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Dosimetric verification of lateral profile with a unique ionization chamber in therapeutic ion

It is essential to consider large-angle scattered particles in dose calculation models for therapeutic ion beams. However, it is difficult to measure the small dose contribution from large-angle scattered particles. Therefore, we developed a parallel-plate ionization chamber consisting of concentric electrodes (ICCE) to efficiently and easily detect small contributions. The ICCE consists of two successive ICs with a common HV plate. The former is a large plane-parallel IC to measure dose distribution integrated over the whole plane, the latter is a 24-channel parallel-plate IC with concentric electrodes to derive the characteristic parameters describing the lateral beam spread. The aim of this study is to evaluate the performance of the ICCE. By taking advantage of the characteristic of ICCE, we studied the recombination associated with lateral beam profile. Also, we measured a carbon pencil beam in several different media by using ICCE. As a result, we confirmed the ICCE could be used as a useful tool to determine the characterization of the therapeutic ion beams. INTRODUCTION The application of charged particles such as proton and carbon-ion beams for radiotherapy has been increased interest around the world. One solution to make optimal use of therapeutic ion beams and to provide flexible dose delivery is three-dimensional (3D) pencil-beam scanning technique [1-3]. It has been utilized since 2011 at the Heavy Ion Medical Accelerator in Chiba (HIMAC), operated by the National Institute of Radiological Sciences (NIRS) [4]. In the scanning irradiation method, since the 3D dose distribution is achieved by superimposing doses of individually weighted pencil beams determined in the treatment planning, the dose calculation must be more accurate. In dose calculation, the lateral beam spread due to multiple scattering that elemental pencil beams undergo in matter is generally assumed by a single-Gaussian model. However, the dose contributions from large-angle scattered (LAS) particles are not properly modeled in the single Gaussian. When the field size is too small for scattered particle equilibrium, it has been reported that the single-Gaussian model cannot express the reduction of the doses at the center of the field [5-7]. Therefore, several studies have proposed the expression for pencil beams of the sum of two or three Gaussians [8-11]. The parameterization of LAS particles by the multi-Gaussian model works very well. Generally, it is necessary to measure the small dose contributions from the LAS particles to derive the parameters. To efficiently and easily measure the small dose contributions, we developed a unique parallel-plate ionization chamber with concentric electrodes (ICCE) [12]. Since the sensitive volume of each channel is increased linearly with radial distance, it is possible to efficiently and easily detect small contributions from the LAS particles. In this paper, both the measurement using the ICCE and the validity of simplified parameterization of LAS particles are described. MATERIALS AND METHODS Specification of the ionization chamber with the concentric electrodes The detailed design of the ICCE was reported previously [12]. Only a simplified explanation is given here. The ICCE consists of two successive ICs with a common HV plate: a large plane-parallel IC to measure dose distribution integrated over the whole plane and a 24-channel parallel-plate IC with concentric electrodes to derive the characteristic parameters describing the lateral beam spread. Figure 1 shows a schematic of measurement system with concentric electrodes. The design of the concentric electrodes is based on the Rayleigh distribution transformed by the bivariate circular Gaussian distribution into polar coordinates. To increase the output at the large off-axis position, we increased the width of the electrode from 0.2 mm to 5.3 mm as a function of radius. Figure 1: Schematic of the ICCE and lateral profile of carbon-ion expressed by the three-Gaussian model. The large plane IC should have enough effective area to intercept all particles scattered. The diameter of the chamber is 150 mm. The gap between the HV plate and multichannel electrometer HV plate Signal plate 24ch B e a m a x is