Bragg Peak Depth Research Articles

Proton relative biological effectiveness for the induction of DNA double strand breaks based on Geant4

Uncertainties in the relative biological effectiveness (RBE) of proton remains a major barrier to the biological optimization of proton therapy. A large amount of experimental data suggest that proton RBE is variable. As an evolving Monte Carlo code toolkit, Geant4-DNA is able to simulate the initial DNA damage caused by particle beams through physical and chemical interactions at the nanometer scale over a short period of time. This contributes to evaluating the radiobiological effects induced by ionizing radiation. Based on the Geant4-DNA toolkit, this study constructed a DNA geometric model containing 6.32Gbp, simulated the relationship between radiochemical yields (G-values) and their corresponding chemical constructors, and calculated a detailed calculation of the sources of damage and the complexity of damage in DNA strand breaks. The damage model constructed in this study can simulate the relative biological effectiveness (RBE) in the proton Bragg peak region. The results indicate that: (1) When the electron energy is below 400 keV, the yield of OH· account for 18.1% to 25.3% of the total water radiolysis yields. (2) Under the influence of histone clearance function, the yield of indirect damage account for over 72.93% of the yield of DNA strand breaks (SBs). When linear energy transfer (LET) increased from 29.79 (keV/μm) to 64.29 (keV/μm), the yield of double strand breaks (DSB) increased from 17.27% to 32.65%. (3) By investigating the effect of proton Bragg peak depth on the yield of direct DSB (DSBdirect) and total DSB (DSBtotal), the RBEDSBtot and RBEDSBdir levels of cells show that the RBE value of protons reaches 2.2 in the Bragg peak region.

Uncertainty-aware spot rejection rate as quality metric for proton therapy using a digital tracking calorimeter

Objective. Proton therapy is highly sensitive to range uncertainties due to the nature of the dose deposition of charged particles. To ensure treatment quality, range verification methods can be used to verify that the individual spots in a pencil beam scanning treatment fraction match the treatment plan. This study introduces a novel metric for proton therapy quality control based on uncertainties in range verification of individual spots. Approach. We employ uncertainty-aware deep neural networks to predict the Bragg peak depth in an anthropomorphic phantom based on secondary charged particle detection in a silicon pixel telescope designed for proton computed tomography. The subsequently predicted Bragg peak positions, along with their uncertainties, are compared to the treatment plan, rejecting spots which are predicted to be outside the 95% confidence interval. The such-produced spot rejection rate presents a metric for the quality of the treatment fraction. Main results. The introduced spot rejection rate metric is shown to be well-defined for range predictors with well-calibrated uncertainties. Using this method, treatment errors in the form of lateral shifts can be detected down to 1 mm after around 1400 treated spots with spot intensities of 1 × 107 protons. The range verification model used in this metric predicts the Bragg peak depth to a mean absolute error of 1.107 ± 0.015 mm. Significance. Uncertainty-aware machine learning has potential applications in proton therapy quality control. This work presents the foundation for future developments in this area.

PETRA: A pencil beam trimming algorithm for analytical proton therapy dose calculations with the dynamic collimation system.

The Dynamic Collimation System (DCS) has been shown to produce superior treatment plans to uncollimated pencil beam scanning (PBS) proton therapy using an in-house treatment planning system (TPS) designed for research. Clinical implementation of the DCS requires the development and benchmarking of a rigorous dose calculation algorithm that accounts for pencil beam trimming, performs monitor unit calculations to produce deliverable plans at all beam energies, and is ideally implemented with a commercially available TPS. To present an analytical Pencil bEam TRimming Algorithm (PETRA) for the DCS, with and without its range shifter, implemented in the Astroid TPS (.decimal, Sanford, Florida, USA). PETRA was derived by generalizing an existing pencil beam dose calculation model to account for the DCS-specific effects of lateral penumbra blurring due to the nickel trimmers in two different planes, integral depth dose variation due to the trimming process, and the presence and absence of the range shifter. Tuning parameters were introduced to enable agreement between PETRA and a measurement-validated Dynamic Collimation Monte Carlo (DCMC) model of the Miami Cancer Institute's IBA Proteus Plus system equipped with the DCS. Trimmer position, spot position, beam energy, and the presence or absence of a range shifter were all used as variables for the characterization of the model. The model was calibrated for pencil beam monitor unit calculations using procedures specified by International Atomic Energy Agency Technical Report Series 398 (IAEA TRS-398). The integral depth dose curves (IDDs) for energies between 70 MeV and 160 MeV among all simulated trimmer combinations, with and without the ranger shifter, agreed between PETRA and DCMC at the 1%/1mm 1-D gamma criteria for 99.99% of points. For lateral dose profiles, the median 2-D gamma pass rate for all profiles at 1.5%/1.5mm was 99.99% at the water phantom surface, plateau, and Bragg peak depths without the range shifter and at the surface and Bragg peak depths with the range shifter. The minimum 1.5%/1.5mm gamma pass rates for the 2-D profiles at the water phantom surface without and with the range shifter were 98.02% and 97.91%, respectively, and, at the Bragg peak, the minimum pass rates were 97.80% and 97.5%, respectively. The PETRA model for DCS dose calculations was successfully defined and benchmarked for use in a commercially available TPS.

