Radiopharmaceutical biodistribution and the factors affecting biodistribution in GA68-PSMA PET/CT imaging

The aim of our study is to determine the physiological and pathophysiological distribution of the pharmaceutical (Ga68-PSMA-617) by determining the range of uptake in the organs and tissues, primary prostate tumor, lymph node, and bone metastasis and investigate whether there are differences in distribution according to the laboratory, histopathological and clinical findings that can affect image evaluation of Ga68-PSMA-617 PET/CT in prostate cancer patients. Also, we aimed to determine cut-off values to distinguish primary prostate tumor from normal prostate tissue, bone and lymph node metastasis from physiological bone and lymph node uptake. 229 prostate cancer patients who underwent Ga68-PSMA PET/CT at our department between January 2018 and May 2018 were retrospectively analyzed. The patients were grouped as negative/positive groups according to the absence/presence of pathological activity on PET/CT. The SUV values of organs and tissues of patients in the negative and positive groups were compared. In the positive group, subgroups were formed according to Gleason score grouping, PSA values, treatments received before imaging, metastatic status, serum ALP, LDH, and creatinine values. The SUV values of the organs, tissues, and pathological lesions of the patients in these subgroups were compared among themselves and also with the negative group. PSMA-617 demonstrates similar biodistribution with PSMA-11 and PSMA-I&T except for minor differences that don't significantly affect the image evaluation. When the mediastinal lymph node and iliac bone SUV values were examined to evaluate physiological uptake in lymph nodes and bone, no significant difference was found between the groups. In the group with patients that received androgen deprivation therapy, the bone metastasis SUV values were found to be higher and the SUV values of the submandibular gland and renal cortex were found to be lower. In the group with patients that received radiotherapy, the normal prostate tissue SUV values were determined to be higher. The SUVmax and SUVmean values of the submandibular gland, cerebellum, breast, testis, muscle, and SUVmean (automated) values of liver and blood pool were determined to be lower in the group of patients with high serum LDH values. In the group of patients with high serum creatinine values, submandibular gland SUVmax and SUVmean and liver, blood pool SUVmean (automated) values were found to be higher but the bladder lumen and renal cortex SUV values were found to be lower. The cut-off SUVmax value was determined to be 6,945 for primary prostate lesion, 4,72 for lymph node metastasis, 4,25 for bone metastasis. The serum PSA values were higher in the positive group than in the negative group and the PSA cut-off value to distinguish these two groups were found to be 1,505. In conclusion, PSMA-617 demonstrates similar biodistribution with other PSMA ligands. The physiological uptake of lymph node and bone which were mostly metastasized in prostate cancer, are not affected by the factors we examined. It should be kept in mind that the normal prostate tissue SUV values may increase in patients receiving radiotherapy, the uptake of the organs that have physiological or pathological uptake may differ due to the changes in PSMA expression in patients receiving androgen deprivation therapy and tumor burden may affect the biodistribution.