Kota Mizushima

Moving target irradiation with fast rescanning and gating in particle therapy.

PURPOSE In moving target irradiation with pencil beam scanning, the interplay effect between the target motion and the scanned beam is a problem because this effect causes over or under dosage in the target volume. To overcome this, we have studied rescanning using a gating technique. METHODS A simulation and experimental study was carried out. In the experiment, we used the fast scanning system developed at the HIMAC to verify the validity of phase controlled rescanning method, in which the time for rescanning irradiation of each slice is matched to the gating duration. RESULTS Simulation and experimental results showed that controlling the scan speed to match the respiration cycle with rescans can deliver the blurred dose distribution. In the comparison between the static measurements and the moving measurements with the phase controlled rescanning method, the dose difference was less than 2% for pinpoint chambers in the target volume. CONCLUSIONS The simulation and experimental results demonstrated that the phase controlled rescanning method makes it possible to deliver the dose distribution close to the expected one. As an experimental result for 3D irradiation, it was estimated that blurring by the probability density function was not only for a lateral distribution, but also for a distal distribution, even in the lateral rescanning.

Performance of the NIRS fast scanning system for heavy‐ion radiotherapy

PURPOSE A project to construct a new treatment facility, as an extension of the existing HIMAC facility, has been initiated for the further development of carbon-ion therapy at NIRS. This new treatment facility is equipped with a 3D irradiation system with pencil-beam scanning. The challenge of this project is to realize treatment of a moving target by scanning irradiation. To achieve fast rescanning within an acceptable irradiation time, the authors developed a fast scanning system. METHODS In order to verify the validity of the design and to demonstrate the performance of the fast scanning prior to use in the new treatment facility, a new scanning-irradiation system was developed and installed into the existing HIMAC physics-experiment course. The authors made strong efforts to develop (1) the fast scanning magnet and its power supply, (2) the high-speed control system, and (3) the beam monitoring. The performance of the system including 3D dose conformation was tested by using the carbon beam from the HIMAC accelerator. RESULTS The performance of the fast scanning system was verified by beam tests. Precision of the scanned beam position was less than +/-0.5 mm. By cooperating with the planning software, the authors verified the homogeneity of the delivered field within +/-3% for the 3D delivery. This system took only 20 s to deliver the physical dose of 1 Gy to a spherical target having a diameter of 60 mm with eight rescans. In this test, the average of the spot-staying time was considerably reduced to 154 micros, while the minimum staying time was 30 micros. CONCLUSIONS As a result of this study, the authors verified that the new scanning delivery system can produce an accurate 3D dose distribution for the target volume in combination with the planning software.

Evaluation of hybrid depth scanning for carbon-ion radiotherapy

PURPOSE In radiotherapy with a scanned carbon-ion beam, its Bragg peak is shifted along the depth direction either by inserting the range shifter plates or by changing the beam-extraction energy of a synchrotron. In the former technique (range shifter scanning: RS), the range shifter plates broaden the beam size and produce secondary fragments through nuclear reactions. In the latter technique (active-energy scanning: ES), it may take several seconds to change the beam energy depending on the synchrotron operation cycle, leading to a long treatment time. The authors propose a hybrid depth scan technique (hybrid scanning: HS), where several beam energies are used in conjunction with the range shifter plates for finer range shift. In this study, HS is evaluated from the viewpoints of dose distribution and treatment time. METHODS Assuming realistic accelerator and beam-delivery systems, the authors performed computer simulations using GEANT4 Monte Carlo code for beam modeling and a treatment planning system to evaluate HS. Three target volumes with the same dimensions of 60 × 60 × 60 mm(3) were generated at depths of 45, 85, and 125 mm in water phantom, and uniform clinical dose was planned for these targets. The sizes of lateral dose falloff and the peak to plateau ratio defined as the ratio of the clinical dose averaged over the target to the clinical dose at the entrance as well as the treatment time were compared among the three depth scan techniques. RESULTS The sizes of lateral dose falloffs at the center of SOBP are 11.4, 8.5, and 5.9 mm for the three targets in RS, while they are 5.7, 4.8, and 4.6 mm in ES and 6.6, 5.7, and 5.0 mm in HS, respectively. The peak to plateau ratios are 1.39, 1.96, and 2.15 in RS, while they are 1.48, 2.04, and 2.19 in ES and 1.47, 2.03, and 2.18 in HS, respectively. The treatment times are 128.7, 128.6, and 128.6 s in ES, while they are 61.2, 54.6, and 47.8 s in RS and 43.2, 44.1, and 44.7 s in HS, respectively. The multiple scattering and the nuclear reaction by range shifter degraded the beam qualities such as lateral dose falloff and peak to plateau ratio, which was especially pronounced for the shallow target in RS. The depth scan timing was limited by accelerator cycle in ES. That increased the treatment time by a few times. CONCLUSIONS This study revealed that HS can provide dose distributions with steeper lateral dose falloffs and higher peak to plateau ratio comparing to RS and comparable to ES. In addition, the treatment time can be considerably reduced in HS compared to ES.