Bragg Curve Detection of Low-Energy Protons by Radiophotoluminescence Imaging in Lithium Fluoride Thin Films.

Lithium fluoride (LiF) crystals and thin films are utilized as radiation detectors for energy diagnostics of proton beams. This is achieved by analyzing the Bragg curves in LiF obtained by imaging the radiophotoluminescence of color centers created by protons. In LiF crystals, the Bragg peak depth increases superlinearly with the particle energy. A previous study has shown that, when 35 MeV protons impinge at grazing incidence onto LiF films deposited on Si(100) substrates, the Bragg peak in the films is located at the depth where it would be found in Si rather than in LiF due to multiple Coulomb scattering. In this paper, Monte Carlo simulations of proton irradiations in the 1-8 MeV energy range are performed and compared to experimental Bragg curves in optically transparent LiF films on Si(100) substrates. Our study focuses on this energy range because, as energy increases, the Bragg peak gradually shifts from the depth in LiF to that in Si. The impact of grazing incidence angle, LiF packing density, and film thickness on shaping the Bragg curve in the film is examined. At energies higher than 8 MeV, all these quantities must be considered, although the effect of packing density plays a minor role.

Detection and discrimination of neutron capture events for NCEPT dose quantification

Neutron Capture Enhanced Particle Therapy (NCEPT) boosts the effectiveness of particle therapy by capturing thermal neutrons produced by beam-target nuclear interactions in and around the treatment site, using tumour-specific ^{10}B or ^{157}Gd-based neutron capture agents. Neutron captures release high-LET secondary particles together with gamma photons with energies of 478 keV or one of several energies up to 7.94 MeV, for ^{10}B and ^{157}Gd, respectively. A key requirement for NCEPT’s translation is the development of in vivo dosimetry techniques which can measure both the direct ion dose and the dose due to neutron capture. In this work, we report signatures which can be used to discriminate between photons resulting from neutron capture and those originating from other processes. A Geant4 Monte Carlo simulation study into timing and energy thresholds for discrimination of prompt gamma photons resulting from thermal neutron capture during NCEPT was conducted. Three simulated 300times 300times 300 mm^3 cubic PMMA targets were irradiated by ^4He or ^{12}C ion beams with a spread out Bragg peak (SOBP) depth range of 60 mm; one target is homogeneous while the others include 10times 10times 10 mm^3 neutron capture inserts (NCIs) of pure ^{10}B or ^{157}Gd located at the distal edge of the SOBP. The arrival times of photons and neutrons entering a simulated 50times 50times 50 mm^3 ideal detector were recorded. A temporal mask of 50–60 ns was found to be optimal for maximising the discrimination of the photons resulting from the neutron capture by boron and gadolinium. A range of candidate detector and thermal neutron shielding materials were simulated, and detections meeting the proposed acceptance criteria (i.e. falling within the target energy window and arriving 60 ns post beam-off) were classified as true or false positives, depending on their origin. The ratio of true/false positives (R_{TF}) was calculated; for targets with ^{10}B and ^{157}Gd NCIs, the detector materials which resulted in the highest R_{TF} were cadmium-shielded CdTe and boron-shielded LSO, respectively. The optimal irradiation period for both carbon and helium ions was 1 µs for the ^{10}B NCI and 1 ms for the ^{157}Gd NCI.

