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

Abstract

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.

Four‐dimensional dose calculations for dynamic tumour tracking with a gimbal‐mounted linear accelerator

“…1, is a gimballed linac capable of DTT by panning and tilting the beam up to ±2.4° in either direction to follow a moving tumor in cranio‐caudal and lateral directions, reaching any point within ±4.2cm in the plane at isocenter perpendicular to the beam 10 . DTT is dependent on building a four‐dimensional (4D) respiration correlation model prior to each fraction using synchronously monitored internal fiducial markers located near the target and external infrared (IR) markers placed on the abdomen at the point where respiratory motion is the largest 9,11,12 . Throughout treatment, the motion of the external IR markers is monitored and the 4D respiration correlation model uses that information to determine the real‐time motion of the internal fiducial markers.…”

“…The linac head is installed on an O‐ring shaped gantry that can rotate around the patient’s inferior‐superior axis and posterior‐anterior axis, enabling non‐coplanar beam deliveries specified by a gantry and ring angle, respectively. The Vero also has two integrated orthogonal sets of kV x‐ray tubes and flat panel detectors for on‐board imaging 9,11,12 …”

“…10 DTT is dependent on building a four-dimensional (4D) respiration correlation model prior to each fraction using synchronously monitored internal fiducial markers located near the target and external infrared (IR) markers placed on the abdomen at the point where respiratory motion is the largest. 9,11,12 Throughout treatment, the motion of the external IR markers is monitored and the 4D respiration correlation model uses that information to determine the real-time motion of the internal fiducial markers. The system pans/tilts the beam according to the internal marker motion assuming the target's displacement is identical.…”

“…The Vero also has two integrated orthogonal sets of kV x-ray tubes and flat panel detectors for onboard imaging. 9,11,12 The RayStation (RaySearch Laboratories, Sweden) treatment planning system (TPS) is commercially available for generating plans for delivery on the Vero. 14,15 However, it does not incorporate the panning and tilting beam geometry that occurs during DTT when calculating a dose distribution for a plan.…”

A method of surface marker location optimization for tumor motion estimation in lung stereotactic body radiation therapy

“…This method has been successfully adopted for some commercial IGRT products, such as the Vero Dynamic Tracking system (BrainLAB AG, Feldkirchen, Germany). 44,45…”