Determination of Optimum Imaging Numbers for ¹⁷⁷Lu-PSMA Radionuclide Treatment Dosimetric Calculation

Introduction: Lutetium-177 ( 177 Lu) prostate-specific membrane antigen (PSMA) was first applied for treating castration-resistant prostate cancer (CRPC) in 2015, and PSA changes, low side effects, and good responses have been reported in the literature.Dosimetric calculations are required to determine the optimum number of treatments and prevent damage to critical organs.The aim of this study was to retrospectively investigate the feasibility of dosimetric calculations with fewer than four scans and to determine the most optimum imaging hours if dosimetric calculations can be performed with fewer than four scans.Methods: Whole body and single-photon emission computed tomography/computed tomography scans (4 th hour, 24 th hour, 48 th hour and 96 th hour) were performed on the patients after 177 Lu-PSMA infusion.A comparison was made between doses calculated using four images, doses calculated using three images, and doses calculated using two images.The calculations were repeated with four images in nine configurations: 1 st , 2 nd , 3 rd and 4 th .Scan configurations were classified as C1-C9.C1 was accepted as the reference and evaluated statistically for significance research between other groups.Results: For an amount of 177 Lu-PSMA activity of 3.7 GBq (100 mCi) per treatment, the mean kidney doses for C1, C2, C3, C4, C4, C5, C6, C7, C8, and C9 were calculated as 1.8±0.54Gy, 1.83±0.57Gy, 1.7±0.47Gy, 1.91±0.57Gy, 1.82±0.54Gy, 1.59±0.47Gy, 1.90±0.58Gy, 1.82±0.57Gy and 1.75±0.52Gy, respectively.A significant difference was found in all groups among C2-C9 compared to C1. Conclusion: Optimum dosimetric calculations for treating CRPC should be performed with C5 (three images taken at the 4 th , 24 th and 48 th hours) after 177 Lu-PSMA injection.The error rate increases in calculations performed with a lower number of images.