An initial systematic study of the linear energy transfer distributions of a proton beam under a transverse magnetic field

To evaluate the biological effectiveness of magnetic resonance (MR)-guided proton beam therapy, comprehensively characterizing the dose and dose-averaged linear energy transfer (LETd ) distributions under a magnetic field is necessary. Although detailed analysis has characterized curved beam paths and distorted dose distributions, the impact of a magnetic field on LETd should also be explored to determine the proton relative biological effectiveness (RBE). Hence, this initial study aims to present a basic analysis of LETd distributions in the presence of a magnetic field using Monte Carlo simulation (MCS). Geant4 MCS (version 10.1.p01) was performed to calculate the LETd distribution of proton beams. The incident beam energies were set to 70.2, 140.8, and 220MeV, and both zero- and finite-emittance pencil beams as well as scanned field were simulated. A transverse magnetic field of 0-3T was applied within a water phantom placed at the isocenter, and the three-dimensional dose and LETd distributions in the phantom were calculated. Then, the depth profiles of LETd along the curved trajectory and the lateral LETd profile at the Bragg peak (BP) depth were analyzed under changing energies and magnetic fields. In addition, for zero- and finite-emittance beams, the correlation of the lateral asymmetries between the dose and LETd distributions were analyzed. Finally, spread-out Bragg peak (SOBP) fields were simulated to assess the depth-dependent asymmetry of the LETd distributions. A transverse magnetic field distorted the lateral LETd distribution of a pencil beam at close to the BP, and the magnitude of the distortion at the BP increased for higher energy beams and larger magnetic fields. For a zero-emittance beam, the differences in LETd between the left and right D20 positions were relatively large; the difference in LETd was 1.5 and 2.3keV/μm at 140.8 and 220MeV, respectively, at a magnetic field of 1.5T. These asymmetries were pronounced at positions where the dose asymmetries were large. The size of the asymmetry was less substantial for a finite-emittance beam and even less for a scanned field. However, a 1.5-keV/μm difference still remained between the left and right D20 positions of a scanned field penumbra for a 220MeV beam under the same magnetic field. For the SOBP field, it was found that the distal region of SOBP had the highest LETd distortions, followed by the central and proximal regions for the middle-sized SOBP (5×5×5cm3 ), whereas the degree of LETd distortion did not vary much with depth for the 10×10×10-cm3 SOBP field. Our results indicate that not only the dose but also LETd distortions should be considered to accurately evaluate the biological effectiveness of MR-guided proton beam therapy.

Microdosimetric measurements as a tool to assess potential in-field and out-of-field toxicity regions in proton therapy

Relative biological effectiveness (RBE) variations are thought to be one of the primary causes of unexpected normal-tissue toxicities during tumor treatments with charged particles. Unlike carbon therapy, where treatment planning is optimized on the basis of the RBE-weighted dose, a constant RBE value of 1.1 is currently used in proton therapy. Assuming a uniform value can lead to under- or over-dosage, not just to the tumor but also to surrounding normal tissue. RBE changes have been linked with dose/fraction, the biological endpoint and beam properties. Understanding radiation quality and the associated RBE can improve the prediction of normal-tissue toxicities.In this study, we exploited microdosimetry for characterizing radiation quality in proton therapy in-field, and off-beam at 20 (beam edge), 50 (close out-of-field) and 100 (far out-of-field) mm from the beam center. We measured the lineal energy y spectra in a water phantom irradiated with 152 MeV protons, from which beam quality as well as the physical dose could be obtained. Taking advantage of the linear quadratic model and a modified version of the microdosimetric kinetic model, the microdosimetric data were combined with radiobiological parameters (α and β) of human salivary gland tumor cells for assessing cell survival RBE and RBE-weighted dose.The results indicate that if a dose of 60 Gy is delivered to the peak, the beam edge receives up to 6 Gy while the close and far out-of-field regions receive doses on the order of 10−3 Gy and 10−4 Gy, respectively. The RBE estimate in-beam shows large variations, ranging from 1.0 ± 0.2 at the entrance channel to 2.51 ± 0.15 at the tail. The beam edge follows a similar trend but the RBE calculated at the Bragg peak depth is 2.27 ± 0.17, i.e. twice the RBE in-beam (1.05 ± 0.15). Out-of-field, the estimated RBE is always significantly higher than 1.1 and increases with increasing lateral distance, reaching the overall highest value of 3.4 ± 0.3 at a depth of 206 mm and a lateral distance of 10 mm. The combination of RBE and dose into the biological dose points to the beam edge and the end-of-range in-beam as the areas with the highest risk of potential toxicities.

