[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%.

Development of a simple control system for uniform proton dose distribution in a dual-ring double scattering method.

In proton radiotherapy with high focusing of irradiation on the tumour, it is important to obtain treatment beams with a highly uniform dose distribution. Uniform dose distribution in the clinical irradiation field can be obtained by the dual-ring double scattering method. This method is superior to the wobbler method, which uses electromagnetic deflection of the proton beams, because of the absence of the temporal structure of irradiation distribution. However, in the dual-ring double scattering method the condition of incident proton beams entering the scatter, especially the accuracy of the position of the incident proton beams with respect to the scatter, markedly affects the uniformity of the beam distribution in the irradiation field. In this study, to ensure the uniformity of dose distribution during treatment, we developed a control system equipped with an automatic fine adjustment of the beam axis and a mechanism for moving the second dual-ring scatter of the double scatters to the optimal position. Using this system, we achieved uniform dose distribution in the irradiation field during proton radiotherapy, with symmetry within +/-1% and flatness within 2%.

[Evaluation of daily quality assurance for proton therapy at National Cancer Center Hospital East.].

It is very important for proton therapy to make and carry out a quality assurance (QA) program. Operators and medical personnel have to avoid any trouble during treatment and provide patients with high quality and reliable treatment. A proton therapy machine consists of an accelerator, gantry and various beam modifying devices. Although QA for proton therapy is complicated and difficult, we tried to realize daily QA for proton therapy. Two main purposes in daily QA are stable operation of the accelerator and reliable control of the irradiation system. In order to determine the daily QA issues, past problems were considered. We made a daily QA program and used it for one year. In this paper, we show the daily QA program and report our results. The daily QA was clearly effective for stable operation of the accelerator and for controlling quality of the irradiation system.