Proton Dose Research Articles

Effects of Rectal Variations on Proton Dose for Treatment of Rectum and Anal Cancers: A Cone Beam CT Based Planning Study

Effects of Rectal Variations on Proton Dose for Treatment of Rectum and Anal Cancers: A Cone Beam CT Based Planning Study

Degradation of proton depth dose distributions attributable to microstructures in lung-equivalent material.

The purpose of the work reported here was to investigate the influence of sub-millimeter size heterogeneities on the degradation of the distal edges of proton beams and to validate Monte Carlo (MC) methods' ability to correctly predict such degradation. A custom-designed high-resolution plastic phantom approximating highly heterogeneous, lung-like structures was employed in measurements and in Monte Carlo simulations to evaluate the degradation of proton Bragg curves penetrating heterogeneous media. Significant differences in distal falloff widths and in peak dose values were observed in the measured and the Monte Carlo simulated curves compared to pristine proton Bragg curves. Furthermore, differences between simulations of beams penetrating CT images of the phantom did not agree well with the corresponding experimental differences. The distal falloff widths in CT image-based geometries were underestimated by up to 0.2 cm in water (corresponding to 0.8-1.4 cm in lung tissue), and the peak dose values of pristine proton beams were overestimated by as much as ˜35% compared to measured curves or depth-dose curves simulated on the basis of true geometry. The authors demonstrate that these discrepancies were caused by the limited spatial resolution of CT images that served as a basis for dose calculations and lead to underestimation of the impact of the fine structure of tissue heterogeneities. A convolution model was successfully applied to mitigate the underestimation. The results of this study justify further development of models to better represent heterogeneity effects in soft-tissue geometries, such as lung, and to correct systematic underestimation of the degradation of the distal edge of proton doses.

Rapid MCNP simulation of DNA double strand break (DSB) relative biological effectiveness (RBE) for photons, neutrons, and light ions

To account for particle interactions in the extracellular (physical) environment, information from the cell-level Monte Carlo damage simulation (MCDS) for DNA double strand break (DSB) induction has been integrated into the general purpose Monte Carlo N-particle (MCNP) radiation transport code system. The effort to integrate these models is motivated by the need for a computationally efficient model to accurately predict particle relative biological effectiveness (RBE) in cell cultures and in vivo. To illustrate the approach and highlight the impact of the larger scale physical environment (e.g. establishing charged particle equilibrium), we examined the RBE for DSB induction (RBEDSB) of x-rays, 137Cs γ-rays, neutrons and light ions relative to γ-rays from 60Co in monolayer cell cultures at various depths in water. Under normoxic conditions, we found that 137Cs γ-rays are about 1.7% more effective at creating DSB than γ-rays from 60Co (RBEDSB = 1.017) whereas 60–250 kV x-rays are 1.1 to 1.25 times more efficient at creating DSB than 60Co. Under anoxic conditions, kV x-rays may have an RBEDSB up to 1.51 times as large as 60Co γ-rays. Fission neutrons passing through monolayer cell cultures have an RBEDSB that ranges from 2.6 to 3.0 in normoxic cells, but may be as large as 9.93 for anoxic cells. For proton pencil beams, Monte Carlo simulations suggest an RBEDSB of about 1.2 at the tip of the Bragg peak and up to 1.6 a few mm beyond the Bragg peak. Bragg peak RBEDSB increases with decreasing oxygen concentration, which may create opportunities to apply proton dose painting to help address tumor hypoxia. Modeling of the particle RBE for DSB induction across multiple physical and biological scales has the potential to aid in the interpretation of laboratory experiments and provide useful information to advance the safety and effectiveness of hadron therapy in the treatment of cancer.

Hydrogen passivation effect on the conversion efficiency of Si solar cells by low-energy proton implantation

We investigated the hydrogen passivation effect on the conversion efficiency of Si solar cells by including low-energy hydrogen ion implantation in the cell fabrication process. The solar cells were implanted with various proton energies and doses just before the co-firing process. The quantum efficiency specifically in the blue wavelengths of the solar cells increased with a low implantation dose and energy indicating improvement of surface passivation. However, quantum efficiency decreased when the implantation dose and energy were increased, indicating a high defect-assisted surface recombination of minority carriers. The cells implanted with a low dose and energy in the presence of SiNx showed interfacial passivation with enhanced photocurrent, minority carrier lifetime and short-circuit current density for improved conversion efficiency.

