Dosimetric Effect of the Duration of Gating Window on Respiratory Phase-Based Gated VMAT

Abstract

When phase-based respiratory gating is applied to volumetric modulated arc therapy (VMAT), dose rate, gantry rotation speed and the motion of MLCs are modulated dynamically to coordinate with the respiratory gating signal during beam delivery. The shorter the duration of gating window, the more demanding is the coordination during the treatment and the higher the potential for introducing dosimetric errors. The purpose of this work is to evaluate the dosimetric effect on respiratory phase-based gated VMAT due to the duration of gating window. A linear accelerator with VMAT capability, a moving platform capable of producing stable periodic movement to simulate respiratory motion, a helical diode array and a 0.057 cc A1SL ionization chamber for dosimetric measurement were used in the study. Ten clinical gated VMAT plans with maximum dose rate 600MU per minute were selected and recalculated with the helical diode array CT images for dosimetric measurement. In the setup the moving platform was placed outside the treatment field. Gated VMAT was achieved by placing motion tracking block on the moving platform to enable the RPM respiratory gating system to track the respiratory signal. The gating window was set at 40-60%. The duration of the gating window can be changed by changing the frequency of the motion of the moving platform. Firstly dosimetric measurements with helical diode array and A1SL ionization chamber were done for all plans without respiratory gating. Such measurements were repeated with respiratory gating using different respiratory signals such that the duration of gating window was varying between 0.4 and 1 second in 0.2 second increments. Gamma tests (3% dose, 3 mm) were carried out to do the absolute dose comparisons between the helical diode array measurement and planning calculation. Passing rate of 90% was set as a threshold for the test. Measurements for all plans without respiratory gating showed good agreement with planning calculation. The average absolute point dose difference was 1.3 ± 1.0% while the average absolute gamma passing rates was 97.7 ± 1.1%. When respiratory gating was applied during the treatment, the average absolute point dose differences were 1.3 ± 1.0%, 1.4 ± 0.9%, 2.0 ± 1.0% and 2.2 ± 1.1% for the duration of gating window equal to 1, 0.8, 0.6 and 0.4 second respectively. The average absolute gamma passing rates were 95.8 ± 0.8%, 94.9 ± 1.5%, 89.4 ± 6.2% and 87.3 ± 7.8% for the duration of gating window equal to 1, 0.8, 0.6 and 0.4 second respectively. Dosimetric errors in gated VMAT treatment delivery increase with decreasing duration of gating window. Our study shows that dosimetric errors become unacceptable when gating window falls below 0.6 second.