SU‐E‐T‐369: Evaluation of Dose Control System Performance on a TomoTherapy Hi‐Art System

Abstract

Purpose: TomoTherapy Hi'¢Art systems may exhibit rotational dose fluctuation and dose drift during treatment delivery. A dose servo system has been developed by Accuray, Inc. to monitor and control the dose output on TomoTherapy Hi'‐Art systems. The Dose Control System (DCS) provides continuous control of the linear accelerator output through a closed loop control system to maintain a constant dose output. The purpose of this study is to evaluate the clinical performance of the DCS and to quantify the stability of the dose output after the installation of DCS on our Hi'¢Art unit. Methods: Daily ion chamber measurements in a solid water phantom for static and IMRT deliveries were compared for a time period of two months before and after the DCS installation. Weekly beam measurements using onboard MVCT detector and ion chamber data acquired by field service engineers were also analyzed. Dose stability and intra‐procedure dose drift were assessed from data before and after the DCS installation. Results: Both the range and standard deviations of daily static and IMRT ion chambermeasurements improved with the DCS. The standard deviation of the daily IMRT measurements improved from 0.96% to 0.5%. The dose drift also improved significantly with the DCS. The intra‐dose dose drift during a 200 second rotating procedure decreased from an average of 1.02% to 0.02% based on on‐board ion chamber data. The data show that the dose output is much more stable overall and more consistent from day to day. Conclusions: Based on dosimetric QA endpoints, the DCS has shown a marked improvement in machine dose stability and intra‐procedure dose drift when compared to data from before DCS was installed. MD Anderson Cancer Center Orlando holds a research grant from Accuray, Inc.