Permanent brachytherapy challenges and solutions : new plastic radioactive seeds and interseed effect correction for online prostate treatment dosimetry

The overall subject of this work was the dosimetric study of new, low energy photon sealed sources made of polymer for clinical brachytherapy application. A key goal was to resolve the problem of inaccuracies in real-time dosimetry that occur as a result of self-shielding by seeds (interseed effect), which is neglected by current treatment planning systems (TPS). Permanent brachytherapy implantation has become a popular treatment option in the management of early stage prostate cancer. Adjuvant stereotactic permanent seed breast implants, similar to those used in the treatment of prostate cancer, have also been developed, with encouraging results. With this mode of therapy, a high radiation dose can be delivered locally to the tumor with rapid dose fall-off in the surrounding normal tissue. Two known isotopes are commonly used: Iodine-125 (mean energy of 27 keV) and Palladium-103 (mean energy of 21 keV). At these low energies, dosimetric characteristics are very dependent on the internal design of the seed; hence, a thorough dosimetric study of any new source design is essential. Most marketed seeds have a metallic shell (titanium), which, however, causes artifacts that can disturb follow-up imaging. This problem prompted companies to develop plastic seeds to reduce these artifacts. The first part of the present work, therefore, describes a dosimetric study of two new seed models produced by the IBt-Bebig group, made with a biocompatible polymeric shell rather than titanium. Measurements with thermoluminescent detectors and Monte Carlo calculations using MCNP codes versions 4C and 5, were performed to determine the dosimetric characteristics of the seeds based on the AAPM Task Group No. 43 Updated (TG-43U1) recommendations. One of the major concerns in radiation treatment is the accuracy of calculated and delivered doses and their distributions relative to the prescribed dose. Currently, dose calculations for patient treatment in brachytherapy are based on the TG-43U1 protocol, which uses line or point source approximation, and assumes homogeneous medium dosimetry and negligible interseed effect because of the complexity of including these factors in the calculations; however, these assumptions do not accurately reflect the dose distribution for brachytherapy using low-energy photon emitters. The interseed effect is defined as the attenuation effect of one seed on the irradiation field of another implanted seed. The complexity of this effect involves many variables, e.g., seed construction, seed positions, distances between seeds and the density of implanted seeds per volume (seed/cm³). Many studies have shown non-negligible perturbations in the dose distributions of 125I and 103Pd coplanar aligned seeds when the interseed attenuation effect is ignored. Therefore, the Monte Carlo TPS is necessary to correct for these differences, and is a more accurate calculation method than TG-43U1 for implant dosimetry. However, general-purpose Monte Carlo codes have prohibitively long computing times, taking about 24h per calculation for one patient depending on computer performance, so this method cannot be used for real-time dosimetry. The second part of this thesis presents a more rapid Monte Carlo dose calculation engine, which has been developed using the MCNP5 code and takes into account the interseed effect. In general, seed dosimetric characteristics are determined using Monte Carlo (MC) simulations. However, such calculations can give different results depending on the MC calculation codes used. These codes can differ in their basic data or in the approximations made in the underlying physics. Experimentally, dosimetry can be performed using thermoluminescent dosimeters (TLDs). However, it is still a challenge to obtain data with high spatial resolution because of the large dose gradient and the very low dose rate (LDR). In this context, there is a need to develop new experimental methods that allow estimation of the dose deposited in the proximity of brachytherapy seeds. The third part of this work concentrates on attempts to develop a new dosimetry method, based on the reconstruction of dose using electron paramagnetic resonance (EPR) imaging (EPRI).