Function Of Proton Beam Energy Research Articles

A comprehensive Monte Carlo study of out-of-field secondary neutron spectra in a scanned-beam proton therapy gantry room

PurposeTo simulate secondary neutron radiation fields that had been measured at different relative positions during phantom irradiation inside a scanning proton therapy gantry treatment room. Further, to identify origin, energy distribution, and angular emission of the secondary neutrons as a function of proton beam energy. MethodsThe FLUKA Monte Carlo code was used to model the relevant parts of the treatment room in a scanned pencil beam proton therapy gantry including shielding walls, floor, major metallic gantry-components, patient table, and a homogeneous PMMA target. The proton beams were modeled based on experimental beam ranges in water and spot shapes in air. Neutron energy spectra were simulated at 0°, 45°, 90° and 135° relative to the beam axis at 2m distance from isocenter for monoenergetic 11×11cm2 fields from 200MeV, 140MeV, 75MeV initial proton beams, as well as for 118MeV protons with a 5cm thick PMMA range shifter. The total neutron spectra were scored for these four positions and proton energies. FLUKA neutron spectra simulations were crosschecked with Geant4 simulations using initial proton beam properties from FLUKA-generated phase spaces. Additionally, the room-components generating secondary neutrons in the room and their contributions to the total spectrum were identified and quantified. ResultsFLUKA and Geant4 simulated neutron spectra showed good general agreement with published measurements in the whole simulated neutron energy range of 10−10 to 103MeV. As in previous studies, high-energy (E≥19.6MeV) neutrons from the phantom are most prevalent along 0°, while thermalized (1meV≤E<0.4eV) and fast (100keV≤E<19.4MeV) neutrons dominate the spectra in the lateral and backscatter direction. The iron of the large bending magnet and its counterweight mounted on the gantry were identified as the most determinant sources of secondary fast-neutrons, which have been lacking in simplified room simulations. ConclusionsThe results helped disentangle the origin of secondary neutrons and their dominant contributions and were strengthened by the fact that a cross comparison was made using two independent Monte Carlo codes. The complexity of such room model can in future be limited using the result. They may further be generalized in that they can be used for an assessment of neutron fields, possibly even at facilities where detailed neutron measurements and simulations cannot be performed. They may also help to design future proton therapy facilities and to reduce unwanted radiation doses from secondary neutrons to patients.

Technical Note: Simulation of dose buildup in proton pencil beams.

The purpose of this study is to characterize the magnitude and depth of dose buildup in pencil beam scanning proton therapy. We simulate the integrated depth-dose curve of realistic proton pencil beams in a water phantom using the Geant4 Monte Carlo toolkit. We independently characterize the electronic and protonic components of dose buildup as a function of proton beam energy from 40 to 400MeV, both with and without an air gap. At clinical energies, electronic buildup over a distance of about 1mm leads to a dose reduction at depth of the basal layer (0.07mm) by up to 6% compared to if no buildup effect were present. Protonic buildup reduces the dose to the basal layer by up to 16% and has effects at depths of up to 150mm. Secondary particles with a mass number A>1 do not contribute to dose buildup. An air gap of 1m has no significant effect on protonic buildup but reduces electronic buildup below 1%. Protonic and electronic dose buildup are relevant for accurate dosimetry in proton therapy although a realistic air gap reduces the electronic buildup to levels where it can be safely neglected. We recommend including electrons and secondary protons in Monte Carlo-based treatment planning systems down to a predicted range of 10-20 μm in order to accurately model the dose at depths of the basal layer, no matter the size of the air gap between nozzle and patient.

