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A Method to Accurately Estimate Out-Of-Field Organ Dose Based On the Calculations From a Commercial Treatment Planning System

G Ding

L Wang , G Ding*, Vanderbilt University Medical Center, Nashville, TN


SU-I-GPD-T-360 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Radiation beam leakage and scatter result in unintended out-of-field doses where organs-at-risk are often located. This study evaluated the out-of-field dose calculated by the Varian Eclipse treatment planning system (v.11 with AAA algorithm) in realistic treatment plans with the goal of improving the accuracy of these estimates.

Methods: Photon beam phase-space files for TrueBeam linear accelerator were provided by Varian. These were used as incident sources in EGSnrc Monte Carlo simulations of radiation transport through the downstream jaws and MLC. Dynamic movements of the MLC leaves were fully modeled based on treatment plans using IMRT or VMAT techniques. The Monte Carlo calculated out-of-field doses were then compare those calculated by Eclipse. The dose comparisons were performed for different beam energies and treatment sites, including head-and-neck, lung and pelvis.

Results: For 6MV (FF/FFF), 10 MV (FF/FFF), and 15MV (FF) beams, Eclipse underestimated out-of-field local doses by 30-50% compared to Monte Carlo calculations when the local dose was < 1% of prescribed dose. The underestimate decreased to 0.5% when the local dose was 1% of prescribed dose. The accuracy of out-of-field dose calculations using Eclipse improved when collimator jaws were set at the smallest possible aperture for MLC openings.

Conclusion: The Eclipse system consistently underestimates out-of-field dose for all beam energies studied, with the amount by which the dose is underestimated depending on the local dose level. These findings can be used to improve the accuracy of out-of-field organ doses estimated by Eclipse when Monte Carlo simulations are not available.

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