Investigation on the impact to beam characteristics of a linear accelerator related to duty cycle of respiratory gating

Abstract

Abstract Background and objective Respiratory-gated techniques are based on controlling the linear accelerator and limiting the beam-on time to only a small portion of the patient’s breathing cycle. For each beam-on, the machine must spend a start-up time (ST) to set up a stable and correct RF field in the waveguide and get the right beam current, etc. That might cause an obvious variation on the beam dosimetric characteristic, including the beam quality and profile, when the duration of ST is un-ignorable to the beam-on time. The purpose of this study is to investigate this kind of impact for various duty cycles (DC) of breathing-gated operation. Materials and method A respiratory motion phantom and a Real-time Position Management System (RPM) were used to simulate the patient respiration and to control the beam gating. Measurements were performed on a Varian 600C/D linac with 6 MV photons, using different DCs of 5%, 7%, 10%, 12%, 20%, 50%, 75%, and 100% to a 6-s breathing cycle (BC) simulation. The beam quality, flatness, symmetry were evaluated for these conditions. Output reproducibility and dose linearity were measured and evaluated for different MUs at 400 MU/min dose rate and 30% DC as well. Results For gated delivery in the above selected DCs and BC, the maximum changes in beam quality, symmetry, flatness were −0.181%, 0.124%, −0.17%, compared with non-gated data respectively. The coefficient of dispersion of output was less than 0.3% and good dose linearity was obtained for the tested gating operation condition. Conclusion For situation similar to our experiment, the duty cycle effect to beam characteristics is ignorable that the beam modeling based on non-gated delivery can be directly used for respiratory gating therapy. Otherwise, the beam characteristics should be carefully checked individually.