Ranges of protons in biological targets

Abstract The paper introduces a simple fitting function for quick assessment of proton ranges in biological targets and human tissues. The function has been found by fitting an extensive data set of Monte Carlo proton ranges obtained with the aid of the SRIM-2013 code. The data has been collected for 28 different targets at 8 energies in the interval from 60 MeV to 220 MeV. The paper shows that at a given kinetic proton-beam energy, the Monte Carlo ranges can be satisfactorily fitted by a power function that depends solely on the target density. This is a great advantage for targets, for which the exact chemical composition is not known, or the mean ionizing potential is not reliably known. The satisfactory fit is meant as the fit that stays within the natural range straggling of the Monte Carlo ranges. In the second step, the energy-scaling yielding a universal fitting formula for proton ranges as a function of proton-beam energy and target density is introduced and discussed.

SU‐E‐T‐547: Modeling Biological Response to Proton Irradiation and Evaluating Its Potential Clinical Consequences

Purpose:In addition to physical uncertainties, proton therapy may also be associated with biologic uncertainties. Currently a generic RBE value of 1.1 is used for treatment planning. In this work the effects of variable RBE, in comparison to a fixed RBE, were evaluated by calculating the effective dose for proton treatments.Methods:The repair misrepair fixation (RMF) model was used to calculate variable proton RBEs. The RBE weighted spread‐out Bragg peak (SOBP) dose in water phantom was calculated using Monte Carlo simulation and compared to 1.1 weighted SOBP dose. A head and neck proton treatment was used to evaluate the potential effects, by comparing the head and neck treatment plan computed with a commercial treatment planning system that incorporates fixed RBE of 1.1 and a Monte Carlo treatment planning system that incorporates variable RBE.Results:RBE calculations along the depth of SOBP showed that the RBE at the entrance is approximately 1 and reaches 1.1 near the center of the SOBP. However, in distal regions the RBE rises to higher values (up to 3.5 depending on the cell type). Comparison of commercial treatment plans using a fixed RBE of 1.1 and Monte Carlo using variable RBE showed noticeable differences in the effective dose distributions.Conclusion:The comparison of the treatment planning with fixed and variable RBE shows that using commercial treatment planning systems that incorporate fixed RBE (1.1) could Result in overestimation of the effective dose to part of head and neck target volumes, while underestimating the effective dose to the normal tissue beyond the tumor. The accurate variable RBE as a function of proton beam energy in patient should be incorporated in treatment planning to improve the accuracy of effective dose calculation.

Study of the Angular Dependence of a Prompt Gamma Detector Response during Proton Radiation Therapy

Abstract Purpose: Several studies have recently shown that the characteristics of prompt gamma (PG) rays emitted during proton radiation therapy are beneficial for verifying proton beam range during treatment delivery. Since PG rays are produced instantaneously upon the proton beam delivery, the viability of in vivo beam range verification using PG rays depends greatly on the design optimization of not only intrinsically highly efficient detectors, but also detector location around the beam to maximize detection efficiency. The purpose of this study is to characterize angular dependence of the PG detection rates as a function of proton beam energy to help develop the design of clinically feasible detectors. Materials and Methods: In this study as a part of the long-term goal of developing a clinically feasible multistage Compton camera, we performed a Monte Carlo–based study of the detector response in a water phantom over the clinical range of beam energies, 50 to 200 MeV, and characterized PG emission s...

