Intra-voxel heterogeneity and its effect on 3d brachytherapy dose distributions

Abstract

Introduction CT based images are used to reconstruct a voxelized dose calculation geometry in 3d brachytherapy treatment planning. Purpose To investigate dose uncertainties due to intra-voxel inhomogeneities of CT based phantoms for brachytherapy sources. Materials and methods In this study, an HDR 192 Ir source is placed in a water phantom. A target volume with the dimensions of either 1.0 × 1.0 × 1.0 mm 3 or 2.0 × 2.0 × 2.0 mm 3 is located at 1 cm away from the source center and divided into a grid of 10 × 10 × 10 cubic sub-voxels. Heterogeneous media within the target volume is obtained by filling each sub-voxel with one of 5 materials; water, adipose, air, lung and bone. In another comparison study, the target volume is filled with only one medium, a kind of material which has an average elemental composition and mass density over sub-voxels at the other simulation geometry. BrachyDose Monte Carlo code is used in all dose calculations. Results r 2 D( r ) values were evaluated to eliminate inverse square dependence. Our results showed that intra-voxel heterogeneities may result in up to 3% or 5% dose difference on the brachytherapy dose distributions for either 1.0 × 1.0 × 1.0 mm 3 or 2.0 × 2.0 × 2.0 mm 3 target volumes, respectively. Conclusion Our conclusion is that the influence of intra-voxel inhomogeneities on brachytherapy dose distributions are significant and they should be taken into account in treatment planning systems. Disclosure This study is partly supported by TUBITAK under project number: 213E028.