A Practical and Simple Method to Estimate Beam-on Delay In Respiratory-Gated Radiotherapy
Abstract
Purpose
To develop a practical and simple method for estimating beam-on delay associated with respiratory-gated radiotherapy and validating the accuracy of the estimated delays
Methods
A respiratory monitoring system (AbchesET, APEX Medical, Japan) tracked the motion of the phantom and delivered gating signals to a linear accelerator (VersaHD, Elekta, Sweden) in real time. Two beams of 6MV and 6FFF and seven amplitude-based gating windows (20, 30, 40, 50, 60, 70, and 80%) were used to acquire gating durations (GD) and delivered MUs for each gating window. The calculated beam-on delays were acquired from an equation using GD and MU. To obtain the reference beam-on delays, a rod with a ball-bearing (BB) marker attached at the end was placed on the moving phantom and captured the moment the BB entered 50% amplitude using a megavoltage imager. The velocity of the BB was 86.1 pixels per second.
Results
The delay averaged from the maximum (80%) and minimum (20%) gating windows was closest to the reference. The calculated (reference) beam-on delays were 230 (244) ms and 80 (95) ms for 6MV and 6FFF beams, respectively. The calculated delays were underestimated by approximately 15 ms compared with the references.
Conclusion
This study demonstrated an experimental and practical method to estimate beam-on delay with acceptable accuracy. Because this method does not require substantial effort or cost typically needed for direct beam-on delay measurements, it could be suitable for periodic machine quality assurance and for evaluating specific treatment cases in a clinical setting.