Measurement of radiation damage of water-based liquid scintillator and liquid scintillator

Liquid scintillating phantoms have been proposed as a means to perform real-time 3D dosimetry for proton therapy treatment plan verification. We have studied what effect radiation damage to the scintillator will have upon this application. We have performed measurements of the degradation of the light yield and optical attenuation length of liquid scintillator and water-based liquid scintillator after irradiation by 201 MeV proton beams that deposited doses of approximately 52 Gy, 300 Gy, and 800 Gy in the scintillator. Liquid scintillator and water-based liquid scintillator (composed of 5% scintillating phase) exhibit light yield reductions of 1.74 ± 0.55 % and 1.31 ± 0.59 % after 0≈ 80 Gy of proton dose, respectively. Whilst some increased optical attenuation was observed in the irradiated samples, the measured reduction to the light yield is also due to damage to the scintillation light production. Based on our results and conservative estimates of the expected dose in a clinical context, a scintillating phantom used for proton therapy treatment plan verification would exhibit a systematic light yield reduction of approximately 0.1% after a year of operation.

Influence of Geant4 parameters on proton dose distribution

Purpose: The proton therapy presents a great precision during the radiation dose delivery. It is useful when the tumor is located in a sensitive area like brain or eyes. The Monte Carlo (MC) simulations are usually used in treatment planning system (TPS) to estimate the radiation dose. In this paper we are interested in estimating the proton dose statistical uncertainty generated by the MC simulations. Methods : Geant4 was used in the simulation of the eye’s treatment room for 62 MeV protons therapy, installed in the Istituto Nazionale Fisica Nucleare Laboratori Nazionali del Sud (LNS-INFN) facility in Catania. This code is a Monte Carlo based on software dedicated to simulate the passage of particles through the matter. In this work, we are interested in optimizing the Geant4 parameters on energy deposit distribution by proton to achieve the spatial resolution of dose distribution required for cancer therapy. We propose various simulations and compare the corresponding dose distribution inside water to evaluate the statistical uncertainties. Results : The simulated Bragg peak , based on facility model is in agreement with the experimental data, The calculations show that the mean statistical uncertainty is less than 1% for a simulation set with 5 × 10 4 events, 10 -3 mm production threshold and a 10 -2 mm step limit. Conclusion : The set of Geant4 cut and step limit values can be chosen in combination with the number of events to reach precision recommended from International Commission on Radiation Units and measurements (ICRU) in Monte Carlo codes for proton therapy treatment.

Monte Carlo comparison of x-ray and proton CT for range calculations of proton therapy beams

Proton computed tomography (CT) has been described as a solution for imaging the proton stopping power of patient tissues, therefore reducing the uncertainty of the conversion of x-ray CT images to relative stopping power (RSP) maps and its associated margins. This study aimed to investigate this assertion under the assumption of ideal detection systems. We have developed a Monte Carlo framework to assess proton CT performances for the main steps of a proton therapy treatment planning, i.e. proton or x-ray CT imaging, conversion to RSP maps based on the calibration of a tissue phantom, and proton dose simulations. Irradiations of a computational phantom with pencil beams were simulated on various anatomical sites and the proton range was assessed on the reference, the proton CT-based and the x-ray CT-based material maps. Errors on the tissue’s RSP reconstructed from proton CT were found to be significantly smaller and less dependent on the tissue distribution. The imaging dose was also found to be much more uniform and conformal to the primary beam. The mean absolute deviation for range calculations based on x-ray CT varies from 0.18 to 2.01 mm depending on the localization, while it is smaller than 0.1 mm for proton CT. Under the assumption of a perfect detection system, proton range predictions based on proton CT are therefore both more accurate and more uniform than those based on x-ray CT.

Study on Effect of Proton Irradiation Energy in AlGaN/GaN Metal-Oxide Semiconductor High Electron Mobility Transistors

The effects of proton irradiation energy on Al2O3/AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOSHEMT) were studied. MOSHEMTs were irradiated at different irradiation energies of 5 MeV, 10 MeV, or 15 MeV with a fixed proton dose of 5 × 1015 cm-2. After the 5 MeV, 10 MeV and 15 MeV proton irradiation, MOSHEMTs’ saturation current at gate voltage of 1V were reduced by 95.3, 68.3 and 59.8% and maximum transconductance were reduced by 88, 54.4, and 40.7% respectively. The carrier removal rate of the irradiation energy employed in this study was in the range of 127-289 cm-1, having lower energy with the highest removal rate.

