Room: Stars at Night Ballroom 1
Purpose: Gated delivery involves repeated beam holds, during which beam properties require pulses to stabilize. As a result, TG-142 recommends measuring output and energy of gated delivery systems for monthly and annual quality assurance, respectively. Detector arrays have proven effective at calculating energy changes from changes in beam profiles. This work studies the robustness of energy calculations derived from beam profiles to beam changes during ramp up for gated beam delivery.
Methods: The Sun Nuclear IC Profiler (ICP) and Quad Wedges were used to measure output and energy of gated 6X, 6X-FFF, and 10X-FFF photon beams. ICP was compared to standard 2 ion chamber PDD20/PDD10 measurement taken at 100 cm SSD using Standard Imaging Exradin A12 and A16 chambers with PC Electrometer. A series of beam pulses consisting of 0.5, 1, 1.5, 2, 2.5, 3, 5, 10, and 50 MU bursts were delivered in 5 successive repetitions using Varian Developer mode on a TrueBeam to simulate gated delivery. Measurements were taken at 125 and 100 ms for ICP and ion chambers, respectively. PDD20/PDD10 measurements were converted to PDD10 for comparison to the ICP PDD10 results. All analysis was performed in Matlab.
Results: Average ICP flatness and symmetry was within 1% for all energies and within 0.5% beyond 2 MU, indicating beam stability for all burst durations. The maximum observed PDD10 energy deviations from baseline were 0.09Â±0.05% and 0.6Â±1.9% for the ICP and ion chamber pair, respectively. Mean ICP deviations for MU bursts <3MU and all ICP standard deviations were an order of magnitude smaller than ion chamber.
Conclusion: Both techniques measured energies within the 2% TG-142 recommendation for all MU bursts. Significant beam asymmetry during ramp up was not observed and ICP provided stable energy and output measurement for very low MU delivery, validating its use for gating QA.