Therapy Beam Line Research Articles

Monte Carlo simulation of a proton therapy beamline for intracranial treatments

A radiation transport code based on a Monte Carlo tool is used to simulate a proton therapy beamline designed to treat paediatric patients with intracranial tumours. The treatments are performed using the IBA gantry at the Proton Therapy Centre of the Institut Curie. The treatment is undertaken at 178.16 MeV using the double scattering technique. The aim of this study is to present the Monte Carlo model of the transported proton beam, beamline and treatment room, as well as the experimental validation of the proton dose distributions calculated by this model. The beamline components and the treatment room are accurately modelled using the Monte Carlo code MCNPX. The proton source at the beamline entrance is defined on the basis of IBA data, measurements and calculations. Measured and calculated relative proton dose distributions in a water phantom are compared for the validation. Depth dose profiles, including pristine Bragg peaks and a spread–out Bragg peak, and lateral dose profiles are studied. A general good agreement was found between calculated and measured distributions with discrepancies of less than 2 mm. Relative proton dose distributions are therefore considered to be correctly described by our simulated geometry and proton source parameters. The Monte Carlo simulation will be used subsequently for radiation protection purposes: calculation of secondary neutron doses received by treated patients of different ages.

First steps towards a fast-neutron therapy planning program

BackgroundThe Monte Carlo code GEANT4 was used to implement first steps towards a treatment planning program for fast-neutron therapy at the FRM II research reactor in Garching, Germany. Depth dose curves were calculated inside a water phantom using measured primary neutron and simulated primary photon spectra and compared with depth dose curves measured earlier. The calculations were performed with GEANT4 in two different ways, simulating a simple box geometry and splitting this box into millions of small voxels (this was done to validate the voxelisation procedure that was also used to voxelise the human body).ResultsIn both cases, the dose distributions were very similar to those measured in the water phantom, up to a depth of 30 cm. In order to model the situation of patients treated at the FRM II MEDAPP therapy beamline for salivary gland tumors, a human voxel phantom was implemented in GEANT4 and irradiated with the implemented MEDAPP neutron and photon spectra. The 3D dose distribution calculated inside the head of the phantom was similar to the depth dose curves in the water phantom, with some differences that are explained by differences in elementary composition. The lateral dose distribution was studied at various depths. The calculated cumulative dose volume histograms for the voxel phantom show the exposure of organs at risk surrounding the tumor.ConclusionsIn order to minimize the dose to healthy tissue, a conformal treatment is necessary. This can only be accomplished with the help of an advanced treatment planning system like the one developed here. Although all calculations were done for absorbed dose only, any biological dose weighting can be implemented easily, to take into account the increased radiobiological effectiveness of neutrons compared to photons.

SU‐E‐T‐664: Accuracy of Patient Dose Calculation in Proton Therapy of Lung Cancer: A Comparison of Monte Carlo and Pencil Beam Computations

Purpose: The analytical, semi‐empirical algorithms are used widely for proton treatment planning in the current clinical practice. The aim of this study is the assessment of possible dosimetric uncertainties in these algorithms in highly heterogeneous areas of the thorax. Methods: In this study we use a detailed Monte Carlo model of the passive scattering proton therapy (PSPT) beam line together with a computed tomography (CT) based model of the patient anatomy to investigate the heterogeneity‐induced dosimetric uncertainties in PSPT treatment planning. A CT scan and a treatment plan were used to assemble the input files for the Monte Carlo code MCNPX. The total energy deposition inside the patient volume was computed with statistical uncertainty of less than 2% and converted to dose‐in‐water. The results were compared to the dose computed by the clinical treatment planning system (TPS). The corresponding dose in the water phantom was computed using both the TPS and the Monte Carlo model and used to calibrate the absolute dosimetry in the Monte Carlo model. Results: While there is acceptable agreement between the TPS and the Monte Carlo dose in the water phantom, there are noticeable differences caused by heterogeneities. The regions of largest differences are the points around and beyond the distal edge and points around the lateral penumbra. These are most pronounced when the beam passes through or is stopped in the low density regions of the lung, and increase with the range of the beam and the amount of the heterogeneities in the beam path. Conclusions: In some lung cases the actual target coverage and the accuracy of dose to lung tissues around and beyond the distal edge of the proton beam, calculated with conventional algorithms, may be suboptimal.

An electron density measurement using an analyzer based imaging system

Using a monochromatic X-ray beam from a synchrotron source the electron density of a homogeneous target was determined by measuring the refraction that occurs at the air–target interface for a known angle of incidence. The angle of deviation that these X-rays undergo at the transition between materials is micro-radian to submicro-radian in scale. Existing analyzer based imaging systems are designed to measure submicro-radian angle changes and commonly use monochromatic hard X-ray beams generated from synchrotron sources. A preliminary experiment using the analyzer based imaging apparatus at the Canadian Light Source Biomedical Imaging and Therapy beamline and a half cylinder shaped plastic target will be presented. By measuring the angle of deviation of the photon beam at several discrete angular positions of the target the electron density of the target material was determined.

