Effects of scattered radiation due to a gold internal fiducial marker for precise setup in external radiotherapy

Monitoring a gold marker position in or near the tumor in the radiotherapy can identify the tumor localization with a high degree of accuracy. However, it has been reported that the dose calculation using Radiation Treatment Planning Systems (RTPS) cannot calculate with the accuracy in the vicinity of high-Z and high-density interfaces. We investigated the effects of the scattered radiation due to a gold internal fiducial marker along the beam axis in a virtual lung phantom. However, practically, it is very important that the effects of scattered radiation near the marker are considered in the patient body. The purpose of this work is to study effects of the scattered radiation due to the gold internal fiducial marker in the virtual lung phantom by comparing the relative dose distributions of vertical direction to the beam axis using Monte Carlo (MC) simulation, film measurement and the RTPS calculation. The Electron Gamma Shower code version 5 (EGS5) was used for MC simulation. Radiochromic Film was used for the measurement. XiO was used for the RTPS calculation. The virtual lung phantom was irradiated by 10 MV photon beam with 5 x 5 cm ² field size. The differences for isodose level greater or equal to 50 % between the MC simulation and the RTPS calculation were up to 23.5 % in front of the gold marker, and up to 7.8 % at behind of gold marker. The causes of the differences between the MC simulation and the RTPS calculation are thought that the RTPS cannot precisely calculate the doses of the backscatter radiation, the frontscatter radiation, and the lateral scatter radiation, the lack of the secondary electron equilibrium, and etc.