Chihiro Omachi

A proton therapy system in Nagoya Proton Therapy Center

The purpose of this paper is to describe an outline of a proton therapy system in Nagoya Proton Therapy Center (NPTC). The NPTC has a synchrotron with a linac injector and three treatment rooms: two rooms are equipped with a gantry and the other one is equipped with a fixed horizontal beamline. One gantry treatment room has a pencil beam scanning treatment delivery nozzle. The other two treatment rooms have a passive scattering treatment delivery nozzle. In the scanning treatment delivery nozzle, an energy absorber and an aperture system to treat head and neck cancer have been equipped. In the passive treatment delivery nozzle, a multi-leaf collimator is equipped. We employ respiratory gating to treat lung and liver cancers for passive irradiation. The proton therapy system passed all acceptance tests. The first patient was treated on February 25, 2013, using passive scattering fixed beams. Respiratory gating is commonly used to treat lung and liver cancers in the passive scattering system. The MLCs are our first choice to limit the irradiation field. The use of the aperture for scanning irradiation reduced the lateral fall off by half or less. The energy absorber and aperture system in scanning delivery is beneficial to treat head and neck cancer.

Evaluation of the influence of double and triple Gaussian proton kernel models on accuracy of dose calculations for spot scanning technique.

PURPOSE The main purpose in this study was to present the results of beam modeling and how the authors systematically investigated the influence of double and triple Gaussian proton kernel models on the accuracy of dose calculations for spot scanning technique. METHODS The accuracy of calculations was important for treatment planning software (TPS) because the energy, spot position, and absolute dose had to be determined by TPS for the spot scanning technique. The dose distribution was calculated by convolving in-air fluence with the dose kernel. The dose kernel was the in-water 3D dose distribution of an infinitesimal pencil beam and consisted of an integral depth dose (IDD) and a lateral distribution. Accurate modeling of the low-dose region was important for spot scanning technique because the dose distribution was formed by cumulating hundreds or thousands of delivered beams. The authors employed a double Gaussian function as the in-air fluence model of an individual beam. Double and triple Gaussian kernel models were also prepared for comparison. The parameters of the kernel lateral model were derived by fitting a simulated in-water lateral dose profile induced by an infinitesimal proton beam, whose emittance was zero, at various depths using Monte Carlo (MC) simulation. The fitted parameters were interpolated as a function of depth in water and stored as a separate look-up table. These stored parameters for each energy and depth in water were acquired from the look-up table when incorporating them into the TPS. The modeling process for the in-air fluence and IDD was based on the method proposed in the literature. These were derived using MC simulation and measured data. The authors compared the measured and calculated absolute doses at the center of the spread-out Bragg peak (SOBP) under various volumetric irradiation conditions to systematically investigate the influence of the two types of kernel models on the dose calculations. RESULTS The authors investigated the difference between double and triple Gaussian kernel models. The authors found that the difference between the two studied kernel models appeared at mid-depths and the accuracy of predicting the double Gaussian model deteriorated at the low-dose bump that appeared at mid-depths. When the authors employed the double Gaussian kernel model, the accuracy of calculations for the absolute dose at the center of the SOBP varied with irradiation conditions and the maximum difference was 3.4%. In contrast, the results obtained from calculations with the triple Gaussian kernel model indicated good agreement with the measurements within ±1.1%, regardless of the irradiation conditions. CONCLUSIONS The difference between the results obtained with the two types of studied kernel models was distinct in the high energy region. The accuracy of calculations with the double Gaussian kernel model varied with the field size and SOBP width because the accuracy of prediction with the double Gaussian model was insufficient at the low-dose bump. The evaluation was only qualitative under limited volumetric irradiation conditions. Further accumulation of measured data would be needed to quantitatively comprehend what influence the double and triple Gaussian kernel models had on the accuracy of dose calculations.

Spot Scanning and Passive Scattering Proton Therapy: Relative Biological Effectiveness and Oxygen Enhancement Ratio in Cultured Cells.

PURPOSE To determine the relative biological effectiveness (RBE), oxygen enhancement ratio (OER), and contribution of the indirect effect of spot scanning proton beams, passive scattering proton beams, or both in cultured cells in comparison with clinically used photons. METHODS AND MATERIALS The RBE of passive scattering proton beams at the center of the spread-out Bragg peak (SOBP) was determined from dose-survival curves in 4 cell lines using 6-MV X rays as controls. Survival of 2 cell lines after spot scanning and passive scattering proton irradiation was then compared. Biological effects at the distal end region of the SOBP were also investigated. The OER of passive scattering proton beams and 6 MX X rays were investigated in 2 cell lines. The RBE and OER values were estimated at a 10% cell survival level. The maximum degree of protection of radiation effects by dimethyl sulfoxide was determined to estimate the contribution of the indirect effect against DNA damage. All experiments comparing protons and X rays were made under the same biological conditions. RESULTS The RBE values of passive scattering proton beams in the 4 cell lines examined were 1.01 to 1.22 (average, 1.14) and were almost identical to those of spot scanning beams. Biological effects increased at the distal end of the SOBP. In the 2 cell lines examined, the OER was 2.74 (95% confidence interval, 2.56-2.80) and 3.08 (2.84-3.11), respectively, for X rays, and 2.39 (2.38-2.43) and 2.72 (2.69-2.75), respectively, for protons (P<.05 for both cells between X rays and protons). The maximum degree of protection was significantly higher for X rays than for proton beams (P<.05). CONCLUSIONS The RBE values of spot scanning and passive scattering proton beams were almost identical. The OER was lower for protons than for X rays. The lower contribution of the indirect effect may partly account for the lower OER of protons.

