Build-down and Build-up of Absorbed Dose at Interfaces between Air and Tissue A Comparative Study between Monte Carlo Simulations and the Helax-TMS Treatment Planning System

Abstract: Purpose: To study the build-down and build up of the absorbed dose at the interface between air and tissue in the head and neck region. Monte Carlo simulations are compared to a treatment planning system that is used at the University Hospital in Lund. Materials and methods A virtual phantom was constructed to resemble an air cavity in the head and neck region. The phantom consisted of a water cylinder with an air cylinder place at the centre. Irradiation with three different field sizes and four different energies were included. The Monte Carlo simulations were done with the EGSnrc user code DOSRZnrc and the treatment planning system was Helax-TMS. In Helax-TMS the dose calculation algorithms Pencil Beam and Collapsed cone were included. Results: The results from the Monte Carlo simulations and the treatment planning system were presented as depth dose curves and dose profiles. The build-up effect is most evident for the smallest field sizes and especially for the field size that has a smaller radius than the air cavity radius. The Pencil Beam algorithm was overestimating the dose compared to the Monte Carlo simulations. The curves for Collapsed Cone and Monte Carlo simulation have a good agreement. Conclusion: Pencil Beam overestimates the dose at the interface between air and water when an air cavity is present. The recommended accuracy limits are not met for this algorithm. Collapsed Cone on the other hand gives a better estimation of the actual absorbed dose in this region and is a better tool to calculate the dose in regions with air cavities. Collapsed cone is however a very time consuming algorithm.