ORGAN DOSE ESTIMATES IN THORAX CT: VOXEL PHANTOM ORGAN MATCHING WITH INDIVIDUAL PATIENT ANATOMY

The main objective of this investigation was to modify existing three-dimensional (3D) voxel phantom models to resemble real patients, towards the personalized patient dosimetry concept. This work focused essentially in one of the most radiosensitive organs in the thorax, the lungs. Additionally, the variations of organ doses when a standard phantom is used instead were studied. A FORTRAN-based program was developed, which is able to semi-automatically modify the volumetric information of organs of interest in a standard voxel phantom (Female ICRP Adult Reference). Monte Carlo (MC) simulations were used to mimic CT scan conditions and access organ dose in both phantoms (ICRP standard and ICRP modified). Voxel models matched to patients’ size and overall anatomy allow increased accuracy in organ dose estimation, which can suffer from up to 20% underestimation and 40% overestimation. This study demonstrates that voxel phantoms developed using single patient data provide more precise organ dose assessments.

MONTE CARLO DOSE DISTRIBUTION CALCULATION AT NUCLEAR LEVEL FOR AUGER-EMITTING RADIONUCLIDE ENERGIES

The distribution of radiopharmaceuticals in tumor cells represents a fundamental aspect for a successful molecular targeted radiotherapy. At microscopic level only a fraction of cells in tumoral tissues incorporate the radiolabel. The most used method to perform cellular dosimetry is the MIRD one, where absorbed doses are calculated for several electron energies, nucleus diameters, and for homogeneous source distributions. However, the radionuclide distribution inside nuclei can be highly non-homogeneous.

The GPSR researchers developed a study to show to what extent a non-accurate cellular dosimetry could lead to misinterpretations of surviving cell fraction vs dose relationship; the modelling of realistic radionuclide localization inside cells (for ^99mTc radionuclide), including a inhomogeneous nuclear distribution, revealed that i) a strong bias in surviving cell fraction vs dose relationships (taking into account different radiobiological models) can arise; ii)  alternative models might contribute to accurate prediction of the radiobiological effects inherent to specific molecular targeted radiotherapy strategies.

IMAGE SEGMENTATION AND VOXELIZATION TECHNIQUES FOR DOSE ASSESSMENT IN X-RAY DIAGNOSTIC PROCEDURES

Computational voxel phantoms are models of the human anatomy used in the field of radiation protection, medical imaging and radiotherapy that enables evaluation of organ doses with a high degree of precision. The gold standard of radiation dosimetry would be to obtain a computational model for each patient involved in radiation processes. Having this in mind, the aim of this work was to try to improve the implementation of a computational voxel phantom starting with a physical one for organ dose assessment and imaging studies in the X-ray diagnostic. The first step was devoted to the segmentation of CT images through thresholding methods and region growing algorithms. The second step consisted in developing a MC model for validation, dose and imaging purposes. Finally, an analysis of SNR for different calcification sizes showed the optimal energy (about 60 keV) that maximizes the image quality for 0.7 cm thick calcification detection.

BIOLOGICAL EFFECTS OF BETA- AND/OR AUGER-EMITTING THERAPEUTIC RADIOPHARMACEUTICALS

In collaboration with the RS Group, we have been involved in the radiobiological evaluation of Auger-emitting (^99mTc) or β⁻ emitting (⁶⁴Cu) radiocompounds, promising candidates for selective and targeted radiotherapy due to the very short range (micrometers to few nanometers) of the emitted β⁻particles and Auger electrons. Continuing our previous work on the ability of ^99mTc-labelled AO-derivatives to cause DNA damage, focusing on late biological effects, an increase on the number of micronuclei (indicative of genetic lesions) induced by ^99mTc-containing compounds was observed. In addition, exposure to ^99mTc led to the decrease of the cellular survival fraction, indicating the occurrence of a persistent lesion, dependent on the activity of the compound. Studies using the mixed β⁻/Auger emitter ⁶⁴Cu have also been initiated and have highlighted the ability of this radionuclide to induce both early and late DNA damage in tumoral cells.