SU‐GG‐J‐79: Experimentally Optimizing Prompt Gamma Detection for Proton Therapy

Purpose: To model and optimize an experimental setup that could potentially provide in‐situ range verification during proton therapy. The approach relies on measuring the prompt gamma emission profile produced by a beam of protons traversing a medium to provide information on the actual distal dose falloff in real time. Method and Materials: Calculations were performed using the GEANT4 Monte Carlo toolkit and a well‐developed model of a therapy beam line at the Francis Burr Proton Therapy Center. A 150 MeV pencil beam of protons was transported through a cylindrical Lucite phantom and the prompt gammas emitted orthogonal to the beam direction were scored. Measurements were performed under similar conditions with beam intensities of approximately 5 nA using both an HPGe spectrometer and different scintillators. The experimental arrangement was then simulated as part of a design study to investigate a more optimized measuring geometry. Results: The measured and calculated gamma energy spectrum displayed several distinctive emission peaks corresponding to 12C, 16O, and 1H as expected. Depth profiles measured with 150 MeV protons using two different detectors exhibited distal falloffs that correlated with the Bragg peak and agreed well with calculated gamma‐ray emission profiles. Calculations found a significant neutron background that may be effectively reduced by energy deposit or possibly timing discrimination. Conclusion: This study highlights the necessity of performing accurate MC simulations to help interpret and design experiments under realistic conditions. Calculations, once properly correlated with measurements aid the effective and efficient use of beam time and will be used to optimize parameters for experiments such as collimation, shielding and detector media.

Ion-optical studies for a range adaptation method in ion beam therapy using a static wedge degrader combined with magnetic beam deflection

Fast radiological range adaptation of the ion beam is essential when target motion is mitigated by beam tracking using scanned ion beams for dose delivery. Electromagnetically controlled deflection of a well-focused ion beam on a small static wedge degrader positioned between two dipole magnets, inside the beam delivery system, has been considered as a fast range adaptation method. The principle of the range adaptation method was tested in experiments and Monte Carlo simulations for the therapy beam line at the GSI Helmholtz Centre for Heavy Ions Research. Based on the simulations, ion optical settings of beam deflection and realignment of the adapted beam were experimentally applied to the beam line, and additional tuning was manually performed. Different degrader shapes were employed for the energy adaptation. Measured and simulated beam profiles, i.e. lateral distribution and range in water at isocentre, were analysed and compared with the therapy beam values for beam scanning. Deflected beam positions of up to ±28 mm on degrader were performed which resulted in a range adaptation of up to ±15 mm water equivalence (WE). The maximum deviation between the measured adapted range from the nominal range adaptation was below 0.4 mm WE. In experiments, the width of the adapted beam at the isocentre was adjustable between 5 and 11 mm full width at half maximum. The results demonstrate the feasibility/proof of the proposed range adaptation method for beam tracking from the beam quality point of view.

Characterization of a boron neutron capture therapy beam line at the University of Missouri Research Reactor

A new research effort aimed at increasing the catalog of boron delivery agents for boron neutron capture therapy (BNCT) has been undertaken by the International Institute of Nano and Molecular Medicine (IINMM) at the University of Missouri. The MU Research Reactor (MURR) and Idaho National Laboratory (INL) have constructed a facility for small animal BNCT experiments. The facility incorporates silicon and bismuth single crystal filters to produce a thermal neutron beam. The thermal beam has a measured thermal flux of 8.8 × 108 n cm−2 s−1 and a gold cadmium ratio of 130. The neutron and photon dose rates were measured using paired ion chambers.

SU‐FF‐T‐449: A Monte Carlo Study of Stray Neutron Background Produced in Ocular Therapy

Purpose: To study the neutron spectral fluence and neutron dose equivalent as a function of the off axis distance to isocenter in an ocular therapy beam line using the Monte Carlo technique and to compare the simulated neutron dose equivalent values with the measurements presented in X. Yan et al, Nuclear Instruments and Methods A 476, 2002. Methods and Materials: A Monte Carlo simulation of an ocular nozzle (Newhauser et al, Physics in Medicine and Biology, 2005) and a cylindrical Lucite phantom were carried out using the FLUKA radiation transport code. Mean initial kinetic energy of protons was 159 MeV. The range of the proton beam was 2.5 cm in water. The neutron track length fluencies were tallied in 8 spherical cells of air of diameter 25.4 cm. The off axis distance to the tallies from the isocenter were between 25 cm and 75 cm. The neutron track length fluencies were converted in to ambient dose equivalent using the conversion coefficient given in ICRP 74. Results: The energy weighted neutron fluence spectra demonstrated characteristic low energy evaporation peak and high energy peak. The strengths of both peaks decreased as the distance from the isocenter increased. The neutron dose equivalent values decreased as the off axis distance from the isocenter increased. At a distance of 65 cm away from the isocenter, the neutron dose equivalent value was about 50% of that of at 25 cm. The neutron dose equivalent values from this study are roughly factor of 2 higher than that of measurements from Yan et al (Reported uncertainty of the measured data is 40%). Contribution to total neutron dose equivalent from high energy neutrons is higher than that from low energy neutrons. Conclusions: A detailed study of neutron background produced in ocular proton therapy is presented.

