Monte Carlo simulation of the ELEKTA SL25 linear accelerator with BEAMnrc for IMRT in head and neck tumors : study of the effects of inhomogeneities and their impact on the accuracy of the new treatment planning system VARIAN AAA

Sterpin, Edmond;Tomsej, Milan;Reynaert, Nick;Vynckier, Stefaan
(2006) 46ième journées scientifiques de la société française des physiciens médicaux — Location: Lyon, France (7.June.2006)

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Introduction : Monte Carlo simulation calculates photon/electron transport using physical laws and it is widely admitted as the most precise calculus tool for delivered doses by linear accelerator. Indeed, Monte Carlo simulation validity is not limited by delivered treatment complexity (small fields, modulated intensity, inhomogeneities, …) in opposition with analytical algorithms where some inaccuracies may occur in low density material, interfaces and in case of a lack of electronic equilibrium. The Monte Carlo calculation is therefore a first choice tool to calculate accurately dose distribution delivered by IMRT in a medium very heterogeneous, as in the head and neck tumors case. Material and methods : We build our Monte Carlo model using BEAMnrc, which allows to model different parts of the linac head. For our study, we used a phantom composed by two sets of thin slabs of low-density material separated by two thicker slabs with a density close to water. This phantom permits us to study phenomena like loss of lateral electronic equilibrium, build down and “rebuild up”. The measurement was achieved by placing films at different positions along depth. The depth dose could be then reconstructed. Our treatment planning system is a VARIAN – ECLIPSE station provided with the new “ Anisotropic Analytical Algorithm (AAA)”, which is a 3D pencil beam convolution/superposition algorithm. Results : We obtain an excellent agreement (dose differences less than 2% of dose maximum) between measurements and Monte Carlo calculus in water for all field sizes concerning profiles and depth doses. Monte Carlo calculation and measurement give also similar results in heterogeneous phantom, even for small fields (2x2 cm²). We can therefore use in the following of the work our model as a reference for the calculus of the dose in scanner of patients for head and neck cancers. Moreover, we compare results with the phantom with the ECLIPSE station’s calculations. We saw good accuracy for depth dose and profiles for 6x6 cm² and 10x10 cm² fields (2-3% maximum disagreement). AAA’s accuracy is somewhat less accurate for 2x2 cm², specially at interfaces, but AAA shows great improvements if we compare to older ECLIPSE algorithms. These results prove that AAA is handling correctly loss of lateral electronic equilibrium, thanks to a history correction kernel. Finally, we compare dose volume histogram calculated with our Monte Carlo model and with ECLIPSE of some patients treated for head and neck cancers in IMRT.

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Sterpin, E., Tomsej, M., Reynaert, N., & Vynckier, S. (2006). Monte Carlo simulation of the ELEKTA SL25 linear accelerator with BEAMnrc for IMRT in head and neck tumors : study of the effects of inhomogeneities and their impact on the accuracy of the new treatment planning system VARIAN AAA. 46ième journées scientifiques de la société française des physiciens médicaux, Lyon, France. https://hdl.handle.net/2078.5/57725