Validation of two calculation options built in Elekta Monaco Monte Carlo based algorithm using MCNP code

Abstract

IntroductionThe calculation algorithm for intensity modulated radiotherapy (IMRT) built in the Elekta Monaco treatment planning system (TPS) is based on Monte Carlo (MC) simulation. Absorbed dose is calculated as dose to medium in medium (Dm,m), but the conversion from Dm,m to dose to water in medium (Dw,m) is enabled. According to published data, differences between these two options exist, particularly in bony structures. In this study, comparisons between dose calculation options built in Elekta Monaco TPS and Monte Carlo N-Particle transport code® (MCNP) in different materials are shown. Furthermore, the majority of clinical experience is based on the dose to water in water (Dw,w) concept provided by analytical algorithms and has represented the standard for dose calculation over the past few decades. Additionally, MCNP calculation was performed to simulate the Dw,w concept. Therefore, the correlation between Dw,w concept, and both calculation options provided by the Elekta Monaco TPS was determined. Materials and methodsTo evaluate the accuracy of TPS calculation options for 6 MV photon beam, MCNP simulation was performed for 13 different materials with mass densities ranging from 0.2 g/cm3 to 2.17 g/cm3 using simplified geometry. The simulation was performed in two ways: with standard material representation taking into account their chemical compositions and corresponding mass densities and using non-standard material representation employing chemical composition of water with varying mass densities. Depth dose curves calculated by MCNP were compared to those obtained by two calculation options Dm,m and Dw,m using root mean square (RMS) deviations. ResultsRMS deviations between depth dose curves, for Dm,m and Dw,m become largest for mass density 2.17 g/cm3, up to RMS = 13%. Comparison for both calculation options to the MCNP defined for Dm,m shows very good agreement, with RMS deviation less than 3% for the majority of examined materials. For the Dw,m calculation option results are acceptable in mass density range from 0.5 g/cm3 (RMS = 1.4%) to 1.06 g/cm3 (RMS = 2.4%). For the rest of examined materials, RMS deviation increases, with a maximal value of 12.4%. Absorbed dose calculation comparison between Dw,m and non-standard MCNP shows large deviations for the majority of used materials, up to RMS = 13.1%. RMS deviations between Dm,m calculation option and non-standard MCNP are much lower than one might expect. For the highest mass density in this research (ρ = 2.17 g/cm3), the RMS deviation is 3.7%. ConclusionAlthough it does not take into account the chemical composition of the medium, TPS calculation option Dm,m shows very good agreement with standard MCNP calculations. Furthermore, it is demonstrated that the Dw,m calculation option differs substantially from Dw,w. Additionally, it was found that for different materials absorbed dose calculated as Dm,m shows better agreement to the algorithms that calculate absorbed dose using Dw,w approach. Although the research was performed on simplified geometry, the results indicate that the use of Dm,m could be preferable in order to allow better consistency with previous clinical data in radiation oncology.