.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
One of the major strengths of FDG-PET is its ability to identify underlying hypermetabolic processes that are not readily apparent on anatomic imaging, even with IV contrast. As such, PET-CT can more readily demonstrate active disease processes that could underlie FUO in critically ill patients. In our example case, we present a 53-year-old male with a history of recent right iliac fossa renal allograft placement and intermittent MRSA bacteremia following prolonged hospital stay. The patient continued to have fever and bacteremia without an identifiable source. FDG-PET showed patchy areas of increased FDG-uptake within the allograft parenchyma and minimal FDG within the collecting system/urine. These findings were consistent with allograft pyelonephritis.

FDG-PET is a useful and underutilized tool for the evaluation of FUO, especially in cases where structural imaging modalities are limited. With the increase in patients with immunosuppression, medical devices, and atypical etiologies, FDG-PET should be considered more readily for the evaluation of FUO. It is imperative for radiologists to understand the proper indications for FDG-PET and how to convey key findings to ensure proper utilization of resources and timely diagnosis.