Eriko Urakabe

Spot Scanning Using Radioactive 11C Beams for Heavy-Ion Radiotherapy

A scheme for spot scanning using 11C beams has been developed in order to form and verify a three-dimensionally conformal irradiation field for cancer radiotherapy. By selecting the momentum spread of a 11C beam, we could considerably decrease the distal falloff of the irradiation field, thus conserving the beam quality. To estimate and optimize the dose distribution in the irradiation field, it is essential to evaluate the precise dose distribution of spot beams. The coupling of the lateral dose and depth-dose distributions originating from a wide momentum spread should be taken into account to calculate the dose distribution of 11C beams. The reconstructed dose distribution of the irradiation field was in good agreement with the experimental results, i.e., within ±0.2%. An irradiation field of 35×35×43 mm3 was optimized and spot scanning using 11C beams was carried out. The flatness was within ±2.3% with an error of 1% in the detector resolution.

Application of an RI-beam for cancer therapy: In-vivo verification of the ion-beam range by means of positron imaging

Abstract In cancer treatment with heavy ions, verification of the ion range in the patients body is important. For this purpose, a positron emitter beam provides the possibility of range verification. To use the positron emitter beam, we have constructed a secondary beam course and its irradiation system. In this paper the constructed system is presented together with some results of beam experiments.

Ridge filter design and optimization for the broad‐beam three‐dimensional irradiation system for heavy‐ion radiotherapy

The broad-beam three-dimensional irradiation system under development at National Institute of Radiological Sciences (NIRS) requires a small ridge filter to spread the initially monoenergetic heavy-ion beam to a small spread-out Bragg peak (SOBP). A large SOBP covering the target volume is then achieved by a superposition of differently weighted and displaced small SOBPs. Two approaches were studied for the definition of a suitable ridge filter and experimental verifications were performed. Both approaches show a good agreement between the calculated and measured dose and lead to a good homogeneity of the biological dose in the target. However, the ridge filter design that produces a Gaussian-shaped spectrum of the particle ranges was found to be more robust to small errors and uncertainties in the beam application. Furthermore, an optimization procedure for two fields was applied to compensate for the missing dose from the fragmentation tail for the case of a simple-geometry target. The optimized biological dose distributions show that a very good homogeneity is achievable in the target.

Numerical simulation design study of a positron camera for heavy-ion radiotherapy

The positron camera system has been designed to measure heavy-ion ranges in patients bodies. The pencil-like beam of positron emitters, such as /sup 11/C, is used to check the range directly and precisely by detecting pairs of annihilation gamma-rays emitted from the end point of the beam trajectory. The positron camera consists of a pair of Anger-type scintillation cameras. The efficiency and the spatial resolution are modeled and simulated by a Monte Carlo method and a numerical calculation so that the positron camera has a high position accuracy for a small amount of irradiation dose. The simulation shows that the optimum effective diameter of the camera crystal is 500 mm and its thickness is 30 mm. The crystal diameter is concluded to be 600 mm by taking the outermost photomultiplier mounting into account. The simulation moreover, indicates that the range can be measured within an accuracy of 1 mm under the limitation that the irradiation dose has to be less than a few percent of the therapeutic one.

Maximum likelihood estimation of proton irradiated field and deposited dose distribution.

In proton therapy, it is important to evaluate the field irradiated with protons and the deposited dose distribution in a patients body. Positron emitters generated through fragmentation reactions of target nuclei can be used for this purpose. By detecting the annihilation gamma rays from the positron emitters, the annihilation gamma ray distribution can be obtained which has information about the quantities essential to proton therapy. In this study, we performed irradiation experiments with mono-energetic proton beams of 160 MeV and the spread-out Bragg peak beams to three kinds of targets. The annihilation events were detected with a positron camera for 500 s after the irradiation and the annihilation gamma ray distributions were obtained. In order to evaluate the range and the position of distal and proximal edges of the SOBP, the maximum likelihood estimation (MLE) method was applied to the detected distributions. The evaluated values with the MLE method were compared with those estimated from the measured dose distributions. As a result, the ranges were determined with the difference between the MLE range and the experimental range less than 1.0 mm for all targets. For the SOBP beams, the positions of distal edges were determined with the difference less than 1.0 mm. On the other hand, the difference amounted to 7.9 mm for proximal edges.

Performance of parallel plate ionization chamber for medical irradiation

We have developed parallel plate ionization chambers (PPIC) to measure not only the cumulative intensity but also the time structure of slow-extracted heavy-ion beams from a medical synchrotron. The characteristics of the PPIC with 3 mm and 1 mm gap distances for 760, 200, 111, and 55 Torr air were investigated with C/sup 6+/ beam (290 MeV/u) at HIMAC. The applied voltage to start the plateau region strongly depends on the beam intensity, and pressure of counter gas. The PPIC can be also used as a useful beam monitor for the time-structure measurement of heavy-ion beams.

Beam-Profile Control Using an Octupole Magnet

For medical irradiation, beams with a uniform distribution have been required. We have proposed a method to control the beam profile using an octupole magnet. We have installed an octupole magnet and confirmed its effectiveness at the Heavy Ion Medical Accelerator in Chiba (HIMAC) beam-transport line. A beam efficiency of 91% can be realized with a flatness of ±4% over the range of ±15 mm. The beam offset at the position of the octupole magnet is required to be within ±0.2 mm to realize this condition. This method is expected to have a beam efficiency higher than that of the ordinary method using wobbler magnets and a scatterer system.