Patient-specific QA and delivery verification of scanned ion beam at NIRS-HIMAC.

PURPOSE To evaluate a patient-specific QA program and system for constancy checking of a scanning delivery system developed at the National Institute of Radiological Sciences. METHODS For the patient-specific QA, all the planned beams are recalculated on a water phantom with treatment planning software (TPS). The recalculated dose distributions are compared with the measured distributions using a 2D ionization chamber array at several depths, and evaluated using gamma index analysis with criteria of 3% and 3 mm and a pass rate of 90%. For the constancy check, the authors developed the multiwire proportional chamber (MWPC), which can record the delivered 2D fluence images in a slice-by-slice manner. During irradiation for dosimetric QA with the 2D ionization chamber array and an accordion-type water phantom, the 2D fluence images are recorded using the MWPC in the delivery system. These recorded images are then compared to those taken in the treatment session to check the constancy check. This analysis also employs gamma index analysis using the same criteria as in the patient-specific QA. These patient-specific QA and constancy check evaluations were performed using the data of 122 patients. RESULTS In the patient-specific QA, the measured dose distributions agreed well with those calculated by the TPS, and the QA criteria were satisfied in all measurements. The additional check of the fluence comparison ensured the constancy of the delivered field during each treatment irradiation. CONCLUSIONS The authors established a patient-specific QA program and additional check of delivery constancy in every treatment session. Fluence comparison is a strong tool for constancy checking of the delivery system.

Development of a compact superconducting rotating-gantry for heavy-ion therapy.

An isocentric superconducting rotating-gantry for heavy-ion therapy is being developed [ 1]. This rotating gantry can transport heavy ions having 430 MeV/u to an isocenter with irradiation angles of over ±180°, and is further capable of performing fast raster-scanning irradiation [ 2]. A layout of the beam-transport line for the compact rotating-gantry is presented in Fig. 1. The rotating gantry has 10 superconducting magnets (BM01-10), a pair of the scanning magnets (SCM-X and SCM-Y) and two pairs of beam profile- monitor and steering magnets (ST01-02 and PRN01-02). For BM01-BM06 and BM09-BM10, the combined-function superconducting magnets are employed. Further, these superconducting magnets are designed for fast slewing of the magnetic field to follow the multiple flattop operation of the synchrotron [ 3]. The use of the combined-function superconducting magnets with optimized beam optics allows a compact gantry design with a large scan size at the isocenter; the length and the radius of the gantry will be to be ∼13 and 5.5 m, respectively, which are comparable to those for the existing proton gantries. Furthermore, the maximum scan size at the isocenter is calculated to be as large as ∼200 mm square for heavy-ion beams at the maximum energy of 430 MeV/u. All of the superconducting magnets were designed, and their magnetic fields were calculated using the Opera-3d code [ 4]. With the calculated magnetic fields, beam-tracking simulations were made. The simulation results agreed well with those of the linear beam-optics calculation, proving validity of the final design for the superconducting magnets. The five out of 10 superconducting magnets, as well as the model magnet were currently manufactured. With these magnets, rotation tests, magnetic field measurements and fast slewing tests were conducted. However, we did not observe any significant temperature increase, which may cause a quench problem. Further, results of the magnetic field measurements roughly agreed with those calculated by the Opera-3d code. The design study as well as major tests of the superconducting magnets was completed, and the construction of the superconducting rotating-gantry is in progress. The construction of the superconducting rotating-gantry will be completed at the end of FY2014, and be commissioned within FY2015. Fig. 1. Layout of the superconducting rotating-gantry. The gantry consists of 10 superconducting magnets (BM01–BM10), a pair of the scanning magnets (SCM-X and SCMY), and two pairs of beam profile-monitor and steering magnets (STR01–STR02 and PRN01–PRN02).

Development of Fast Scanning Magnets and Their Power Supply for Particle Therapy

At the Heavy Ion Medical Accelerator in Chiba (HIMAC), more than 8000 patients have been successfully treated by carbon ion beams since 1994. The successful results of treatments have led us to construct a new treatment facility equipped with a 3-D pencil beam scanning irradiation system. For the implementation of this irradiation technique, we developed a fast scanning magnet and its power supply, which are required to have field ramp rate of more than 250 T/s. The scanning magnets set at the entrance of the irradiation system are two dipole magnets to scan the beam in both horizontal and vertical directions and used to form the irradiation field to be tumor shape. Before the installation, performance test including both static and dynamic field measurement was carried out. Severe delay of the field and temperature rise were not observed as expected. Measured field ramp rate reached up to 300 T/s. After the installation, we verified the accuracy and stability of the scanned ion beam through the commissioning with the beam. As a successful result of the commissioning, treatment using the new scanning system was started in 2011.

