Improvement of dose monitor system in rotational gantry for proton therapy at national cancer center hospital East

Quality assurance and quality control of delivered dose are important in proton therapy. The delivered proton dose is controlled using a dose monitor system which consists of two ionization chambers filled with free air. Response of the monitor for the delivered dose changes under the influence of temperature and pressure of surrounding air. In the dose monitor system at the National Cancer Center Hospital East, the monitor is near an X-tray tube which operates at a high temperature and is frequently used in every patient setup. Therefore the response of the dose monitor changes a lot with the temperature of the X-ray tube. In this report, temperature and pressure influence on the dose monitor was investigated in detail. Furthermore, a new method was presented to correct the monitor response using the real-time data of temperature and pressure at any time for each patient. This method improved the accuracy of delivered dose from 1% to 0.5%.