R. Nakada

Direct impact analysis of multi-leaf collimator leaf position errors on dose distributions in volumetric modulated arc therapy: a pass rate calculation between measured planar doses with and without the position errors*

We propose a new method for analyzing the direct impact of multi-leaf collimator (MLC) leaf position errors on dose distributions in volumetric modulated arc therapy (VMAT). The technique makes use of the following processes. Systematic leaf position errors are generated by directly changing a leaf offset in a linac controller; dose distributions are measured by a two-dimensional diode array; pass rates of the dose difference between measured planar doses with and without the position errors are calculated as a function of the leaf position error. Three different treatment planning systems (TPSs) were employed to create VMAT plans for five prostate cancer cases and the pass rates were compared between the TPSs under various leaf position errors. The impact of the leaf position errors on dose distributions depended upon the final optimization result from each TPS, which was explained by the correlation between the dose error and the average leaf gap width. The presented method determines leaf position tolerances for VMAT delivery for each TPS, which may facilitate establishing a VMAT quality assurance program in a radiotherapy facility.

SU‐E‐J‐12: Flex Compensation for a KV Cone Beam Computed Tomography System Integrated with Linear Accelerator: A Comparison with and without Add‐On Micro Multi‐Leaf Collimator

Purpose: Due to the weight of the accelerator head and the additional weight of the kV x‐ray tube along with its flat panel detector, flex compensation is required to match the kV cone beam CTimage center to the MV isocenter. However, the flex compensation has not been compared with and without an add‐on micro multi‐leaf collimator. The purpose of this study is to evaluate the impact of the add‐on micro multi‐leaf collimator on the flex compensation or flex map calculation. Methods: Before attaching an add‐on micro multi‐leaf collimator to Elekta Synergy, MV radiation center was determined by a ball bearing and the flex map was obtained. Subsequently, a micro multi‐leaf collimator was attached to the Synergy head and Winston‐Lutz test was performed to locate the most reasonable MV isocenter. Then a ball bearing was positioned at the MV isocenter., and another flex map was obtained and compared to the previously obtained one acquired before attaching the micro multi‐leaf collimator. Results: MV isocenter discrepancy with and without the micro multi‐leaf collimator was 0.5 mm in GT direction which is less than our tolerance limit. The flex map discrepancy with and without the micro multi‐leaf collimator was not significant, either. Conclusions: We could not detect significant flex map discrepancy with and without the micro multi‐leaf collimator. However, flex map measurement needs to be repeated after fully unifying the method for determining the MV isocenter between the two configurations.