Toshiyuki Terunuma

Measurement of neutron ambient dose equivalent in passive carbon-ion and proton radiotherapies.

Secondary neutron ambient dose equivalents per the treatment absorbed dose in passive carbon-ion and proton radiotherapies were measured using a rem meter, WENDI-II at two carbon-ion radiotherapy facilities and four proton radiotherapy facilities in Japan. Our measured results showed that (1) neutron ambient dose equivalent in carbon-ion radiotherapy is lower than that in proton radiotherapy, and (2) the difference to the measured neutron ambient dose equivalents among the facilities is within a factor of 3 depending on the operational beam setting used at the facility and the arrangement of the beam line, regardless of the method for making a laterally uniform irradiation field: the double scattering method or the single-ring wobbling method. The reoptimization of the beam line in passive particle radiotherapy is an effective way to reduce the risk of secondary cancer because installing an adjustable precollimator and designing the beam line devices with consideration of their material, thickness and location, etc., can significantly reduce the neutron exposure. It was also found that the neutron ambient dose equivalent in passive particle radiotherapy is equal to or less than that in the photon radiotherapy. This result means that not only scanning particle radiotherapy but also passive particle radiotherapy can provide reduced exposure to normal tissues around the target volume without an accompanied increase in total body dose.

Experimental evaluation of validity of simplified Monte Carlo method in proton dose calculations

It is important for proton therapy to calculate dose distributions accurately in treatment planning. Dose calculations in the body for treatment planning are converted to dose distributions in water, and the converted calculations are then generally evaluated by the dose measurements in water. In this paper, proton dose calculations were realized for a phantom simulating a clinical heterogeneity. Both dose calculations in the phantom calculated by two dose calculation methods, the range-modulated pencil beam algorithm (RMPBA) and the simplified Monte Carlo (SMC) method, and dose calculations converted to dose distributions in water by the same two methods were verified experimentally through comparison with measured distributions, respectively. For the RMPBA, though the converted calculations in water agreed moderately well with the measured ones, the calculated results in the actual phantom produced large errors. This meant that dose calculations in treatment planning should be evaluated by the dose measurements not in water but in the body with heterogeneity. On the other hand, the results calculated in the phantom, even by the less rigorous SMC method, reproduced the experimental ones well. This finding showed that actual dose distributions in the body should be predicted by the SMC method.

Induction of in situ DNA double-strand breaks and apoptosis by 200 MeV protons and 10 MV X-rays in human tumour cell lines

Purpose: To clarify the properties of clinical high-energy protons by comparing with clinical high-energy X-rays. Materials and methods: Human tumor cell lines, ONS76 and MOLT4, were irradiated with 200 MeV protons or 10 MV X-rays. In situ DNA double-strand breaks (DDSB) induction was evaluated by immunocytochemical staining of phosphorylated histone H2AX (γ-H2AX). Apoptosis was measured by flow-cytometry after staining with Annexin V. The relative biological effectiveness (RBE) was obtained by clonogenic survival assay. Results: DDSB induction was significantly higher for protons than X-rays with average ratios of 1.28 (ONS76) and 1.59 (MOLT4) at 30 min after irradiation. However the differences became insignificant at 6 h. Also, apoptosis induction in MOLT4 cells was significantly higher for protons than X-rays with an average ratio of 2.13 at 12 h. However, the difference became insignificant at 20 h. RBE values of protons to X-rays at 10% survival were 1.06 ± 0.04 and 1.02 ± 0.15 for ONS76 and MOLT4, respectively. Conclusions: Cell inactivation may differ according to different timings and/or endpoints. Proton beams demonstrated higher cell inactivation than X-rays in the early phases. These data may facilitate the understanding of the biological properties of clinical proton beams.

Multi-layer energy filter for realizing conformal irradiation in charged particle therapy

A new type of filter for charged particle radiotherapy is developed to reduce unwanted dose transfer to the normal tissues around a tumor. The new filter can make a static irradiation field where the width of the spread-out Bragg peak (SOBP) is two-dimensionally adjusted. That makes the field conformal to the tumor three-dimensionally. The filter is made of many layers produced by using stereolithography. The layer has a miniaturized structure that has geometrical similarity to the conventional ridge filter. Shapes of cone and pyramid are also usable for the unit-cell constructing the layer. The spread of the field in the depth direction is decided by the thickness of the filter, or by the number of layers. The experimental result of the irradiation using the ridge-type construction shows a good agreement with an estimate by the Monte Carlo calculation. By combining this technique with intensity modulation that has lateral position dependence, the conformal irradiation can be achieved by a simple procedure.

Simplified Monte Carlo Dose Calculation for Therapeutic Proton Beams

A simplified Monte Carlo (SMC) method has been developed for dose calculation of therapeutic proton beams. It uses the depth-dose distribution in water measured by a broad proton beam to calculate the energy loss in a material easily and accurately. It employs the water-equivalent model of inhomogeneous materials. In addition, the multiple scattering effect in the materials is also calculated using the water-equivalent thickness. The accuracy of dose calculations by the SMC method is verified by comparison with dose measurements in a heterogeneous phantom. Results of the measured dose distributions agree well with calculations by the SMC method, though those determined by the dose calculation method based on the pencil beam algorithm show a large discrepancy. Therefore, the dose-calculation method by the SMC method will be useful for application to the treatment planning for proton therapy.

