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In our laboratory we have confirmed that the capacitance of barium titanate-based capacitors changes due to radiation. Since a commercially available capacitor is very small and inexpensive, it may be used as a multidimensional dose meter in which a large number of capacitor elements are arranged, or may be embedded in the body and used as an in-vivo dose meter. In this study we examined the usefulness of a dosimeter using the capacitance change of a barium titanate capacitor. As a basic property, it was confirmed that the dose linearity was good. With regard to dose rate characteristics and response to fractionated irradiation, capacitance change due to aging affects measurement accuracy, but online measurement of capacitance change immediately before irradiation can be performed to correct aging effects during irradiation. By doing this, we confirmed that the dose rate characteristics and the response to fractionated radiation are improved.

Accelerator-based neutron sources have been developed and installed in recent decades for boron neutron capture therapy (BNCT) in several clinical facilities. Lithium is one of the targets that can produce epithermal neutrons from the 7 Li(p,n)7 Be near-threshold reaction, and accelerator-based BNCT systems employing a Li target are promising for cancer treatment. The accurate evaluation of the characteristics of an accelerator-based neutron source is a key to estimating the therapeutic effects of the accelerator-based BNCT. Particle and Heavy Ion Transport code System (PHITS) is a general-purpose Monte Carlo code, which can simulate a variety of diverse particle types and nuclear reactions. The latest PHITS code enables simulating the generation of neutrons from the 7 Li(p,n)7 Be reactions by using the Japanese Evaluated Nuclear Data Library 4.0 high-energy file. Thus, the PHITS code can be adopted for dose estimation during treatment planning for the accelerator-based BNCT. In this study, we evaluated the neutron fluence using the PHITS code by comparing it to reference data. The subsequent neutron transport simulations were performed to evaluate the boron trifluoride detector responses and the recoiled proton fluence detected by a CR-39 plastic detector. These comparative studies confirmed that the PHITS code can accurately simulate neutrons generated from an accelerator using a Li target. The PHITS code has a significant potential for a detailed evaluation of neutron fields and for predicting the therapeutic effects of the accelerator-based BNCT.

We evaluate the track segment yield G′ of typical water radiolysis products (eaq−, ·OH and H2O2) under heavy ions (He, C and Fe ions) using a Monte Carlo simulation code in the Geant4-DNA. Furthermore, we reproduce experimental results of ·OH of He and C ions around the Bragg peak energies ( 10 MeV/u), the simulation results using Geant4-DNA have agreed with experimental results. However, the G-values of water radiolysis species have not been properly evaluated around the Bragg peak energies, at which high ionizing density can be expected. Around the Bragg peak energy, dense continuous secondary products are generated, so that it is necessary to simulate the radical–radical reaction more accurately. To do so, we added the role of secondary products formed by irradiation. Consequently, our simulation results are in good agreement with experimental results and previous simulations not only in the high-energy region but also around the Bragg peak. Several future issues are also discussed regarding the roles of fragmentation and multi-ionization to realize more realistic simulations.

This study describes measurements on secondary particles produced by a 290 MeV/n Spread Out Bragg Peak (SOBP) carbon beam. Microdosimetric distributions of secondary fragments from the SOBP carbon beam have been measured by using a new tissue equivalent proportional counter (TEPC) system at the Heavy Ion Medical Accelerator in Chiba of the National Institute of Radiological Sciences. The new TEPC system consists of a TEPC, two solid-state detectors (SSD) and a scintillation counter (FSC:forward scintillation counter).The SSDs and FSC can separately identfy charged fragments and secondary neutrons produced by the incident carbon ions. Microdosimetric distributions were measured for secondary particles including neutrons produced by a body-simulated phantom consisting of various PMMA plates (Thickness: 0,34.81,55.2,60.95,64.83,95.03,114.79,124.69,135.2 and 144.98mm, respectively)to cover the SOBP(at 60-125mm depth). The new system can separately determine produced fragments from the incident SOBP carbon beam in a body-simulated phantom.

Absorbed doses from main charged-particle beams and charged-particle fragments have been measured with high accuracy for particle therapy, but there are few reports for doses from neutron components produced as fragments. This study describes the measurements on neutron doses produced by carbon beams; microdosimetric distributions of secondary neutrons produced by 290 MeV/nucleon carbon beams have been measured by using a tissue equivalent proportional counter at the Heavy Ion Medical Accelerator in Chiba, Japan at the National Institute of Radiological Sciences. The microdosimetric distributions of the secondary neutron were measured on the distal and lateral faces of a body-simulated acrylic phantom (300 mm heightx300 mm width x253 mm thickness). To confirm the dose measurements, the neutron energy spectra produced by incident carbon beams in the acrylic phantom were simulated by the particle and heavy ion transport code system. The absorbed doses obtained by multiplying the simulated neutron energy spectra with the kerma factor calculated by MCNPX agree with the corresponding experimental data fairly well. Downstream of the Bragg peak, the ratio of the neutron dose to the carbon dose at the Bragg peak was found to be a maximum of 1.4x10−4 and the ratio of neutron dose was a maximum of 3.0x10−7 at a lateral face of the acrylic phantom. The ratios of neutrons to charged particle fragments were 11% to 89% in the absorbed doses at the lateral and the distal faces of the acrylic phantom. We can conclude that the treatment dose will not induce serious secondary neutron effects at distances greater than 90 mm from the Bragg peak in carbon particle therapy.

Microdosimetric spctra were measured in order to gain the microdosimetric parameters of some epithermal neutron fields.Changes in dose mean lineal energy yd as a function of depth of heaby water showed a trend of softening with heaby water of the beam. The neutron absorbed dose was obtained by using the freqency mean lineal energy.Results show good agreement with measurements with the activation method using gold foil. This study demonstrated how microdosimetric parameters cahnge in radiation quality as a function of heaby water depth.

13th SYmposium on Microdosimetry

13th SYmposium on Microdosimetry