Normal Organ Uptake Variability on PSMA-Targeted 18F-DCFPyL PET/CT

519 Objectives 18F-DCFPyL is a small-molecule, PSMA-targeted PET radiotracer that shows promise in the detection of primary and metastatic prostate cancer (PC) lesions. As a new radiotracer used in clinical patients, it is desirable to understand the variability in normal organ uptake so as to judge the ability to quantify a 18F-DCFPyL PET scan. It is also essential to calculate the normal range of variability of 18F-DCFPyL tissue uptake for therapeutic monitoring. Methods Fifty-two patients with a history of PC and a variety of disease states including biochemical recurrence, oligometastatic disease, and widespread metastatic disease underwent 18F-DCFPyL PET/CT imaging. Volumes of interest (VOI) were manually drawn covering the entire organ volume on the PET scans. Mean standardized uptake values (SUV) corrected for lean body mass were measured in both lacrimal glands, all four major salivary glands, the liver, the spleen, and both kidneys for all patients. The average mean SUV across all scans, standard deviations, and coefficients of variation (COV) for each organ were calculated. The same parameters were also derived for a 3 cm sphere drawn in the center of the liver, a potentially more convenient VOI for clinical application. Results 18F-DCFPyL mainly accumulated in salivary glands, liver, spleen, kidney and lacrimal glands. The average Mean SUV for the liver (3.75 ± 0.57) was higher than for the spleen (3.01 ± 1.21). The average Mean SUV of the 3 cm sphere in the liver was 3.79 ± 0.59, very close to that of whole liver organ. The highest uptake organs were the kidneys, with no significant difference between the right and left kidney, with the average mean SUV 14.85 ± 3.60 for the right kidney and 14.31 ± 3.49 for the left kidney, respectively. The uptake in the salivary glands was also high with the average mean SUV for right parotid 6.81 ± 1.64, left parotid 6.79 ± 1.66, right submandibular gland 6.94 ± 1.57, and left submandibular gland 6.79 ± 1.54. We also noticed a high accumulation in the bilateral lacrimal glands (4.71 ± 1.02 on the right and 4.55 ± 1.09 on the left). In regards to variability we found the COV of the liver was lowest (15.20%), while that of spleen was highest (40.10%). COV of the 3 cm sphere in the liver was also very low (15.57%). COV of both kidneys were ~ 24% (24.24% on the right and 24.38% on the left). Both lacrimal glands and four major salivary glands showed similar COV, from 21% to 24.5% (right parotid 24.04%, left parotid 24.52%, right submandibular 22.59%, left submandibular 22.68%, right lacrimal 21.58%, and left lacrimal 24.07%). Conclusions Radiotracer accumulation in 18F-DCFPyL PET/CT imaging mainly occurs in the salivary and lacrimal glands, liver, spleen, and kidney. Variability in normal organ uptake across all patients was lowest in the liver, suggesting that liver uptake may serve as an internal metric on quantification of 18F-DCFPyL PET/CT studies. The coefficient of variation of the liver was only 15.2%, which is lower than that reported with 18F-FDG PET, emphasizing the potential to quantify reliably this new radiotracer. Furthermore, a 3 cm sphere drawn in normal liver recapitulates average whole liver uptake, an important finding for clinical translation of assessing normal organ uptake.