Development of four-dimensional Monte Carlo dose calculation system for dynamic tumor-tracking irradiation with a gimbaled X-ray head

Abstract

The purpose of this study was to propose a dose calculation system for a dynamic tumor-tracking irradiation by gimbals mechanism in Vero4DRT.

First, 6-MV photon beam delivered by the Vero4DRT was simulated using EGSnrc. Difference of output characteristic between at the maximum and minimum rotation angles without setup error was estimated from each of the simulated lateral doses (10x10 cm2 at 100 mm depth). Subsequently, a moving phantom was driven at a frequency of 0.25 Hz with amplitude of 10 mm, horizontally. The phantom position directly measured by a laser displacement gauge; rotational angles and MUs responded by Vero4DRT; and the time were recorded in a log file. Then, a phase space data at any angles was created from both the log file and the particle data under the MLC. Finally, both irradiation with and without tumor-tracking were simulated with irradiation size of 4x4 cm² at 50 mm depth under the SSD of 950 mm. The corresponding film measurement was performed as well.

Next, dose calculation at each CT data of ten phases with the X-ray head rotated was performed to simulate the dynamic tumor-tracking irradiation. Subsequently, deformable image registration between the end-ofexhale phase and at other phases was performed using MIMvista to accumulate dose distribution. Isodose curves and DVHs of the dynamic tumor-tracking plan and the ITV plan were calculated.

The difference between doses at the minimum and at the maximum angles was less than 0.5% in flat region. The averaged differences between the simulated and the measured doses with stationary and tumor-tracking irradiation were 1.9% for both plans. The mean-lungdose in the dynamic tumor-tracking plan is 15% lower than that of the ITV.

This study has demonstrated our proposed system has acceptable accuracy for dynamic tumor-tracking irradiation using the Vero4DRT.