Validation of PTSIM for clinical usage

The Geant4 simulation toolkit has been widely accepted in particle therapy domain for more accurate treatment planning. In Japan, the PTSIM, Particle Therapy System Simulation Framework, has been developed by the fund from the Core Research for Evolutional Science and Technology of Japan Science and Technology Agency, JST/CREST. The PTSIM provides a common platform to model beam delivery systems including a DICOM data handling for hadron therapy facility. The PTSIM can simulate existing six irradiation systems in the world and try extending the scope for clinical usage. In this paper, the performance of PTSIM is described for the use of clinical applications as a dose engine. The PTSIM was examined at the National Cancer Center (NCC) and Hyogo Ion Beam Medical Center (HIBMC) in Japan. The dose calculations had been performed on CT images with the treatment parameters determined by a treatment planning system. The dose distributions were compared with pencil beam algorithm.

Estimation of influence of material assignment in CT data for hadron therapy using GEANT4

The influence of material assignment in a patient geometry for hadron therapy had been studied using GEANT4 simulation. For the purpose of evaluating the influence of material differences, the dose distributions by proton and carbon beams were compared in a homogeneous medium for water and tissue materials, where all mass densities were normalized to 1 g/cm3. The differences were observed in the depth of Bragg peak positions. For clinical treatments, the dose distributions in CT data of sample patients were studied with actual treatment parameters. The tissue materials were assigned by dividing CT space into material groups. Within the group, the mass density was varied with the CT value using a look up table in a treatment planning system. The dose distributions in the sample patients were consistent each other between material assignments using inhomogeneous water and heterogeneous medium of tissue materials.

Recent updates and plan in Geant4 based particle therapy system simulation framework

Particle therapy system simulation framework, PTSIM, is a simulation framework based on Geant4 Monte Carlo simulation. It has been developed in the project, “Development of simulation framework for advanced radiotherapy”, funded by the Japan Science and Technology Agency (JST) in the program of Core Research for Evolutional Research and Technology (CREST), from 2003 to 2010. The PTSIM has provided a common platform to model proton and ion therapy facilities, allowing users who are not Geant4 experts to accurately and efficiently run Geant4 simulations with the pre-build configurations. Efforts on further development of PTSIM are still under way to include more functionality and improve the performance. The PTSIM has been upgraded with the extensions in the DICOM-RT interface to coordinate with hospital information system, the Web interface with universal grid environment, etc. In this paper, we report on our activities about these updates and plan for extending the PTSIM functionality with new requirements from users.

Evaluation of dosimetric advantages of using patient‐specific aperture system with intensity‐modulated proton therapy for the shallow depth tumor

Abstract In this study, we evaluate dosimetric advantages of using patient‐specific aperture system with intensity‐modulated proton therapy (IMPT) for head and neck tumors at the shallow depth. We used four types of patient‐specific aperture system (PSAS) to irradiate shallow regions less than 4 g/cm2 with a sharp lateral penumbra. Ten head and neck IMPT plans with or without aperture were optimized separately with the same 95% prescription dose and same dose constraint for organs at risk (OARs). The plans were compared using dose volume histograms (DVHs), dose distributions, and some dose indexes such as volume receiving 50% of the prescribed dose (V50), mean or maximum dose (Dmean and Dmax) to the OARs. All examples verified in this study had decreased V50 and OAR doses. Average, maximum, and minimum relative reductions of V50 were 15.4%, 38.9%, and 1.0%, respectively. Dmax and Dmean of OARs were decreased by 0.3% to 25.7% and by 1.0% to 46.3%, respectively. The plans with the aperture over more than half of the field showed decreased V50 or OAR dose by more than 10%. The dosimetric advantage of patient‐specific apertures with IMPT was clarified in many cases. The PSAS has some dosimetric advantages for clinical use, and in some cases, it enables to fulfill dose constraints.

Correction of stopping power and LET quenching for radiophotoluminescent glass dosimetry in a therapeutic proton beam

To measure the absorbed dose to water D w in proton beams using a radiophotoluminescent glass dosimeter (RGD), a method with the correction for the change of the mass stopping power ratio (SPR) and the linear energy transfer (LET) dependence of radiophotoluminescent efficiency [Formula: see text] is proposed. The calibration coefficient in terms of D w for RGDs (GD-302M, Asahi Techno Glass) was obtained using a 60Co γ-ray. The SPR of water to the RGD was calculated by Monte Carlo simulation, and [Formula: see text] was investigated experimentally using a 70 MeV proton beam. For clinical usage, the residual range R res was used as a quality index to determine the correction factor for the beam quality [Formula: see text] and the LET quenching effect of the RGD [Formula: see text]. The proposed method was evaluated by measuring D w at different depths in a 200 MeV proton beam. For both non-modulated and modulated proton beams, [Formula: see text] decreases rapidly where R res is less than 4 cm. The difference in [Formula: see text] between a non-modulated and a modulated proton beam is less than 0.5% for the R res range from 0 cm to 22 cm. [Formula: see text] decreases rapidly at a LET range from 1 to 2 keV µm-1. In the evaluation experiments, D w using RGDs, [Formula: see text] showed good agreement with that obtained using an ionization chamber and the relative difference was within 3% where R res was larger than 1 cm. The uncertainty budget for [Formula: see text] in a proton beam was estimated to investigate the potential of RGD postal dosimetry in proton therapy. These results demonstrate the feasibility of RGD dosimetry in a therapeutic proton beam and the general versatility of the proposed method. In conclusion, the proposed methodology for RGDs in proton dosimetry is applicable where R res  >  1 cm and the RGD is feasible as a postal audit dosimeter for proton therapy.