Absorbed dose maps of patients submitted to 68Ga-PSMA-11 PET/CT

Abstract

Although nuclear medicine (NM) procedures are highly effective diagnostic tools, they have been contributing significantly, together with other medical diagnostic and therapeutic methodologies, to the increase in exposure to ionizing radiation in recent years. There is an urgent need to opti mize NM techniques, to maintain diagnostic quality at a minimum possible radiation absorbed dose. 68Ga-prostate-specific membrane antigen positron emission tomography/computed tomog raphy (68Ga-PSMA-11 PET/CT) imaging has rapidly gained notoriety in the NM field and, at the same time, personalized dosimetry studies using voxel-based methods have been performed. This study aimed to calculate the absorbed dose at the voxel level in the kidneys, liver, spleen, and red bone marrow, compare the results with other studies and draw conclusions regarding the safety of using 68Ga-PSMA-11 in NM clinics. Whole-body PET/CT images from six patients were acquired after a single 68Ga-PSMA 11 injection. After registration of the CT and PET images, the target organs were manually seg mented in the CT and resampled to the PET voxel size. Voxel S-values were computed for spe cific tissues using the Monte-Carlo N-Particle transport 6.1 code. The absorbed dose rates were obtained by convolution of the PET activity images with the specific S-values of each tissue. A time integral was then applied to each distribution to account for all 68Ga decay. Statistical dose values were computed and compared with the available literature. Considering all the patients included in this study, the kidneys received the highest radi ation, with a mean overall absorbed dose of 0.0561 mGy/MBq and a median overall absorbed dose of 0.0499 mGy/MBq. In contrast, the red bone marrow received the lowest absorbed dose values (mean dose: 0.0015 mGy/MBq, median dose: 0.0013 mGy/MBq). The present study showed lower dosimetry values than the literature, resulting in deviations ranging from -38.1% (in the liver) to -91.3% (in the red bone marrow). The present study employs a voxel-based approach, which considers a non-uniform bio distribution of the radiopharmaceutical in the organs and leads to dosimetry estimates closer to the real ones. The reasonable low absorbed doses in the four organs herein studied is an argument in favor of using 68Ga-PSMA-11 in NM clinics. In the Future Work chapter, a more specific dynamic NM imaging methodology, taking into consideration the radiopharmaceutical pharma cokinetics, is presented.