Determination of optimal Ga-PSMA PET/CT imaging time in prostate cancers by total-body dynamic PET/CT.

Background:  Ga-PSMA PET/CT has been widely used in patients with prostate cancer. Due to the limited axial field of view of conventional PET scanners, whole-body dynamic  Ga-PSMA PET/CT has not been performed. We investigated the time-activity curves (TACs) of prostate cancer pathological lesions and physiologic bladder activity to determine the optimal  Ga-PSMA PET/CT imaging time by total-body (TB) PET/CT.

Methods: Dynamic TB-PET performed on 11 patients with prostate cancer was analyzed. TACs were obtained by drawing regions of interest in normal organs and pathological lesions (primary prostate lesions and lymph nodes and bone metastases). We evaluated the  Ga-PSMA uptake pattern of normal organs, urinary bladder, and pathological lesions.

Results: The urinary bladder TAC increased slowly between 180 and 330 s post-injection and then rapidly between 5.5 and 60.0 min post-injection. The pathological lesion uptake increased rapidly during the first 5 min post-injection and then slowly through the remaining 55 min. Six minutes post-injection was the optimal time with the highest pathological lesion SUV values still higher than the urinary bladder activity value. However, these prostate lesion, lymph node metastasis, and bone metastasis SUV values were one-third, one-half, and one-half the corresponding values 60 min post-injection, suggesting that early imaging might miss low PSMA uptake lesions. A minimum of 35 min post-injection was required for the pathological lesions to have SUV values similar to the corresponding values at 60 min post-injection (all P > 0.05), even though the pathological lesion SUV values showed a continuous upward trend through the 60 min.

Conclusions: Combining early dynamic  Ga-PSMA PET (75-360 s) and conventional static imaging 60 min post-injection could avoid the urinary bladder activity interference to better detect pathological lesions and lesions with relatively low PSMA uptake. The pathological lesion SUV values at 35-59 min and 60 min post-injection were similar, so  Ga-PSMA PET imaging could also be made at 35-59 min post-injection.