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
(SUVmax), and TBR (tumor SUVmax/ 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.