Patient-specific Monte Carlo-based dose-kernel approach for inverse planning in afterloading brachytherapy.
ABSTRACT Brachytherapy planning software relies on the Task Group report 43 dosimetry formalism. This formalism, based on a water approximation, neglects various heterogeneous materials present during treatment. Various studies have suggested that these heterogeneities should be taken into account to improve the treatment quality. The present study sought to demonstrate the feasibility of incorporating Monte Carlo (MC) dosimetry within an inverse planning algorithm to improve the dose conformity and increase the treatment quality.
The method was based on precalculated dose kernels in full patient geometries, representing the dose distribution of a brachytherapy source at a single dwell position using MC simulations and the Geant4 toolkit. These dose kernels are used by the inverse planning by simulated annealing tool to produce a fast MC-based plan. A test was performed for an interstitial brachytherapy breast treatment using two different high-dose-rate brachytherapy sources: the microSelectron iridium-192 source and the electronic brachytherapy source Axxent operating at 50 kVp.
A research version of the inverse planning by simulated annealing algorithm was combined with MC to provide a method to fully account for the heterogeneities in dose optimization, using the MC method. The effect of the water approximation was found to depend on photon energy, with greater dose attenuation for the lower energies of the Axxent source compared with iridium-192. For the latter, an underdosage of 5.1% for the dose received by 90% of the clinical target volume was found.
A new method to optimize afterloading brachytherapy plans that uses MC dosimetric information was developed. Including computed tomography-based information in MC dosimetry in the inverse planning process was shown to take into account the full range of scatter and heterogeneity conditions. This led to significant dose differences compared with the Task Group report 43 approach for the Axxent source.
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ABSTRACT: An accelerated Monte Carlo code [Monte Carlo dose calculation for prostate implant (MCPI)] is developed for dose calculation in prostate brachytherapy. MCPI physically simulates a set of radioactive seeds with arbitrary positions and orientations, merged in a three-dimensional (3D) heterogeneous phantom representing the prostate and surrounding tissue. MCPI uses a phase space data source-model to account for seed self-absorption and seed anisotropy. A "hybrid geometry" model (full 3D seed geometry merged in 3D mesh of voxels) is used for rigorous treatment of the interseed attenuation and tissue heterogeneity effects. MCPI is benchmarked against the MCNP5 code for idealized and real implants, for 103Pd and 125I seeds. MCPI calculates the dose distribution (2-mm voxel mesh) of a 103Pd implant (83 seeds) with 2% average statistical uncertainty in 59 s using a single Pentium 4 PC (2.4 GHz). MCPI is more than 10(3) and 10(4) times faster than MCNP5 for prostate dose calculations using 2- and 1-mm voxels, respectively. To illustrate its usefulness, MCPI is used to quantify the dosimetric effects of interseed attenuation, tissue composition, and tissue calcifications. Ignoring the interseed attenuation effect or slightly varying the prostate tissue composition may lead to 6% decreases of D100, the dose delivered to 100% of the prostate. The presence of calcifications, covering 1%-5% of the prostate volume, decreases D80, D90, and D100 by up to 32%, 37%, and 58%, respectively. In conclusion, sub-minute dose calculations, taking into account all dosimetric effects, are now possible for more accurate dose planning and dose assessment in prostate brachytherapy.Medical Physics 01/2006; 32(12):3688-98. · 2.91 Impact Factor