SU‐E‐T‐343: Respiratory‐Gated Volumetric Modulated Arc Therapy for Lung SBRT with an Elekta Digital Accelerator: A Pre‐Clinical Evaluation

Abstract

Purpose: To evaluate the machine‐delivery capabilities and dosimetric accuracy of respiratory‐gated VMAT delivered with an Elekta digital accelerator. Method and Materials: Three lung patients previously treated with SBRT using un‐gated 6 MV VMAT were used for this evaluation. The treatment plans were created in Monaco treatment planning system (ver. 3.0). For each case, one un‐gated and two gated deliveries were delivered to a static ion‐chamber‐array phantom. Surface motion was used as a surrogate for breathing motion and was simulated using a moving chest plate driven by a motor with a constant amplitude range of +/− 3 mm and a period of 4 seconds using a cos6ωt waveform. Two gating windows (GWs), 77% and 66% around the end of exhalation were used for the gated deliveries. An ion chamber array was used and comparisons were made between the measured and the planned coronal dose distributions. The dosimetric accuracy of gated VMAT plans was determined using gamma‐index analyses (3 mm/3% passing rate criteria). Results: There were insignificant differences between the two gated deliveries and the un‐gated delivery. For the planar dose comparisons between the measured and the planned, the gamma passing rates were all higher than 99.0%. For the two gated deliveries, the true duty cycles were calculated as 57% and 39%, as against the corresponding GWs 77% and 66%. This was mainly due to the beam‐on latencies, which were estimated as 0.25 second and 0.40 second for GWs 77% and 66%, respectively. Conclusions: Respiratory‐gated VMAT was demonstrated for the first time using an Elekta digital accelerator with a pre‐clinical gating interface provided by Elekta and a novel surface imaging system as a respiratory input. High dosimetric accuracy was demonstrated in three SBRT lung cases for as many as 213 beam interruptions during a single 360°‐arc delivery. This work is sponsored by a research grant from Elekta AB.