Three‐dimensional gamma criterion for patient‐specific quality assurance of spot scanning proton beams

The purpose of this study was to evaluate the effectiveness of full three‐dimensional (3D) gamma algorithm for spot scanning proton fields, also referred to as pencil beam scanning (PBS) fields. The difference between the full 3D gamma algorithm and a simplified two‐dimensional (2D) version was presented. Both 3D and 2D gamma algorithms are used for dose evaluations of clinical proton PBS fields. The 3D gamma algorithm was implemented in an in‐house software program without resorting to 2D interpolations perpendicular to the proton beams at the depths of measurement. Comparison between calculated and measured dose points was carried out directly using Euclidian distance in 3D space and the dose difference as a fourth dimension. Note that this 3D algorithm faithfully implemented the original concept proposed by Low et al. (1998) who described gamma criterion using 3D Euclidian distance and dose difference. Patient‐specific proton PBS plans are separated into two categories, depending on their optimization method: single‐field optimization (SFO) or multifield optimized (MFO). A total of 195 measurements were performed for 58 SFO proton fields. A MFO proton plan with four fields was also calculated and measured, although not used for treatment. Typically three different depths were selected from each field for measurements. Each measurement was analyzed by both 3D and 2D gamma algorithms. The resultant 3D and 2D gamma passing rates are then compared and analyzed. Comparison between 3D and 2D gamma passing rates of SFO fields showed that 3D algorithm does show higher passing rates than its 2D counterpart toward the distal end, while little difference is observed at depths away from the distal end. Similar phenomenon in the lateral penumbra was well documented in photon radiation therapy, and in fact brought about the concept of gamma criterion. Although 2D gamma algorithm has been shown to suffice in addressing dose comparisons in lateral penumbra for photon intensity‐modulation radiation therapy (IMRT) plans, results here showed that a full 3D algorithm is required for proton dose comparisons due to the existence of Bragg peaks and distal penumbra. A MFO proton plan with four fields was also measured and analyzed. Sharp dose gradients exist in MFO proton fields, both in the middle of the modulation and toward the most distal layers. Decreased 2D gamma passing rates at locations of high dose gradient are again observed as in the SFO fields. Results confirmed that a full 3D algorithm for gamma criterion is needed for proton PBS plan's dose comparisons. The 3D gamma algorithm is implemented by an in‐house software program. Patient‐specific proton PBS plans are measured and analyzed using both 3D and 2D gamma algorithms. For measurements performed at depths with large dose gradients along the beam direction, gamma comparison passing rates using 2D algorithm is lower than those obtained with the full 3D algorithm.PACS number: 87.53.Bn, 87.53.Jw, 87.55.de, 87.55.kd, 87.55.ne, 87.55.Qr

Effects of 340 keV proton irradiation on InGaN/GaN blue light-emitting diodes

The effects of proton irradiation on optical and electrical performances of InGaN/GaN blue light-emitting diodes (LEDs) were investigated. The InGaN/GaN blue LEDs were irradiated with protons at a fixed energy of 340 keV and doses ranging from 5 × 1010 to 1 × 1014/cm2. Both current–voltage (I-V) and light output–current (L-I) characteristics of InGaN/GaN blue LEDs were gradually degraded as increasing the proton doses. The optical performances of LED were much more sensitive to the proton irradiation than that of electrical performances. The electroluminescence spectra and the light output performances before and after proton irradiations had similar trends in degradation. Then, the reverse recovery time before and after 1 × 1014/cm2 proton irradiation slightly decreased from 31.0 to 27.6 ns.

Effect of proton irradiation energy on AlGaN/GaN metal-oxide semiconductor high electron mobility transistors

The effects of proton irradiation energy on dc characteristics of AlGaN/GaN metal-oxide semiconductor high electron mobility transistors (MOSHEMTs) using Al2O3 as the gate dielectric were studied. Al2O3/AlGaN/GaN MOSHEMTs were irradiated with a fixed proton dose of 5 × 1015 cm−2 at different energies of 5, 10, or 15 MeV. More degradation of the device dc characteristics was observed for lower irradiation energy due to the larger amount of nonionizing energy loss in the active region of the MOSHEMTs under these conditions. The reductions in saturation current were 95.3%, 68.3%, and 59.8% and reductions in maximum transconductance were 88%, 54.4%, and 40.7% after 5, 10, and 15 MeV proton irradiation, respectively. Both forward and reverse gate leakage current were reduced more than one order of magnitude after irradiation. The carrier removal rates for the irradiation energies employed in this study were in the range of 127–289 cm−1. These are similar to the values reported for conventional metal-gate high-electron mobility transistors under the same conditions and show that the gate dielectric does not affect the response to proton irradiation for these energies.