Reconstruction of dose distributions for fine carbon-ion beams using iterative approximation toward carbon-knife

For the practical application of carbon-knife with fine carbon-ion beams, the quantification of the dose distribution is essential and requires a high spatial resolution. We propose a novel method to quantify dose distributions with a spatial resolution smaller than the dosimeter size. The proposed method innovates the iterative reconstruction technique. Using a diode dosimeter with a sensitive area of 1 mm2, two-dimensional dose-area-product (DAP) distributions were measured at a 0.1 mm step at the surface and near the Bragg peak depths for fine carbon-ion beams of ∼1 mm size at the full width at half maximum (FWHM). Then, the dose distributions were reconstructed with a spatial resolution of 0.1 × 0.1 mm2 from the measured DAP distributions. However, an unnaturally high noise was observed in the reconstructed dose distributions, which were considered to originate from the measurement reproducibility errors of the DAP distributions estimated to be 2.5%–3%. Therefore, a low-pass filtering process was implemented to reduce the errors on the reconstructed dose distributions. The optimum cut-off frequencies of the low-pass filter were estimated depending on the amplitude of the induced noise. Using the filtering process with the obtained optimum cut-off frequency, the dose distribution was quantified with an average error of approximately 3% or less with respect to the peak value, when the actual measurement had an error of 3%. In the reconstructed dose rate distributions, a steep penumbra P80-20 ∼ 0.2 mm was observed at the surface, and a dose rate at the center axis of ∼90 Gy s−1 and a beam size of ∼1.1 mm at FWHM near the Bragg peak were obtained. The proposed method is expected to be useful for the measurement-based determination of microbeam models for commissioning and dose distribution calculations toward carbon-knife applications.

Calculation of direct DNA damages by a new approach for carbon ions and protons using Geant4-DNA

A simulation to predict DNA damages due to 114.2 MeV/u carbon ions and 62 MeV protons at different depths was performed by a new approach to make a comparison between those different beams. An atomic model of DNA was used to build the different levels of DNA organization in the cell nucleus. A new damage diagnostic algorithm was designed to identify break yields and has been used with the Geant4 toolkit. The single strand break (SSB) ratios for carbon ions compared to proton were 18.5 and 19 at 5 mm and 30.7 mm depth in water respectively. For Double-Strand Breaks (DSBs) the ratios were higher and they varied from 27 at 5 mm to 55 at the Bragg peak depth. The DSB yields for the same depths of carbon ion and proton beams were evaluated. The results indicated that the entrance DSB yield were 0.84 and 1.19 (GyGbp)−1 (at 5 mm depth) for protons and carbon ions, respectively, while at the Bragg peak depth were 1.15 and 3.11 (GyGbp)−1. It has been found that the DSB yield at 5 mm for the carbon beam was about 1.4 times higher than for the proton beam and regarding the Bragg peak depth, this ratio was 2.7. Our new damage diagnostic algorithm can help the identification of breaks. Also, results confirmed that carbon ions are more effective in biological effects than protons.

Measurement of the Bragg Peak Depth by a Digital Imaging Detector and PPC-40 Ionization Camera

The principle of operation of a detector that belongs to a new class is described. This detector, whose operation is based on recording ultraweak radiation from radioluminescence in an aqueous phantom, is designed for simulating proton therapy sessions at the Prometheus accelerator. The developed new detector is a digital imaging detector featuring a special electronic path. It is intended for applications in nuclear- and particle-physics experiments, together with traditional detectors. The principle for constructing a control scheme for the addition of pixels of the adaptive electronic path of the digital imaging detector is considered. The experimental measurements of the Bragg peak depth with an air ionization camera and the digital imaging detector are compared at the proton energies of 50, 100, and 170 MeV. The Bragg peak depths measured with a certified PPC-40 air camera and the digital imaging detector are found to agree within the error allowed by the IAEA requirements.

