Comparison Of Accuracy Of Two Dose Algorithm In Small Air Cavity Interface Dose

The effect of dose distribution influenced by tissue inhomogeneity is particularlyimportant in clinical practics. Human body contains several cavities. Studies haveshown that when photon beams pass through air cavity, the charged particledisequilibrium will appear near the air tissue interface. Larger than5%dose errors willoccur when this effect was ignored. The dose calculation accuracy directly affects thetreatment result of largest part of tumor patients. Monaco algorithm and Xio are twokinds of the most commonly used commercial TPS system.The accuracy of XVMC of Monaco and MGS of Xio near the interface in lateraland longitudinal direction was compared in our study. A Monte Carlo simulation on theElekta linear accelerator is performed with EGS4code. And measurements were madeusing IBA Dosimetry “blue phantom”3D scanning systems to verify Monte Carlosimulation. Results confirm the reliability of this Monte Carlo algorithm.Central axisdepth dose distribution and profiles in a water equivalent phantom with variously-sizedair cavities were calculated by XVMC and MGS, and compared with Monte Carlosimulation and EBT2film, respectively. The influence of cavity shape, cavity size, fieldsize, energy, the number of field on the XVMC and MGS dose calculation errors wasinvestigated. Our study has shown that XVMC and MGS both overestimate the dosenear the air tissue interface, XVMC performs better than MGS. Prediction errors arebase on filed size, energy photon beam the number of filed cavity density and grid does.The results of this study can be used as a guide in the clinical. The XVMC and MGSalgorithm should be enhanced to fully account for electron disequilibrium near aircavity interface.