Effects of 340 Kev Proton Irradiation on InGaN/GaN Blue Light-Emitting Diodes

The development of GaN-based light-emitting diodes (LEDs) holds promise for various applications such as televisions, mobile phones, traffic signals, automotive lighting and indoor/outdoor lighting. Recently, the application of GaN-based LEDs has been extended to satellite communication systems for weather forecasting or broadband data transmission due to their high radiation hardness. GaN-based devices with small lattice constant (a = 3.189 Å, c = 5.186 Å for wurtzite structure) show superior radiation resistance due to the higher displacement energies compared with other semiconductor systems such as GaAs (a = 5.653 Å). In this work, the low energy (340 keV) proton irradiation effects on optical and electrical performances of InGaN/GaN blue LEDs were investigated. The commercial InGaN/GaN blue LEDs with the emission peak of 441nm were used. The epitaxial structures consisted of p- and n-type GaN layer and InGaN/GaN multi quantum wells grown by metal organic chemical vapor deposition on sapphire substrates. Different proton doses from 5 × 1010 to 1 × 1014 /cm2 were employed to investigate the effects of irradiation doses on the device performances. The irradiated samples were intentionally tilted 7° away from the normal direction of the proton beam to prevent the channeling effect. The Stopping and Range of Ions in Matter (SRIM) simulation was used to estimate the penetration depth of protons and the distribution of vacancies generated by proton irradiations. The L-I-V characteristics of InGaN/GaN blue LEDs were measured with Si photodiode connected to a HP 4156 parameter analyzer. The electroluminescence (EL) spectra were obtained by an optical fiber connected to the Ocean Optics USB 4000 spectrometer. The reverse recovery characteristics were measured with oscilloscope and pulse generator. Figure 1 shows the current-voltage (I-V) of InGaN/GaN blue LEDs before and after proton irradiations with various doses, respectively. The LED diode I-V characteristics of InGaN/GaN blue LEDs were drastically degraded after high dose (1 × 1014 /cm2) proton irradiation. This degradation was caused by the displacement damage, resulting in the reduction of carrier concentration and mobility. However, the degradation of light output was much faster than that of I-V characteristics as shown in Figure 2. This faster degradation of light output indicates that defects generated by energetic protons can act as non-radiative recombination centers. The reduction of light output was also confirmed using EL spectra. In addition, the reverse recovery time, which is related to the minority carrier life time, was also measured by switching the current from 0mA to 50mA. The reverse recovery time before and after 1 × 1013 and 1 × 1014 /cm2 proton irradiations were 80, 79 and 73 ns, respectively. This degradation resulted from the reduction of minority carrier lifetime caused by the introduction of irradiation-induced defects in InGaN/GaN blue LEDs after proton irradiation. In conclusion, the effects of 340 keV proton irradiations on InGaN/GaN blue LEDs were investigated using L-I-V characteristics, EL spectra and reverse recovery characteristics. The optical performances severely degraded compared with the electrical performances due to the radiation-induced defects served as the non-radiative recombination centers. Figure 1

Study on Effect of Proton Irradiation Energy in AlGaN/GaN Metal-Oxide Semiconductor High Electron Mobility Transistors