Study on the use of electron-tracking Compton gamma-ray camera to monitor the therapeutic proton dose distribution in real time

Radiation therapy with proton and heavy-ion beams has been better established lately and the patient throughput is increasing. Although the therapy beam is controlled with high accuracy, it is difficult to know the location of distal dose falloff in the body. If real-time monitoring of the location is realized, the treatment quality would be improved. We have developed an electron-tracking Compton camera (ETCC) for real-time monitoring on the proton therapy. Our ETCC has a wide energy dynamic range of 200-1300 keV and a wide field of view. Therefore, ETCC has a potential as a quality assurance tool for proton therapy. We simulated and conducted an experiment with a 155 MeV proton beam and a water phantom. We succeeded in imaging a Bragg peak with prompt gamma rays.

Comparison of adverse effects of proton and X-ray chemoradiotherapy for esophageal cancer using an adaptive dose–volume histogram analysis

Cardiopulmonary late toxicity is of concern in concurrent chemoradiotherapy (CCRT) for esophageal cancer. The aim of this study was to examine the benefit of proton beam therapy (PBT) using clinical data and adaptive dose–volume histogram (DVH) analysis. The subjects were 44 patients with esophageal cancer who underwent definitive CCRT using X-rays (n = 19) or protons (n = 25). Experimental recalculation using protons was performed for the patient actually treated with X-rays, and vice versa. Target coverage and dose constraints of normal tissues were conserved. Lung V5–V20, mean lung dose (MLD), and heart V30–V50 were compared for risk organ doses between experimental plans and actual treatment plans. Potential toxicity was estimated using protons in patients actually treated with X-rays, and vice versa. Pulmonary events of Grade ≥2 occurred in 8/44 cases (18%), and cardiac events were seen in 11 cases (25%). Risk organ doses in patients with events of Grade ≥2 were significantly higher than for those with events of Grade ≤1. Risk organ doses were lower in proton plans compared with X-ray plans. All patients suffering toxicity who were treated with X-rays (n = 13) had reduced predicted doses in lung and heart using protons, while doses in all patients treated with protons (n = 24) with toxicity of Grade ≤1 had worsened predicted toxicity with X-rays. Analysis of normal tissue complication probability showed a potential reduction in toxicity by using proton beams. Irradiation dose, volume and adverse effects on the heart and lung can be reduced using protons. Thus, PBT is a promising treatment modality for the management of esophageal cancer.

Waveform simulation based on 3D dose distribution for acoustic wave generated by proton beam irradiation

A pulsed proton beam is capable of generating an acoustic wave when it is absorbed by a medium. This phenomenon suggests that the acoustic waveform produced may well include information on the three-dimensional (3D) dose distribution of the proton beam. We simulated acoustic waveforms by using a transmission model based on the Green function and the 3D dose distribution. There was reasonable agreement between the calculated and measured results. The results obtained confirm that the acoustic waveform includes information on the dose distribution.

Dose distribution resulting from changes in aeration of nasal cavity or paranasal sinus cancer in the proton therapy.

BACKGROUND AND PURPOSE Aeration in the nasal cavity and paranasal sinus (NCPS) was investigated during the course of proton therapy (PT), and the influence of aeration on the dose distribution was determined. MATERIAL AND METHODS Twenty patients with NCPS cancer (10 nasal cavity, 10 paranasal sinus) were analyzed. All the patients received a total proton beam irradiation dose of 38-78.4 Gray equivalents (GyE). Two to five CT examinations were performed during the course of treatment. The aeration ratio inside the cavity/sinus was calculated for each CT observation. Moreover, a simulation study supposing that the first treatment plan had been continued until the end of treatment was performed using the subsequent CT findings. RESULTS The aeration ratio was increased in 18 patients. The largest increase was from 15% to 82%. Three patients had a simulated maximum cumulative dose in the brainstem of beyond 60 GyE, while 10 patients had a simulated maximum cumulative dose in the optic chiasm of beyond 50 GyE. The shortest simulated time period to reach the dose limitation was 21 days. CONCLUSIONS Aeration in the NCPS is altered during the course of PT treatment and can greatly alter the dose distribution in the brainstem and optic chiasm.

Measurement of depth-dose distribution of protons by an imaging plate

By using an imaging plate (IP), we have developed a novel, simple method for depth-dose distribution measurement of clinical proton beams. When protons are obliquely incident upon the IP, the energy is deposited closer to the surface of the photo-stimulated luminescence (PSL) material than in the case of normal incidence. On the other hand, the spatial distribution of deposited energy varies considerably with variations in proton energy and results in a dependence of the IP response on linear-energy transfer (LET). Through a combination of these factors, for the oblique incidence, IP response to protons may be relatively enhanced around the Bragg peak because the observed PSL intensity depends on the position (depth) at which the energy is deposited. To examine this geometrical enhancement, a simple calculation is performed. The effect is thought to reduce the dependence of IP response on LET so that the response of an IP may approach that of a parallel-plate ionization chamber. Experimental results are given for protons of various angles of incidence (θ) on the IP. The proposed method may be useful for conducting quick checks of depth-dose distributions in proton-therapy facilities.