Assesment of radiation dose distributions in heterogenous media for low energy gamma rays used in brachytherapy treatment by Monte Carlo method on micro scaled phantoms

CT based images are used to reconstruct a voxelized dose calculation geometry in 3D brachytherapy treatment planning. Our purpose in this study is to investigate dose uncertainties due to intra-voxel inhomogeneities of CT based phantoms for brachytherapy sources. In order to achieve this purpose, an HDR 192 Ir source was placed in a water phantom. A target volume with the dimensions of either 1.0x1.0x1.0mm 3 or 2.0x2.0x2.0mm 3 was located at either 0.5 cm or 1 cm away from the source center and on the transverse axis of the source. In addition, the target volume was divided into a grid of 10x10x10 cubic sub-voxels. Heterogeneous media within the target volume was obtained by filling each sub-voxel with one of 5 different materials (i.e. water, air, adipose, bone or lung tissue). In another comparison study, the target volume was 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 was used in all dose calculations. In order to eliminate inverse square dependence, r 2 D(r) values were evaluated. Our results shows that intra-voxel heterogeneities may result in up to 3% or 5% dose difference on the brachytherapy dose distributions for either 1.0x1.0x1.0mm 3 or 2.0x2.0x2.0mm 3 target volumes, respectively.