27-Pole 4.2 T wiggler for biomedical imaging and therapy beamline at the Canadian light source

The main goal of the wiggler is creation of high photon flux in energy range 10–100 keV for BioMedical and Therapy (BMIT) beamline for advanced soft tissue imaging for medicine including high-precision radiation cancer therapies. Total radiation power is about 10 kW for electron beam current of 0.2 A and 2.9 GeV energy. In November 2007 the wiggler was installed in the CLS storage ring. The cryostat design enabled long-term (up to one year) machine operation without liquid helium refilling. Consumption of liquid helium does not exceed 0.03 l/h. The main parameters of the magnet and the cryogenic systems, as well as experimental test results during machine operation on the CLS storage ring are presented.

Radiological considerations for POE-1 photon shutters, collimators and beam stops of the Biomedical Imaging and Therapy beamline at the Canadian Light Source

A study of radiation levels due to primary and secondary gas bremsstrahlung is carried out for the BioMedical Imaging and Therapy (BMIT) beamline at the Canadian Light Source (CLS). The BMIT beamline, being built at present, is a major research and diagnostic tool for X-ray imaging and X-ray radiation therapy for animals and humans. For the BMIT beamline to be as flexible as possible, a movable tungsten collimator is designed. This can move vertically and assumes two positions; up and down. The BMIT beamline is, thus, able to perform two modes of operation: one white beam, the other monochromatic. Gas bremsstrahlung produced in the vacuum chamber propagates with synchrotron radiation and may enter the imaging or therapy hutch. In this study, the dose behind the collimator is investigated in each mode by assessing the energy deposition in a water phantom that surrounds the entire copper shutter-tungsten collimator unit. When estimating the dose, particular attention is given to the opening area of the collimator, since this passage leads to the imaging or therapy hutch. Also examined are the doses when a tungsten safety shutter is closed.

SU-FF-T-108: Clinical Use of Strip Ionization Chamber Detector as Online Proton Beam Monitor

Purpose: In proton therapy it is important to deliver uniform dose distribution in tumor volume. The parameters which indicate the beam geometry have to be evaluated and the beam has to be controlled during radiation. For this reason a detector system has been developed for online beam monitoring at the Centro di AdroTerapia e Applicazioni Nucleari Avanzate (CATANA) within a collaboration with the Istituto Nazionale di Fisica Nucleare- Torino (INFN-To). Shallow tumors (32 mm maximum depth) like uveal melanomas have been treated since spring 2002 in this center. Method and Materials: The 62 MeV proton beam, extracted from LNS Superconducting Cyclotron, is delivered based on double foils scattering system. A Range shifter followed by an energy modulator is placed downstream of the scattering system to provide the Spread Out Bragg Peak (SOBP) at the tumor position. The detector has been placed upstream of the last collimator; it consists of two parallel plate strip ionization chambers segmented in vertical and horizontal orientation respectively. Each anode consists of 256 0.5 mm wide strips with 12.8 × 12.8 cm2 sensitive area. Results: The detector has been checked in different beam conditions and is currently used in clinical practice. The beam symmetry and integrated fluence are measured with this detector. The value of skewness and centre of gravity have been tested in different clinical beam settings and the ranges of allowed values have been defined. During treatment these parameters are evaluated and checked against the set limits to ensure the correct delivery of the dose. Conclusion: A strip ionization chamber detector has been developed to be used as online beam monitor in the proton therapy beam line at LNS (Catania, Italy). The beam is monitored with frequency of the order of one Hertz and it can be stopped in case of misbehavior during treatment.

Development of a pixel ionization chamber for beam monitor in proton therapy

We have developed a detector to be used as monitor for proton therapy beam lines. The detector is a 2-D parallel plate ionization chamber, with the anode segmented in 1024 square pixels arranged in a 32×32 matrix. The detector characterization is presented.

