Normal Distribution Pattern and Physiological Variants of Ga-68-PSMA PET/CT

Objectives Glu-NH-CO-NH-Lys-(Ahx)-[68Ga(HBED-CC)] (Ga-68-PSMA) is a novel PET tracer and binds to extracellular component of Glutamate carboxypeptidase II, which is also called prostate specific membrain antigene (PSMA) due to high expression in prostate cancer. Initial experience with Ga-68-PSMA suggests that PSMA PET/CT can be used for imaging of advanced prostate cancer. In this study we aimed to present detailed biodistribution of Ga-68-PSMA, as well as physiological and benign variants of this novel tracer.

Methods We performed retrospective analysis of 40 patients who underwent PSMA PET/CT between January and August 2015 at our department and who had no evidence of residual or metastatic disease on PSMA PET/CT and conventional imaging methods. Additionaly 16 patients who underwent PSMA PET/CT for any indications other than prostate cancer included in the study to evaluate physiological uptake in the normal prostate gland. Median, minimum-maximum, mean, standart deviation of SUVmax and SUVmean values were calculated for visceral organs, bone marrow and lymph nodes and mucosal areas. Any physiological variants or benign lesions with Ga-68-PSMA were noted.

Results Ga-68-PSMA uptake was noted in kidneys, parotid and submandibular glands, duodenum, small intestines, spleen, liver, lacrimal glands, mucosal uptake in nasopharynx, vocal cords, pancreas, stomach, mediastinal blood pool, thyroid gland, adrenal gland, rectum, vertebral bone marrow, testes and calculated meanSD of SUVmax were 55.623, 19.66.5, 19.595.3, 18.36.6, 17.77.4, 13.515.87, 12.94.9, 11.44.1, 5.562.63, 5.641.87, 5.52.5, 5.12.3, 4.31.67, 4.21.96, 3.61.6, 3.62.25, 3.61.58, 2.860.93 respectively. Celiac lymph nodes showed slightly Ga-68-PSMA uptake in 24 of 40 patients without presence of any other pathologic lymph nodes in abdominal and pelvic areas and calculated meanSD of SUVmax were 3.62.02. Kidneys demonstrated DMSA-like high cortical uptake in 39 patients and showed reduced uptake in atrophic kidney of one patient. High uptake was observed in the gallbladder wall of 4 patients (SUVmax ranged from 10.7 to 40.1). In 2 of 40 patients, calcified choroid plexus showed minimal but visible uptake. Slightly elevated uptake was noted in breast parenchyma in 3 of 8 patients who had gynecomastia. Osteophytes of 2 patients showed slight uptake. Analysis of normal-sized prostate glands, in patient group who had PSMA PET/CT for indications other than prostate cancer (n=16), demonstrated a slight physiological uptake in prostate glands (SUVmax: 5.82.28, range:3.5-12.3).

Conclusions This study demonstrates normal distribution pattern, range of SUVs and physiological variants of Ga-68-PSMA. Additionally, several potential pitfalls were documented.