MEASUREMENTS OF THE BRAGG PEAK DEPTH BY A PPC-40 IONIZATION CHAMBER AND A DIGITAL IMAGING DETECTOR

Experimental measurements of the Bragg peak depth by two methods: the air ionization chamber and the digital imaging detector (DIDE) for proton energies of 50, 100, and 170 MeV are compared. The principle of operation of a new detector type, i.e., the DIDE for measuring the energy release region of the proton beam passing through a water phantom is considered. The DIDE detector is a coordinate meter of the Bragg peak depth profile which, using a known energy dependence of the dose, i.e., “quality curve,” can recalculate the coordinate to the doze depth profile (with controlled accelerator output stability). It is indicated that the binning (pixel summing) mode is most important for light collection from radioluminescence. In measuring the coordinate region of energy release (Bragg peak) by the detector, the result appears almost instantaneously and with an accuracy regulated by the IAEA.

First application of a novel SRAM-based neutron detector for proton therapy

In proton therapy, neutrons produced in collimators or in the patient body will contribute to unwanted additional radiation dose to the patient. This neutron dose is primarily associated with an increased risk of secondary cancer after treatment. Neutron detection in proton therapy has previously, to a large extent, been based on the use of passive detectors or detectors of large physical size which are not applicable for measurements inside phantoms. In this study, we present the first application of a neutron detector based on registration of radiation induced effects in Static Random Access Memories (SRAMs) which enables in-phantom fast neutron measurements. Measurements were performed in a water phantom irradiated by a 178 MeV proton pencil beam in a setup mimicking internally produced neutrons in pencil beam scanning (PBS) proton therapy. A neutron energy response model was developed for the SRAM detector to increase the accuracy of measurements in radiation fields with a broad range of neutron energies, and thereby make the detector applicable in the proton therapy setting. FLUKA Monte Carlo (MC) simulations and thermal neutron measurements with thermoluminescence detectors (TLDs) were conducted to evaluate the SRAM detector performance.Both experimental results (SRAM detector and TLDs) and MC simulations indicated a steep decrease in the neutron fluence with increasing lateral distance from the beam axis, while less variation was observed with depth. At Bragg peak depth, experimental values of fast neutron dose (H*(10)) were reduced from 1.4 mSv/Gy at 5.2 cm lateral distance to the beam axis to 0.22 mSv/Gy at 13.7 cm lateral distance. The MC simulations also showed that the SRAM detection threshold of 3 MeV was sufficiently low to capture approximately 90% of the neutron dose in PBS proton therapy. The differences in detector response due to variations in the neutron energy spectrum were small (<10%), and may be corrected using position-specific calibration factors. The results demonstrate the potential for estimating spatial out-of-field neutron dose based on the SRAM detector measurements in combination with the neutron energy response model.

Energy Spectra of Protons and Generated Secondary Electrons around the Bragg Peak in Materials of Interest in Proton Therapy.

The number and energy of secondary electrons generated around the trajectories of swift protons interacting with biological materials are highly relevant in proton therapy, due to the prominent role of low-energy electrons in the production of biodamage. For a given material, electron energy distributions are determined by the proton energy; and it is imperative that the distribution of proton energy at depths around the Bragg peak region be described as accurately as possible. With this objective, we simulated the energy distributions of proton beams of clinically relevant energies (50-300 MeV) at depths around the Bragg peak in liquid water and the water-equivalent polymer poly(methyl methacrylate) (PMMA). By using a simple model, this simulation has been conveniently extended to account for nuclear fragmentation reactions, providing depth-dose curves in excellent agreement with available experimental data. Special care has been taken to describe the electronic excitation spectrum of the target, taking into account its condensed phase nature. A predictive formula has been obtained for the mean value and the width of the proton energy distribution at the Bragg peak depth, quantities which are found to grow linearly with the initial energy of the beam, in good agreement with available data. To accurately characterize (in number and energy) the electrons generated around the proton paths, the energy distributions of the latter at each depth have been convoluted with the energy-dependent ionization inverse mean free paths. This results in a number of low-energy electrons around the Bragg peak larger than when only the proton beam average energy at the given depths is considered. The convoluted ionization inverse mean free path closely resembles the Bragg curve shape. The average energy of the secondary electrons is nearly constant (∼55 eV for liquid water and ∼43 eV for PMMA) in the plateau of the Bragg curve, independent of the proton incident energy and suddenly decaying once the Bragg peak is reached. These findings highlight the importance of a precise calculation of the proton beam energy distribution as a function of the target depth to reliably characterize the secondary electrons generated around the Bragg peak region.