TU‐A‐9A‐09: Proton Beam X‐Ray Fluorescence CT

Purpose:To evaluate x‐ray fluorescence computed tomography induced with proton beams (pXFCT) for imaging of gold contrast agent.Methods:Proton‐induced x‐ray fluorescence was studied by means of Monte Carlo (MC) simulations using TOPAS, a MC code based on GEANT4. First, proton‐induced K‐shell and L‐shell fluorescence was studied as a function of proton beam energy and 1) depth in water and 2) size of contrast object. Second, pXFCT images of a 2‐cm diameter cylindrical phantom with four 5‐ mm diameter contrast vials and of a 20‐cm diameter phantom with 1‐cm diameter vials were simulated. Contrast vials were filled with water and water solutions with 1‐5% gold per weight. Proton beam energies were varied from 70‐250MeV. pXFCT sinograms were generated based on the net number of gold K‐shell or L‐shell x‐rays determined by interpolations from the neighboring 0.5keV energy bins of spectra collected with an idealized 4π detector. pXFCT images were reconstructed with filtered‐back projection, and no attenuation correction was applied.Results:Proton induced x‐ray fluorescence spectra showed very low background compared to x‐ray induced fluorescence. Proton induced L‐shell fluorescence had a higher cross‐section compared to K‐shell fluorescence. Excitation of L‐shell fluorescence was most efficient for low‐energy protons, i.e. at the Bragg peak. K‐shell fluorescence increased with increasing proton beam energy and object size. The 2% and 5% gold contrast vials were accurately reconstructed in K‐shell pXFCT images of both the 2‐cm and 20‐cm diameter phantoms. Small phantom L‐shell pXFCT image required attenuation correction and had a higher sensitivity for 70MeV protons compared to 250MeV protons. With attenuation correction, L‐shell pXFCT might be a feasible option for imaging of small size (∼2cm) objects. Imaging doses for all simulations were 5‐30cGy.Conclusion:Proton induced x‐ray fluorescence CT promises to be an alternative quantitative imaging technique to the commonly considered XFCT imaging with x‐ray beams.

Assessment of radionuclidic impurities in cyclotron produced 99mTc

The commercial viability of cyclotron-produced (99m)Tc as an alternative to generator-produced (99m)Tc depends on several factors. These include: production yield, ease of target processing and recycling of (100)Mo, radiochemical purity, specific activity as well as the presence of other radionuclides, particularly various Tc radioisotopes that cannot be separated chemically and will remain in the final clinical preparation. These Tc radionuclidic impurities are derived from nuclear interactions of the accelerated protons with other stable Mo isotopes present in the enriched (100)Mo target. The aim of our study was to determine experimentally the yields of Tc radioisotopes produced from these stable Mo isotopes as a function of incident beam energy in order to predict radionuclidic purity of (99m)Tc produced in highly enriched (100)Mo targets of known isotopic composition. Enriched molybdenum targets of (95)Mo, (96)Mo, (97)Mo, (98)Mo and (100)Mo were prepared by pressing powdered metal into an aluminum target support. The thick targets were bombarded with 10 to 24MeV protons using the external beam line of the U-120M cyclotron of the Nuclear Physics Institute, Řež. The thick target yields of (94)Tc, (94m)Tc, (95)Tc, (95m)Tc, (96m+g)Tc and (97m)Tc were derived from their activities measured by γ spectrometry using a high purity Ge detector. These data were then used to assess the effect of isotopic composition of highly enriched (100)Mo targets on the radionuclidic purity of (99m)Tc as a function of proton beam energy. Estimates were validated by comparison to measured activities of Tc radioisotopes in proton irradiated, highly enriched (100)Mo targets of known isotopic composition. The measured thick target yields of (94)Tc, (94m)Tc, (95)Tc, (95m)Tc, (96m+g)Tc and (97m)Tc correspond well with recently published values calculated via the EMPIRE-3 code. However, the measured yields are more favourable with regard to achievable radionuclidic purity of (99m)Tc. Reliability of the measured thick target yields was demonstrated by comparison of the estimated and measured activities of (94)Tc, (95)Tc, (95m)Tc, and (96m+g)Tc in highly enriched (100)Mo (99%) targets that showed good agreement, with maximum differences within estimated uncertainties. Radioisotopes (94m)Tc and (97m)Tc were not detected in the irradiated (100)Mo targets due to their low activities and measurement conditions; on the other hand we detected small amounts of the short-lived positron emitter (93)Tc (T(½)=2.75h). In addition to (99m)Tc and trace amounts of the various Tc isotopes, significant activities of (96)Nb, (97)Nb and (99)Mo were detected in the irradiated (100)Mo targets. Radioisotope formation during the proton irradiation of Mo targets prepared from different, enriched stable Mo isotopes provides a useful data base to predict the presence of Tc radionuclidic impurities in (99m)Tc derived from proton irradiated (100)Mo targets of known isotopic composition. The longer-lived Tc isotopes including (94)Tc (T(½)=4.883h), (95)Tc (T(½)=20.0h), (95m)Tc (T(½)=61 d), (96m+g)Tc (T(½)=4.24 d) and (97m)Tc (T(½)=90 d) are of particular concern since they may affect the dosimetry in clinical applications. Our data demonstrate that cyclotron production of (99m)Tc, using highly enriched (100)Mo targets and 19-24MeV incident proton energy, will result in a product of acceptable radionuclidic purity for applications in nuclear medicine.