AlGaN/GaN high electron mobility transistors (HEMTs) showed its promising features in high power and high frequency applications such as inverter units in hybrid electric vehicles, advanced radar systems, and satellite-based communication networks with its high density of sheet carrier concentration, high electron mobility, and radiation hardness. However, the high gate leakage and current collapse limits the reliability and performance of the HEMTs. In the other hand, using oxide as gate dielectric solves the problems at the same time not degrading the device performances. In this paper, we investigated the effect of the proton energy dependency on dc characteristics of atomic layer deposition (ALD) deposited Al2O3 gate dielectric AlGaN/GaN MOS-HEMTs. AlGaN/GaN MOS-HEMTs were irradiation with fixed proton dose of 5 × 1015 cm-2 at 5MeV, 10MeV, and 15MeV. The MOS-HEMTs’ dc characteristics were compared before and after the irradiation to assess the damage done by irradiation. AlGaN/GaN epilayer were grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD), consisting of 5 μm GaN buffer, 21 nm of undoped Al0.24Ga0.76N, and a 5 nm undoped GaN cap. On-wafer Hall measurement was used to measure sheet resistance, sheet carrier concentration, and mobility of 479 Ω/□, 9.55 × 1012 cm-2, and 1363 cm2/V·s respectively. Mesa isolation process was achieved by Unaxis Shuttlelock Reactive Ion Etcher with Inductively Coupled Plasma Module (ICP) with Cl2/Ar plasma. Consisting of Ti/Al/Ni/Au (25 nm/125 nm/45 nm/100 nm), 5 μm distance of source to drain Ohmic metallization was achieved by lift-off process of electron-beam evaporator deposited Ohmic metal. Rapid thermal annealing at 850ºC with N2 ambient for 45 seconds was followed by Ohmic metallization. A 10 nm Al2O3 layer was deposited by ALD at 300ºC. Ni/Au (20nm/80nm) gate metal with 100 μm width and 1 μm length was deposited by liftoff of e-beam evaporator. After fabricating MOS-HEMT, proton irradiation was projected at the Korean Institute of Radiological & Medical Sciences (KIRAMS) using a MC 50 (Scanditronix) cyclotron with proton fluence of 5 × 1015 cm-2 at 5MeV. MOS-HEMT’s dc characteristics were measured by 4156 HP parameter analyzer. Transmission line measurement (TLM) was used to measure sheet resistance (RS), transfer resistance (RT), and contact resistance (RC) before and after proton irradiation. Stopping and range of ions in matter simulations (SRIMS) was used to simulate the penetration depth of the protons into AlGaN/GaN MOS-HEMTs at different proton energies. Figure 1 shows the effect of 5 × 1015 cm-2 proton irradiation at 10MeV on drain I-V characteristics before and after. The saturation current, IDSS, was reduced by 68.3% and mobility, which was calculated from linear region of drain I-V curve using charge control model, was reduced by 69.3%. These reductions were due to the displacement damage induced by ion bombardment from proton irradiation. Table 1 shows the summary of MOS-HEMTs device degradation in IDSS, transconductance, threshold voltage shift, mobility, and sheet carrier concentration. In conclusion, irradiation-induced damage causes the reductions of carrier mobility and carrier concentration. The severe degradation in lower irradiation energy is observed due to the more acceptor type defects generated near the 2 dimensional electron gas channel. Figure 1

A Monte Carlo study of the relationship between the time structures of prompt gammas and the in-vivo radiation dose in proton therapy

For in-vivo range verification in proton therapy, attempts have been made to measure the spatial distribution of the prompt gammas generated by the proton-induced interactions and to determine the proton dose distribution. However, the high energies of prompt gammas and background gammas are still problematic in measuring the distribution. In this study, we suggested a new method for determining the in-vivo range by utilizing the time structure of the prompt gammas formed during the rotation of a range modulation wheel (RMW) in passive scattering proton therapy. To validate the Monte Carlo code simulating the proton beam nozzle, we compared the axial percent depth doses (PDDs) with the measured PDDs for varying beam range from 4.73 to 24.01 cm. Also, we assessed the relationship between the proton dose rate and the time structure of the prompt gammas in a water phantom. The results of the PDD showed agreement within relative errors of 1.1% in the distal range and 2.9% in the modulation width. The average dose difference in the modulation was assessed as less than 1.3% by comparison with the measurements. The time structure of prompt gammas was well-matched, within 0.39 ms, with the proton dose rate, and this enabled an accurate prediction of the in-vivo range.