Monte Carlo evaluation of magnetically focused proton beams for radiosurgery

The purpose of this project is to investigate the advantages in dose distribution and delivery of proton beams focused by a triplet of quadrupole magnets in the context of potential radiosurgery treatments. Monte Carlo simulations were performed using various configurations of three quadrupole magnets located immediately upstream of a water phantom. Magnet parameters were selected to match what can be commercially manufactured as assemblies of rare-earth permanent magnetic materials. Focused unmodulated proton beams with a range of ~10 cm in water were target matched with passive collimated beams (the current beam delivery method for proton radiosurgery) and properties of transverse dose, depth dose and volumetric dose distributions were compared. Magnetically focused beams delivered beam spots of low eccentricity to Bragg peak depth with full widths at the 90% reference dose contour from ~2.5 to 5 mm. When focused initial beam diameters were larger than matching unfocused beams (10 of 11 cases) the focused beams showed 16%–83% larger peak-to-entrance dose ratios and 1.3 to 3.4-fold increases in dose delivery efficiency. Peak-to-entrance and efficiency benefits tended to increase with larger magnet gradients and larger initial diameter focused beams. Finally, it was observed that focusing tended to shift dose in the water phantom volume from the 80%–20% dose range to below 20% of reference dose, compared to unfocused beams. We conclude that focusing proton beams immediately upstream from tissue entry using permanent magnet assemblies can produce beams with larger peak-to-entrance dose ratios and increased dose delivery efficiencies. Such beams could potentially be used in the clinic to irradiate small-field radiosurgical targets with fewer beams, lower entrance dose and shorter treatment times.

Characterization of a Terbium-Activated Gadolinium Oxysulfide Plastic Optical Fiber Sensor in Photons and Protons

A characterization study was carried out to determine if a novel, millimetre sized Terbium-activated Gadolinium Oxysulfide optical fibre detector has potential for future use in dosimetry. 1.25 MeV photons from Co-60 decay and 74 MeV protons from the TRIUMF facility studies were used. Dose response, field size response and Cerenkov contributions, as well as raw and spread-out Bragg peak depth doses were investigated.

Proton range verification in homogeneous materials through acoustic measurements

Clinical proton beam quality assurance (QA) requires a simple and accurate method to measure the proton beam Bragg peak (BP) depth. Protoacoustics, the measurement of the pressure waves emitted by thermal expansion resulting from proton dose deposition, may be used to obtain the depth of the BP in a phantom by measuring the time-of-flight of the pressure wave. Rectangular and cylindrical phantoms of different materials (aluminum, lead, and polyethylene) were used for protoacoustic studies. Four different methods for analyzing the protoacoustic signals are compared. Data analysis shows that, for Methods 1 and 2, plastic phantoms have better accuracy than metallic ones because of the lower speed of sound. Method 3 does not require characterizing the speed of sound in the material, but it results in the largest error. Method 4 exhibits minimal error, less than 3 mm (with an uncertainty ⩽1.5 mm) for all the materials and geometries. Psuedospectral wave-equation simulations (k-Wave MATLAB toolbox) are used to understand the origin of acoustic reflections within the phantom. The presented simulations and experiments show that protoacoustic measurements may provide a low cost and simple QA procedure for proton beam range verification as long as the proper phantoms and calculation methods are used.

Beam angle selection incorporation of anatomical heterogeneities for pencil beam scanning charged-particle therapy

In charged particle therapy with pencil beam scanning (PBS), localization of the dose in the Bragg peak makes dose distributions sensitive to lateral tissue heterogeneities. The sensitivity of a PBS plan to lateral tissue heterogeneities can be reduced by selecting appropriate beam angles. The purpose of this study is to develop a fast and accurate method of beam angle selection for PBS. The lateral tissue heterogeneity surrounding the path of the pencil beams at a given angle was quantified with the heterogeneity number representing the variation of the Bragg peak depth across the cross section of the beams using the stopping power ratio of body tissues with respect to water. To shorten the computation time, one-dimensional dose optimization was conducted along the central axis of the pencil beams as they were directed by the scanning magnets. The heterogeneity numbers were derived for all possible beam angles for treatment. The angles leading to the minimum mean heterogeneity number were selected as the optimal beam angle. Three clinical cases of head and neck cancer were used to evaluate the developed method. Dose distributions and their robustness to setup and range errors were evaluated for all tested angles, and their relation to the heterogeneity numbers was investigated. The mean heterogeneity number varied from 1.2 mm–10.6 mm in the evaluated cases. By selecting a field with a low mean heterogeneity number, target dose coverage and robustness against setup and range errors were improved. The developed method is simple, fast, accurate and applicable for beam angle selection in charged particle therapy with PBS.

