RESUMO
PURPOSE: This study aimed to assess the accuracy of patient-specific absorbed dose calculations for tumours and organs at risk in radiopharmaceutical therapy planning, utilizing hybrid planar-SPECT/CT imaging. METHODS: Three Monte Carlo (MC) simulated digital patient phantoms were created, with time-activity data for mIBG labelled to I-123 (LEHR and ME collimators) and I-131 (HE collimator). The study assessed the accuracy of the mean absorbed doses for I-131-mIBG therapy treatment planning. Multiple planar whole-body (WB) images were simulated (between 1 to 72 h post-injection (p.i)). The geometric-mean image of the anterior and posterior WB images was calculated, with scatter and attenuation corrections applied. Time-activity curves were created for regions of interest over the liver and two tumours (diameters: 3.0 cm and 5.0 cm) in the WB images. A corresponding SPECT study was simulated at 24 h p.i and reconstructed using the OS-EM algorithm, incorporating scatter, attenuation, collimator-detector response, septal scatter and penetration corrections. MC voxel-based absorbed dose rate calculations used two image sets, (i) the activity distribution represented by the SPECT images and (ii) the activity distribution from the SPECT images distributed uniformly within the volume of interest. Mean absorbed doses were calculated considering photon and charged particle emissions, and beta emissions only. True absorbed doses were calculated by MC voxel-based dosimetry of the known activity distributions for reference. RESULTS: Considering photon and charged particle emissions, mean absorbed dose accuracies across all three radionuclide-collimator combinations of 3.8 ± 5.5% and 0.1 ± 0.9% (liver), 5.2 ± 10.0% and 4.3 ± 1.7% (3.0 cm tumour) and 15.0 ± 5.8% and 2.6 ± 0.6% (5.0 cm tumour) were obtained for image set (i) and (ii) respectively. Considering charged particle emissions, accuracies of 2.7 ± 4.1% and 5.7 ± 0.7% (liver), 3.2 ± 10.2% and 9.1 ± 1.7% (3.0 cm tumour) and 13.6 ± 5.7% and 7.0 ± 0.6% (5.0 cm tumour) were obtained for image set (i) and (ii) respectively. CONCLUSION: The hybrid WB planar-SPECT/CT method proved accurate for I-131-mIBG dosimetry, suggesting its potential for personalized treatment planning.
RESUMO
The biodistribution of an N2 N2 ' tetradentate gold(III) chelate, which is known to be cytotoxic towards a range of human cancer cell lines, was determined by a radiolabelled equivalent of the compound. The (198) Au-labelled gold(III) chelate of a bis(pyrrolide-imine) Schiff base ligand with a three-carbon di(azomethine) linkage was successfully synthesised with a high radiochemical yield of 73% and radiochemical purity of >95%. The high energy γ-ray emitted by the (198) Au nucleus was used to follow the biodistribution of the compound in vivo in six male Sprague Dawley rats on a gamma camera. The log Po/w value of the (nat) Au analogue, -1.92(2), showed that the compound is hydrophilic and therefore likely to largely remain in the blood pool. This was confirmed by the biodistribution study, which showed 21% of the injected dose (ID) remained in the blood pool 4.5 h after injection. This decreased to 10.8% over a 24-h period. The activity measured in the lungs, 1.48%ID/g, remained relatively constant over a 24-h period suggesting that the complex had accumulated in the lungs in the form of particulates, and could not be cleared by the test subjects. The t½ for the heart and lungs was greater than 24 h. Excretion of the test compound is seemingly via the kidneys, but is slow with approximately 30% of the ID excreted within 24 h.