Kazuhiro Tsukuda

Development of a new concept automatic frequency controller for an ultrasmall C-band linear accelerator guide

We are developing a four-dimensional, image-guided radiotherapy system with a gimbaled x-ray head. The system has pursuing irradiation capability in addition to precise irradiation capability, owing to its agile x-ray head. The moving x-ray head requires a very small C-band accelerator guide. The heat intensity of the accelerator guide is much higher than that of conventional S-band medical linear accelerators. The resonance frequency varies over almost 1.0 MHz with a thermal time constant of about 30 s. An automatic frequency controller (AFC) is employed to compensate for this variation in resonance frequency. Furthermore, we noted that fast AFC response is important for step-and-shoot intensity modulation radiotherapy (IMRT), in which the beam is turned on and off frequently. Therefore, we invented a digital AFC, based on a new concept, to provide effective compensation for the thermal characteristics of the accelerator guide and to ensure stable and optimized x-ray treatment. An important aspect of the performance of the AFC is the capture-frequency range over which the AFC can seek, lock on to, and track the resonance frequency. The conventional, analog AFC used in S-band medical linear accelerators would have a capture-frequency range of about 0.9 MHz, if applied to our accelerator guide, and would be inappropriate. Conversely, our new AFC has a capture-frequency range of 24 MHz, which is well suited to our accelerator guide. The design concept behind this new AFC has been developed and verified. A full prototype system was constructed and tested on an existing accelerator guide at the rated x-ray output (500 cGy/min) of our radiotherapy system, with a pulse-repetition frequency of 300 Hz. The AFC acquired the resonance frequency of the accelerator guide within 0.15 s after beam-on, and provided stable tracking and adjustment of the frequency of the microwave source to the resonance frequency of the accelerator guide. With a planned improvement of the initialization of the AFC it should be able to acquire the resonance frequency within 33 ms.

SU‐FF‐T‐155: Development of An Ultra‐Small C‐Band Linear Accelerator Guide and Automatic Frequency Controller

Purpose: The aim of this study was to evaluate the performance of our newly developed C‐band linear accelerator guide and digital AFC. Method and Materials: We are developing an image‐guidedradiotherapy system with a gimbaled X‐ray head. The system has the capability of pursuing irradiation in addition to the capability of precise irradiation with the help of the agile moving X‐ray head on the gimbals. The moving X‐ray head requires a very small C‐band accelerator guide and control system to stabilize the operation of the accelerator guide. Especially an automatic frequency controller (AFC) is needed to compensate properly for the thermal characteristics of the accelerator guide and assure a stable and optimized performance of the treatment X‐ray. We invented a small size C‐band accelerator and new concept digital AFC. Basic beam performance, leakage radiation and AFC control performance were evaluated with an existing radiotherapy system. Results: The accelerator guide is 38 cm long including the electron gun and weighs is about 10kg. The output beam current was 75mA with a transmission efficiency of 58%. The average energy was 5.3MeV. The beam profile was 0.7mm × 0.9mm FWHM width at the 88mm point on the axis of the accelerator guide. The maximum radiation leakage of the accelerator guide at 100cm from the axis of the accelerator guide was 0.33cGy/min at the rated X‐ray output of 500 cGy/min. This leakage requires no radiation shielding. The AFC acquired the resonance frequency of the accelerator guide within 0.15 s after the beam on and gave a stable tracking and adjustment of the frequency of the microwave source to the resonance frequency of the accelerator guide. Conclusion: An ultra‐small C‐band linear accelerator guide and new concept digital AFC were developed and the performance of this system was confirmed.