211 - 4D dose calculations: Tetrahedral meshes versus voxel-based structures

Abstract

Purpose: The estimation of the distribution patterns of energy and dose in respiratory-induced organ motion represents a technical challenge for hadron therapy treatment planning, notably in the case of lung cancer in which many difficulties arose, like tissue densities variation and the tumor position shifting during breathing. This study focuses on the comparison between deformable tetrahedral meshes and voxel-based structures used as computational phantoms in four-dimensional dose calculations. The former use a continuous representation of tissue densities by respecting mass conservation principle, while the latter is a discrete grid of density values (CT-scan). Methods: The movement used to simulate breathing is generated with deformable image registration (DIR) of CT images (Castillo, 2010) (Klein, 2010) (Shamonin, 2013). Tissue tracking for tetrahedral model is implicitly performed by the fact that the meshes maintain their topology during deformations. The dose distribution is calculated using the time-dependent tetrahedral density map issued from 4D-CT scans (Petru Manescu, 2014). Unlike image-based methods, the deposited energy is accumulated inside each deforming tetrahedron of the meshes. An implementation of this dose computation method on a deformable anatomy in the case of a passive scattering beam line is demonstrated using the Geant4 code (Agostinelli, 2003). Besides, energy values in voxel-based structures are calculated for each time step and accumulated using the transformations provided by the registration. Then, values are accumulated back onto the reference image and divided by the mass to obtain the 4D dose map. Figure 1 illustrates the process used to accumulate dose in respiratory-induced simulations. Results: The tetrahedral mesh dose distribution was compared to the conventional voxel-based structure using a thoracic 4D-CT data of a patient case. Preliminary results show that dose distributions for both representations are in a good agreement (figure 2), and dose homogeneity is about the same (table1). However, motion-induced dose accumulations are more intuitive using a tetrahedral model since they do not introduce additional uncertainties with image resampling and interpolation methods, and also for the fact that they respect mass conservation principle. Conclusion: We have developed a 4D tetrahedral model for Monte Carlo dose calculations alongside its implementation on the Geant4 platform. Results of comparison with conventional methods based on voxels have shown that dose distributions are in good agreement. This novel structure can be of a great aid for treatment planning of moving targets. An experimental validation based on 4D anthropomorphic phantom (e.g. LuCa phantom developed in paul scherrer institute) (Neihart, 2013) would draw a clear conclusion regarding the performance of the presented method in comparison with the classical methods. Nevertheless, the main advantage of this method is that, coupled with a patient-specific biomechanical model, it could be used in the future to correct motion artefacts in treatment planning.