Unusual Soft Tissue Uptake of F-18 Sodium Fluoride in Three Patients Undergoing F-18 NaF PET/CT Bone Scans for Prostate Cancer

F-18 NaF contrast-enhanced CT (a, d), PET (b, e), and fused PET/CT images (c, f) through the kidneys demonstrate an enhancing, non-calcified NaF avid aortacaval lymph node (long arrow, images a–c) and a less-avidly enhancing NaF avid retrocaval lymph node (long arrow, images d–f) which is separate from physiologic NaF activity seen in the right ureter (short arrow, images d–f; ureteral uptake in (f) is misregistered slightly such that it overlies the psoas muscle). These nodes and NaF avid iliac chain nodes were present in a 71 year old male with a stable prostate cancer metastasis to the acetabulum, not on androgen deprivation therapy, and with slowing rising PSA (10.1 ng/mL at time of the first scan). The nodes’ NaF activity increased on subsequent scans, for example the retrocaval node SUVmax was 7.6 in September 2013 (10.7 mCi F-18 NaF, uptake time 58 min), and SUVmax 12.6 in February 2014 (10.7 mCi F-18 NaF, uptake time 61 min). Both scans were performed according to established guidelines [1]. NaF activity was significantly higher than background blood pool, liver, and spleen activity (SUVmax 1.8, 0.79 and 0.95 respectively) [2]. It is conceivable that malignant infiltration of a lymph node could result in increased NaF deposition due to increased perfusion, as may sometimes be seen with Tc99m methylene diphosphonate bone scans. A follow-up F-18 FDG PET/CT was suggested to assess for hypermetabolic lymph node metastases but was not pursued by the referring oncologist, likely due to patient’s overall clinical stability. Sheth et al. reported NaF uptake in a mediastinal node in a renal cell carcinoma patient and speculated that the etiology may be related to presence of microscopic calcifications [3]. It has been shown that F-18 NaF activity can be elevated in the blood pool and coronary arteries from atherosclerosis and inflammation. However, the true significance of NaF uptake in lymph nodes remains unknown [4