Experimental verification of gain drop due to general ion recombination for a carbon‐ion pencil beam

PURPOSE Accurate dose measurement in radiotherapy is critically dependent on correction for gain drop, which is the difference of the measured current from the ideal saturation current due to general ion recombination. Although a correction method based on the Boag theory has been employed, the theory assumes that ionized charge density in an ionization chamber (IC) is spatially uniform throughout the irradiation volume. For particle pencil beam scanning, however, the charge density is not uniform, because the fluence distribution of a pencil beam is not uniform. The aim of this study was to verify the effect of the nonuniformity of ionized charge density on the gain drop due to general ion recombination. METHODS The authors measured the saturation curve, namely, the applied voltage versus measured current, using a large plane-parallel IC and 24-channel parallel-plate IC with concentric electrodes. To verify the effect of the nonuniform ionized charge density on the measured saturation curve, the authors calculated the saturation curve using a method which takes into account the nonuniform ionized charge density and compared it with the measured saturation curves. RESULTS Measurement values of the different saturation curves in the different channels of the concentric electrodes differed and were consistent with the calculated values. The saturation curves measured by the large plane-parallel IC were also consistent with the calculation results, including the estimation error of beam size and of setup misalignment. Although the impact of the nonuniform ionized charge density on the gain drop was clinically negligible with the conventional beam intensity, it was expected that the impact would increase with higher ionized charge density. CONCLUSIONS For pencil beam scanning, the assumption of the conventional Boag theory is not valid. Furthermore, the nonuniform ionized charge density affects the prediction accuracy of gain drop when the ionized charge density is increased by a higher dose rate and/or lower beam size.

SU-F-J-190: Time Resolved Range Measurement System Using Scintillator and CCD Camera for the Slow Beam Extraction

PURPOSE To investigate the time structure of the range, we have verified the rang shift due to the betatron tune shift with several synchrotron parameters. METHODS A cylindrical plastic scintillator block and a CCD camera were installed on the black box. Using image processing, the range was determined the 80 percent of distal dose of the depth light distribution. The root mean square error of the range measurement using the scintillator and CCD system is about 0.2 mm. Range measurement was performed at interval of 170 msec. The chromaticity of the synchrotron was changed in the range of plus or minus 1% from reference chromaticity in this study. All of the particle inside the synchrotron ring were extracted with the output beam intensity 1.8×108 and 5.0×107 particle per sec. RESULTS The time strictures of the range were changed by changing of the chromaticity. The reproducibility of the measurement was sufficient to observe the time structures of the range. The range shift was depending on the number of the residual particle inside the synchrotron ring. CONCLUSION In slow beam extraction for scanned carbon-ion therapy, the range shift is undesirable because it causes the dose uncertainty in the target. We introduced the time-resolved range measurement using scintillator and CCD system. The scintillator and CCD system have enabled to verify the range shift with sufficient spatial resolution and reproducibility.

A Patient-Specific QA Procedure for Moving Target Irradiation in Scanned Ion Therapy

Three-dimensional (3D) pencil-beam scanning technique has been utilized since 2011 in NIRS-HIMAC. Beam delivery system and treatment planning software (TPS) require dosimetric patient-specific QA to check each individual plan. Any change in the scanned beams will result in a significant impact on the irradiation dose. Therefore, patient-specific QA for moving target irradiation requires additional procedure. In an additional QA for moving target irradiation, we placed 2D ionization chamber on the PMMA plate tilted with respect to the beam axis. The PMMA plate was set on the stage of the moving phantom. The moving phantom was moved according to patient data. We measured the dose distribution for both the static target and the moving target. We compared the results for the moving target with those for the static targets by means of a gamma index analysis. In the additional patient-specific QA, the gamma analysis between the moving and static targets showed the good agreement. We confirmed that this new technique was a beneficial QA procedure for moving target irradiation.

Development of QA System for the Rotating Gantry for Carbon Ion Therapy at NIRS

At the National Institute of Radiological Sciences (NIRS), we have been developing the rotating-gantry system for the carbon-ion radiotherapy. This system is equipped with a three-dimensional pencil beam scanning irradiation system. To ensure the treatment quality, calibration of the primary dose monitor, range check, dose rate check, machine safety check, and some mechanical tests should be performed efficiently. For this purpose, we have developed a measurement system dedicated for quality assurance (QA) of this gantry system. The ion beam’s dose output are calibrated by measurement using an ionization chamber. A Farmer type ionization chamber is inserted into the center of a water equivalent phantom. The thickness of the phantom could be changed so that employ both calibration of the output at entrance and output checking at center of the irradiation field. The ranges of beams are verified using a scintillator and a CCD camera system. From the taken images, maximum gradient points are determined by some image processing and compared with reference data. In this paper, we describe consideration of the daily QA for the rotating-gantry.