Radiation dosimetry of Tc99m-PSMA I&S: a single-center prospective study

Abstract

Introduction: Tc-99m-labeled Mas3-y-nal-k(Sub-KuE) (Tc-99m-PSMA I&S) is a prostate specific membrane antigen (PSMA) tracer that can be used for planar and SPECT/CT gamma imaging and radio-guided surgery (RGS). The primary aim of this study was to estimate the dosimetry of Tc-99m-PSMA I&S using a hybrid method (sequential gamma planar imaging and one single SPECT/CT) in healthy volunteers. The secondary aim was to depict the tracer biodistribution and tumor-to-background ratios (TBR) in patients with prostate cancer (PCa). Methods: Dosimetry of Tc-99m-PSMA I&S was investigated in four healthy volunteers. Whole-body planar imaging was acquired at 1, 2, 3, 6 and 24 hours, and SPECT/CT at 6h after tracer injection. Contours of organs were drawn on all acquisitions to determine organ activity at each timepoint. Absorbed dose was estimated using two Methods: 1) independent curve-fitting manual method (Levenberg-Marquardt-based algorithm using dose factors from Radiation Dose Assessment Resource (RADAR) web site) and 2) OLINDA/EXM® v 2.0 software (HERMES Medical Solutions). Biodistribution of Tc-99m-PSMA I&S was assessed in ten patients with PCa on SPECT/CT images at 6 h. Tumor uptake (SUV_max), and TBR (tumor SUV_max/ background organ SUVmean) using muscle (T/M), bladder (T/B) and intestine (T/I) as background organs were determined. Results: The mean injected activity of Tc-99m-PSMA I&S was 717 MBq (range: 562-828). No adverse events related to the injection of Tc-99m-PSMA-I&S were reported. The average radiation effective dose was 0.0055 mSv/MBq with the RADAR manual method and 0.0052 mSv/MBq with OLINDA/EXM®. Total body effective dose ranged between 3.33-4.42 and 3.11-4.23 mSv, respectively. All PCa patients showed high tracer uptake in primary and metastatic lesions with T/M, T/B, T/I ranging from 5.29 - 110, 0.11 – 7.02 and 0.96 – 16.30, respectively. Conclusion: Effective doses of Tc-99m-PSMA I&S were comparable to those known for most of the Tc-99m tracers and was lower than ⁶⁸Ga-labelled and ¹⁸F-labelled agents. Tc-99m-PSMA I&S SPECT/CT showed high TBR in PCa patients. This study can provide required data for translation and approval of Tc-99m-PSMA I&S by regulatory agencies.