A pixel ionization chamber used as beam monitor at the Institut Curie—Centre de Protontherapie de Orsay (CPO)

The Dipartimento di Fisica Sperimentale and the Istituto Nazionale di Fisica Nucleare (INFN), Torino, in collaboration with the Institut Curie—Centre de Protontherapie de Orsay (CPO), have developed and built a pixel parallel plate ionization chamber to be used as monitor for the proton therapy beam line at the Institut Curie—CPO (Orsay, France). The sensitive area of the detector is (160×160) mm 2, with the anode segmented in 1024 square pixels arranged in a 32×32 matrix; the area of each pixel is (5×5) mm 2. The detector has been placed on the beam line just upstream of the last collimator to monitor the beam shape and to measure the stability and reproducibility of the delivery system. In this paper, we present a detailed description of the detector and the results of a set of preliminary tests.

Po‐Poster ‐ 35: The BioMedical Imaging and Therapy Beamline at the Canadian Light Source Inc.

The BioMedical Imaging and Therapy (BMIT) Beamline is a multidisciplinary, multiuse facility that is being constructed at the Canadian Light Source, Canada's National Synchrotron Facility. The BMIT Beamline will have two specific research uses, non‐destructive imaging of tissues and radiation therapy in living organisms. The total cost of this project will be approximately $17M. Funding for this project has been obtained from the Canadian Foundation for Innovation, the provincial governments and charitable foundations. The BMIT Beamline is projected to be ready for operations in 2007.

Implementation of a new Monte Carlo-GEANT4 Simulation tool for the development of a proton therapy beam line and verification of the related dose distributions

Using the Monte Carlo simulation tool GEANT4, we simulated the hadron-therapy beam line of the Centro di AdroTerapia ed Applicazioni Nucleari Avanzate (CATANA) center. This is the unique Italian hadron-therapy facility in which 62-MeV proton beams are used for the radiotherapeutic treatment of choroidal and iris melanomas. All the elements, such as diffusers, range shifters, collimators, and detectors, typical of a proton-therapy line were modeled. The beam line provides an ideal environment for the experimental testing and validation of the software developed. The software architecture was developed, and the validation of the software is in its final stage. Simulated ranges, energy distribution, depth and lateral dose distributions for full energy proton beams will be compared to the experimental results obtained at Laboratori Nazionali del Sud (LNS) with different detectors.

BANG®-Polymergeldosimetrie in der Protonentherapie von Augentumoren

The use of polymer gel makes it possible to measure three-dimensional dose distributions of ionizing radiation. Phantoms with BANG-1 and BANG-3 gel were irradiated using a 68 MeV proton beam at the eye tumour therapy beam line of the Hahn Meitner Institute in Berlin. Up to twelve treatment fields could be applied to one gel phantom. The investigations consisted of mono-energetic Bragg curves, spread-out Bragg curves of circular fields (diameter 20 mm), and spread-out Bragg curves of patient fields. Magnetic resonance imaging was used to obtain the gel dose distributions. The results were compared to measurements of a water-phantom ionization chamber. The BANG polymer gels showed a significant quenching of Bragg peaks compared to ionization chamber measurements. The BANG-3 gel was found to be unsuitable for further investigations with 68 MeV protons. The use of BANG-1 allowed the verification of wedge slopes and irregular field forms. On the basis of our experience, polymer gels are well suited for quality assurance in proton therapy in principle.

298 poster Implementation of a GEANT4 Monte Carlo code for the development of a proton therapy beam line and verification of proton beams dose distributions

298 poster Implementation of a GEANT4 Monte Carlo code for the development of a proton therapy beam line and verification of proton beams dose distributions

An experimental approach to the design of a scattering system for a proton therapy beam line dedicated to opthalmological applications

The development of a scattering system for a proton therapy beam line dedicated to ophthalmological applications. A protontherapy beam line has been developed for the treatment of uveal melanoma at the Orsay synchrocyclotron. The original 200 MeV proton beam is degraded to 76 MeV and the final beam characteristics (range, modulation, flatness, collimation) are obtained with beam modifiers in the treatment room. A passive scattering system is used to obtain a uniform dose distribution in the beam cross-section throughout 30 mm in diameter, with minimal losses in energy and dose rate. We have used an experimental approach for the scattering study. An elliptical ring shaped from 0.1-mm thick lead is the solution we have adopted for the scattering system. For a modulated beam, a flatness of 1% is obtained on transverse profiles. The energy loss introduced by this scatterer is only 0.5 MeV, with no appreciable change in the range over the treatment field. For an unmodulated beam, 21% of intensity is lost when the scatterer is used. The distal and the lateral dose fall-off (90-10%) for a modulated beam are 2.6 mm. These last values are independent of the range and the modulation currently used for the ophthalmic applications. A specific passive scattering system can be adapted to a particular beam emittance. A systematic experimental approach can easily be undertaken to obtain the scatterer adapted for small irradiation fields in proton therapy.