Study of spatial resolution and electron density measurement for proton computed tomography

PurposeProton computed tomography (PCT) is an imaging technique using a high-energy proton beam penetrating the human body. Accurate electron density distribution can be derived with PCT. However, the deterioration of the spatial resolution owing to multiple Coulomb scattering is a crucial issue. The spatial resolution of the PCT image is determined by magnitudes of multiple Coulomb scattering in detector material and the resolution of the position measurement of the tracking detector. To avoid deterioration of spatial resolution by multiple Coulomb scattering, reconstructing the single proton trajectory is a promising method to collect as much information as possible. Therefore, we chose a single-sided silicon strip detector as a tracking detector, and we developed a prototype system. In this report, the result of the beam test is introduced.MethodsWe performed the beam test at the HIMAC SB2 beamline, the National Institute of Radiological Sciences, Japan. All measurements were done with a proton beam of 160 MeV. The object for imaging is small(～25 mm in thickness) and made of polyethylene and acrylic. The proton energy was measured with a calorimeter at the most downstream position. Because the data-taking speed of the total system is low(～200Hz), the phantom was not rotated during the data taking. Study of the spatial resolution is done with the projected images. We also confirmed the accuracy of the density resolution of the PCT image.ResultsWe demonstrated an improvement in spatial resolution by reconstructing the proton trajectory. A spatial resolution of 0.45 mm is achieved for a 25-mm-thick polyethylene object.This result implies that those methods are useful for improving spatial resolution.ConclusionWe confirmed that the spatial resolution is improved by reconstructing the proton trajectory. The next step is to drastically improve the data-taking speed.The 7th Korea-Japan Joint Meeting on Medical Physic

Effect of asymmetric lateral beam spread of a pencil beam and oscillation of spot position on fluence distribution for particle pencil beam scanning

A pencil beam scanning system for carbon ion radiotherapy has operated at National Institute of Radiological Sciences (NIRS) in Japan since 2011. The pencil beam scanning system makes conformal dose distribution to a target by superposition of a pencil beam. Then it is required to accurately irradiate the pencil beam with 2-dimensional Gaussian distribution with symmetric variance in the lateral direction to a planned spot position. However, the actual lateral beam spread of the pencil beam is slightly different and the actual beam irradiation position is also slightly different from the planned spot position. The different lateral beam spread and the different beam irradiation position are suppressed to the acceptance level in the beam commissioning process to remain the uniformity of dose distribution. However, the range of the acceptance level has been unclear and determined experimentally. The purpose of this study is to objectively determine the acceptance level about the symmetry of the lateral beam spread of the pencil beam and the accuracy of the beam irradiation position. We calculated a fluence distribution by superposing the individual pencil beam with asymmetric variances in the lateral direction and/or with oscillation of the beam irradiation position. We changed the difference of the lateral beam spread and the amplitude of the oscillation and studied the effect of these variations on the fluence distribution. We present the calculation results and summarize the acceptance level about the symmetry of the pencil beam spread and the accuracy of the beam irradiation position.54th Annual Conference of the Particle Therapy Co-Operative Group (PTCOG54

Experimental verification of small field with low energy carbon-ion scanning in NIRS-HIMAC

To make the best use of the characteristics of a carbon-ion beam and provide flexible dose delivery, three-dimensional (3D) pencil-beam scanning is an ideal irradiation technique. To suppress beam spread due to multiple scattering and nuclear reactions, we then developed a full energy scanning method. In some case such as eye treatment, the irradiation fields are very small and short range. Thus, we prepared a minimum low energy carbon-ion beam corresponding to the water-equivalent residual ranges less than 2 mm. In our presentation, we introduce the experimental verification for low energy carbon-ion beam.13th European Conference on Accelerators in Applied Research and Technology (ECAART13

Experimental verification of short-range low-energy carbon-ion scanning in NIRS-HIMAC

Three-dimensional (3D) pencil-beam scanning is an ideal irradiation technique to make the best use of the characteristics of a carbon-ion beam and to provide flexible dose delivery. To suppress beam spread due to multiple scattering and nuclear reactions, we developed a full energy scanning method. In some cases, such as eye treatments, the irradiation fields are very small and short ranged. Accordingly, we prepared a minimum low-energy carbon-ion beam corresponding to water-equivalent residual ranges of less than 2 mm. We performed experimental verification for low-energy carbon-ion beams ranging from 55.6 to 96.0 MeV/u. The accuracy of 3D dose delivery with the low-energy carbon-ion beam was verified by measuring the dose distributions for different target volumes. The results confirmed that the measured dose distributions agree well with the calculated ones. Then, the first eye treatment with low-energy carbon-ion beam to a patient was performed in 2018

Early Experience with recursion optimization in an Extensible Rewriter

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Range verification system using edge detection method for a scintillator and a CCD camera system

Purpose: Three-dimensional irradiation with a scanned carbon-ion beam has been performed from 2011 at our facility. We have been developed the rotating-gantry equipped with the scanning irradiation system. The number of combinations of beam properties to measure for the commissioning is more than 7200, i.e. 201 energy steps, 3 intensities, and 12 gantry angles. To compress the commissioning time, quick and simple range verification system is required. In this work, we develop quick range verification system using scintillator and CCD (charge-coupled device) camera and estimate the accuracy of the range verification.Methods: A cylindrical plastic scintillator block and a CCD camera were installed on the black box. The optical spatial-resolution of the system is 0.2 mm/pixel. The camera control system was connected and communicates with the measurement system that is part of the scanning system. The range was determined by image processing. Reference range for each energy beam was determined by a difference of Gaussian (DOG) method and the 80 percent of distal dose of the depth dose distribution that were measured by a large parallel-plate ionization chamber. We compared a threshold method and a DOG method. Results: We found that the edge detection method (i.e., the DOG method) is best for the range detection. The accuracy of range detection using this system is within 0.2 mm, and the reproducibility of same energy measurement is within 0.1 mm without setup error. Conclusions: The results of this study demonstrate that our range check system is capable of quick and easy range verification with sufficient accuracy