Designing accelerator-based epithermal neutron beams for boron neutron capture therapy.

The 7Li(p,n)7Be reaction has been investigated as an accelerator-driven neutron source for proton energies between 2.1 and 2.6 MeV. Epithermal neutron beams shaped by three moderator materials, Al/AlF3, 7LiF, and D2O, have been analyzed and their usefulness for boron neutron capture therapy (BNCT) treatments evaluated. Radiation transport through the moderator assembly has been simulated with the Monte Carlo N-particle code (MCNP). Fluence and dose distributions in a head phantom were calculated using BNCT treatment planning software. Depth-dose distributions and treatment times were studied as a function of proton beam energy and moderator thickness. It was found that an accelerator-based neutron source with Al/AlF3 or 7LiF as moderator material can produce depth-dose distributions superior to those calculated for a previously published neutron beam design for the Brookhaven Medical Research Reactor, achieving up to approximately 50% higher doses near the midline of the brain. For a single beam treatment, a proton beam current of 20 mA, and a 7LiF moderator, the treatment time was estimated to be about 40 min. The tumor dose deposited at a depth of 8 cm was calculated to be about 21 Gy-Eq.

Charge transfer into the metastable2Slevel of hydrogen by protons colliding with K and Na

We measure the charge transfer cross section for protons in potassium and sodium with the final state being $\mathrm{H}(2s)+{A}^{+}$. Using a crossed-beam experimental method we look at this cross section as a function of proton beam energy from 500 to 2500 eV. Our results for potassium agree with previous experiments and theories in showing two main peaks in the cross section: one of (3.9 \ifmmode\pm\else\textpm\fi{} 1.5)\ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ at a proton beam energy of 800 eV, and another of (2.8\ifmmode\pm\else\textpm\fi{} 1.1)\ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ at a proton beam energy of 2.5 keV. Our results for sodium show a steadily rising cross section with a value of (7.3\ifmmode\pm\else\textpm\fi{} 2.5)\ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ at a proton beam energy of 2.5 keV in agreement with the prediction of Kubach and Sidis; we do not find the maximum at 700 eV reported by Nagata that agrees with the theory of Kimura, Olson, and Pascale.

Reactively sputtered tantalum thin film resistors Part 2. Low energy, low fluence proton radiation damage

Tantalum thin film resistors which were reactively sputtered in oxygen and nitrogen simultaneously were bombarded with 150 keV protons to obtain the conductance-fluence characteristics. The conductance of heat treated films increased as a linear function of fluence at room temperature, for values of fluence up to the measured value of 10 16 p/cm 2. This linear conductance increase may be accounted for by radiation-produced defect levels within the intergranular crystalline oxide regions of the discontinuous films. The rate of conductance change with fluence was a maximum for a total reactive partial pressure in the range of 2 × 10 -5 torr, falling off for both greater and smaller total reactive partial pressures. For non heat treated films, the conductance increased as a non-linear function of fluence. This non-linear increase may be accounted for by defect trapping within the amorphous inter-granular oxide. The damage rate as a function of proton beam energy followed the Rutherford law over the range of 50 keV to 150 keV.