SU‐F‐T‐514: Evaluation of the Accuracy of Free‐Breathing and Deep Inspiration Breath‐Hold Gated Beam Delivery Using An Elekta Linac

Abstract

Purpose:In this study, we evaluated the performance of an Elekta linac in the delivery of gated radiotherapy. We examined whether the use of either a short gating window or a long beam hold impacts the accuracy of the deliveryMethods:The performance of an Elekta linac in the delivery of gated radiotherapy was assessed using a 20cmX 20cm open field with the radiation delivered using a range of beam‐on and beam‐off time periods. Two SBRT plans were used to examine the accuracy of gated beam delivery for clinical treatment plans. For the SBRT cases, tests were performed for both free‐breathing based gating and for gated delivery with a simulated breath‐hold. A MatriXX 2D ion chamber array was used for data collection, and the gating accuracy was evaluated using gamma score.Results:For the 20cmX20cm open field, the gated beam delivery agreed closely with the non‐gated delivery results. Discrepancies in the agreement, however, began to appear with a 5‐to‐1 ratio of the beam‐off to beam‐on. For these tight gating windows, each beam‐on segment delivered a small number of monitor units. This finding was confirmed with dose distribution analysis from the delivery of the two VMAT plans where the gamma score(±1%,2%/1mm) showed passing rates in the range of 95% to 100% for gating windows of 25%, 38%, 50%, 63%, 75%, and 83%. Using a simulated sinusoidal breathing signal with a 4 second period, the gamma score of freebreathing gating and breath‐hold gating deliveries were measured in the range of 95.7% to 100%.Conclusion:The results demonstrate that Elekta linacs can be used to accurately deliver respiratory gated treatments for both free‐breathing and breath‐hold patients. The accuracy of beams delivered in a gated delivery mode at low small MU proved higher than similar deliveries performed in a non‐gated (manually interrupted) fashion.