Dose accuracy research on air cavity interface in treatment planning system

Abstract

Objective To study the accuracy of collapsed cone convolution (CCC) and anisotropic analytical algorithm (AAA) in dosimetric calculation on the air cavity interface. Methods A BEAMnrc/EGSnrc Monte Carlo (MC) simulation was performed on a Varian Trilogy linear accelerator. The IBA Dosimetry blue 3D scanning system was used to verify the accuracy and reliability of the MC simulation. Central axis depth dose distribution and lateral dose profile in a water-equivalent phantom with variously sized air cavities were calculated by CCC and AAA. The obtained depth dose distribution and lateral dose profile were compared with those by MC simulation and EBT2 film, respectively. Results Both CCC and AAA overestimated the dose on the air cavity interface. In spite of some errors, CCC had a higher accuracy than AAA. The errors were mainly related to computational grid, field size, photon energy, cavity size, and the number of fields. Conclusion Electronic disequilibrium on the air cavity interface should be taken into account when CCC and AAA are used for dosimetric calculation in treatment planning system. Key words: Electron disequilibrium; Collapsed cone convolution; Anisotropic analytical algorithm; Monte Carlo algorithm; EBT2 film