Optimal theranostic SPECT imaging protocol for 223radium dichloride therapy.

223Radium dichloride image-based individual dosimetry requires an optimal acquisition and reconstruction protocol and proper image correction methods for theranostic applications. To assess this problem, radium-223 dichloride SPECT images were acquired from a Jaszczak simulator with a dual-headed gamma camera, LEHR collimator, 128 × 128 matrix, and total time of 32 minutes. A cylindrical PMMA phantom was used to calibrate the measurements performed with Jaszczak. The image quality parameters (noise coefficient, contrast, contrast-to-noise ratio and recovery coefficient) and septal penetration correction were calculated by MATLAB®. The best results for the investigated image quality parameters were obtained with an 89 keV energy window (24% wide) produced together with OSEM/MLEM reconstruction (8 subsets and 4 iterations) applying a Butterworth filter (order 10 and cutoff frequency of 0.48 cycles·cm-1). The successfully performed recovery coefficient parameter evaluation allows uptake correction for future patient dosimetry applications.

A cell-based dosimetry model for radium-223 dichloride therapy using bone micro-CT images and GATE simulations.

Dosimetry at the cellular level has outperformed macrodosimetry in terms of agreement with toxicity effects in clinical studies. This fact has encouraged dosimetry studies aiming to quantify the absorbed doses needed to reach radiotoxicity at the cellular level and to inform recommendations on the administration of radium-223. The aim of this work is to qualitatively and quantitatively evaluate the absorbed doses of radium-223 and the interactions of the doses at the cellular level. The analysis was performed by Monte Carlo simulations in GATE using micro-CT image of a mouse. Two physics lists available in the GATE code were tested. The influence of single and multiple scattering models on the absorbed dose distribution and number of particle hits was also studied. In addition, the fuzzy c-means clustering method was used for data segmentation. The segmentation method was suitable for these analyses, particularly given that it was unsupervised. There was no significant difference in the estimated absorbed dose between the two proposed physics lists. The absorbed dose values were not significantly influenced by scattering, although single scattering resulted in twice as many interactions as multiple scattering. The absorbed dose histogram at the voxel level shows heterogeneous absorbed dose values within each shell, but the observations from the graph of the medians were comparable to those in the literature. The interaction histogram indicates 104 events, although some voxels had no interactions with alpha particles. However, the voxels did not show absorbed doses capable of deterministic effects in the deepest part of the bone marrow. The absorbed dose distribution in images of mouse trabecular bone was compatible with simple geometric models, with absorbed doses capable of deterministic effects near the bone surface. The interaction distributions need to be correlated with in vivo studies for better interpretation.

Influence of the SPECT calibration source position on the absorbed dose calculation for 131I-NaI therapy using GATE simulations.

Many research groups have studied nuclear medicine image quantification to improve its accuracy in dose estimation. This work aims to evaluate the influence of the source calibration position for absorbed dose calculation for a 131I-NaI therapy using Monte Carlo (MC) simulations. The calibration approach consisted of a cylindrical phantom filled with water. A cylindrical 131I source with 361.1 ± 3.6 kBq ml-1 was positioned at the center of the phantom and its outer part. Images were acquired with 150 00 counts per projection image acquired with SPECT detector (high counts density-HCD) and 3000 counts per projection (low counts density-LCD). MC simulations, performed with GATE code, were validated by comparing the S values of a water sphere uniformly filled with 131I, as from the sphere model of OLINDA/EXM 1.1. Calibration factors deviation between central and peripheral calibrations is more significant for HCD (18.3%) than for LCD images (3.7%). The 3D dose distribution map obtained from GATE resulted in a dose factor equal to 1.5 × 10-3 mGy/(MBq.s). For both HCD and LCD images, the commonly used approach, which employs the central source calibration to obtain the dose from a peripheral source, resulted in dose overestimation. Results suggest that organ dose calculation can be improved considering the organ position in the field of view. Finally, patients' radiation protection in dosimetry studies could be improved considering the calibration source position, due to the superior accuracy in dose calculation.