Poster ‐ 25: Neutron Spectral Measurements around a Scanning Proton Beam

We describe the measurements of neutron spectra that we undertook around a scanning proton beam at the Skandion proton therapy clinic in Uppsala, Sweden. Measurements were undertaken using an extended energy range Nested Neutron Spectrometer (NNS, Detec Inc., Gatineau, QC) operated in pulsed and current mode. Spectra were measured as a function of location in the treatment room and for various Bragg peak depths. Our preliminary unfolded data clearly show the direct, evaporation and thermal neutron peaks and we can show the effect on the neutron spectrum of a water phantom in the primary proton beam.

Experimental Comparison of Knife-Edge and Multi-Parallel Slit Collimators for Prompt Gamma Imaging of Proton Pencil Beams

More and more camera concepts are being investigated to try and seize the opportunity of instantaneous range verification of proton therapy treatments offered by prompt gammas emitted along the proton tracks. Focusing on one-dimensional imaging with a passive collimator, the present study experimentally compared in combination with the first, clinically compatible, dedicated camera device the performances of instances of the two main options: a knife-edge slit (KES) and a multi-parallel slit (MPS) design. These two options were experimentally assessed in this specific context as they were previously demonstrated through analytical and numerical studies to allow similar performances in terms of Bragg peak retrieval precision and spatial resolution in a general context. Both collimators were prototyped according to the conclusions of Monte Carlo optimization studies under constraints of equal weight (40 mm tungsten alloy equivalent thickness) and of the specificities of the camera device under consideration (in particular 4 mm segmentation along beam axis and no time-of-flight discrimination, both of which less favorable to the MPS performance than to the KES one). Acquisitions of proton pencil beams of 100, 160, and 230 MeV in a PMMA target revealed that, in order to reach a given level of statistical precision on Bragg peak depth retrieval, the KES collimator requires only half the dose the present MPS collimator needs, making the KES collimator a preferred option for a compact camera device aimed at imaging only the Bragg peak position. On the other hand, the present MPS collimator proves more effective at retrieving the entrance of the beam in the target in the context of an extended camera device aimed at imaging the whole proton track within the patient.

SU‐F‐T‐129: Impact of Radial Fluctuations in RBE for Therapeutic Proton Beams

Purpose:To evaluate the off axis relative biological effectiveness (RBE) for actively scanned proton beams and determine if a constant radial RBE can be assumed.Methods:The PHITS Monte Carlo code paired with a microscopic analytical function was used to determine probability distribution functions of the lineal energy in 0.3µm diameter spheres throughout a water phantom. Twenty million primary protons were simulated for a 0.6cm diameter pencil beam. Beam energies corresponding to Bragg Peak depths of 50, 100, 150, 200, 250, and 300mm were used and evaluated transversely every millimeter and radially for annuli of 1.0, 2.0, 3.0, 3.2, 3.4, 3.6, 4.0, 5.0, 10.0, 15.0, 20.0 and 25.0mm outer radius. The acquired probability distributions were reduced to dose‐mean lineal energies and applied to the modified microdosimetric kinetic model, for human submandibular gland (HSG) cells, to calculate relative biological effectiveness (RBE) compared to 60Co beams at the 10% survival threshold.Results:RBE was generally seen to increase as distance from the central axis (CAX) increased. However, this increase was only seen in low dose regions and its overall effects on the transverse biological dose remains low. In the entrance region of the phantom (10mm depth), minimum and maximum calculated RBEs varied between 15.22 and 18.88% for different energies. At the Bragg peak, this difference ranged from 3.15 to 26.77%. Despite these rather large variations the dose‐weighted RBE and the CAX RBE varied by less than 0.14% at 10mm depth and less than 0.16% at the Bragg peak. Similarly small variations were found at all depths proximal of the Bragg peak.Conclusion:Although proton RBE does vary radially, its overall effect on biological dose is minimal and the use of a radially constant RBE in treatment planning for scanned proton beams would not produce large errors.