Magnetoresistance of Proton Irradiated Si0.97Ge0.03 Whiskers

Whiskers are a new material that is characterized by high structural perfection, chemical resistance and strength which reaches the theoretically possible limit for crystals of small transverse dimensions. The test whiskers were synthesized by the method of chemical transport reactions in a closed bromide system using gold as the initiator of growth. The crystals were irradiated by protons with an energy of 6 MeV and doses of 5 · 10 13 , 10 15 and 1 · 10 17 p + /cm 2 at 40 ° C in a U − 120 cyclotron. The effects of proton irradiation and high magnetic fields on the magnetoresistance of Si 1 − x Ge x ( x = 0,03) whiskers in the 4,2—300 K temperature range has been studied. A slight decrease in the electrical resistance of the crystals in the 4,2—40 K temperature range during irradiation with small proton doses and a significant increase in their resistance in the entire investigated temperature range for a dose of 1·10 17 p + /cm 2 have been found. The ionization energy of the impurity atoms in different magnetic fields has been calculated. It has been revealed that the energy level of the impurity depends on the magnetic field but slightly which in turn indicates a independence of the concentration of holes on the magnetic field. It has been shown that a significant magnetoresistance at all studied temperatures was due to the magnetic field − caused decrease in the mobility of free charge carriers (holes). It has been found that the concentration of holes depends on magnetic field but a little. Conclusion has been made about a negligible expansion of the band gap in magnetic fields of up to 8 T.

SU‐E‐T‐673: Recent Developments and Comprehensive Validations of a GPU‐Based Proton Monte Carlo Simulation Package, GPMC

Purpose:A GPU‐based Monte Carlo (MC) simulation package gPMC has been previously developed and high computational efficiency was achieved. This abstract reports our recent improvements on this package in terms of accuracy, functionality, and code portability.Methods:In the latest version of gPMC, nuclear interaction cross section database was updated to include data from TOPAS/Geant4. Inelastic interaction model, particularly the proton scattering angle distribution, was updated to improve overall simulation accuracy. Calculation of dose averaged LET (LETd) was implemented. gPMC was ported onto an OpenCL environment to enable portability across different computing devices (GPUs from different vendors and CPUs). We also performed comprehensive tests of the code accuracy. Dose from electro‐magnetic (EM) interaction channel, primary and secondary proton doses and fluences were scored and compared with those computed by TOPAS.Results:In a homogeneous water phantom with 100 and 200 MeV beams, mean dose differences in EM channel computed by gPMC and by TOPAS were 0.28% and 0.65% of the corresponding maximum dose, respectively. With the Geant4 nuclear interaction cross section data, mean difference of primary proton dose was 0.84% for the 200 MeV case and 0.78% for the 100 MeV case. After updating inelastic interaction model, maximum difference of secondary proton fluence and dose were 0.08% and 0.5% for the 200 MeV beam, and 0.04% and 0.2% for the 100 MeV beam. In a test case with a 150MeV proton beam, the mean difference between LETd computed by gPMC and TOPAS was 0.96% within the proton range. With the OpenCL implementation, gPMC is executable on AMD and Nvidia GPUs, as well as on Intel CPU in single or multiple threads. Results on different platforms agreed within statistical uncertainty.Conclusion:Several improvements have been implemented in the latest version of gPMC, which enhanced its accuracy, functionality, and code portability.

SU‐E‐J‐135: Feasibility of Using Quantitative Cone Beam CT for Proton Adaptive Planning

Purpose:To investigate the feasibility of using scatter corrected cone beam CT (CBCT) for proton adaptive planning.Methods:Phantom study was used to evaluate the CT number difference between the planning CT (pCT), quantitative CBCT (qCBCT) with scatter correction and calibrated Hounsfield units using adaptive scatter kernel superposition (ASKS) technique, and raw CBCT (rCBCT). After confirming the CT number accuracy, prostate patients, each with a pCT and several sets of weekly CBCT, were investigated for this study. Spot scanning proton treatment plans were independently generated on pCT, qCBCT and rCBCT. The treatment plans were then recalculated on all images. Dose‐volume‐histogram (DVH) parameters and gamma analysis were used to compare between dose distributions.Results:Phantom study suggested that Hounsfield unit accuracy for different materials are within 20 HU for qCBCT and over 250 HU for rCBCT. For prostate patients, proton dose could be calculated accurately on qCBCT but not on rCBCT. When the original plan was recalculated on qCBCT, tumor coverage was maintained when anatomy was consistent with pCT. However, large dose variance was observed when patient anatomy change. Adaptive plan using qCBCT was able to recover tumor coverage and reduce dose to normal tissue.Conclusion:It is feasible to use qu antitative CBCT (qCBCT) with scatter correction and calibrated Hounsfield units for proton dose calculation and adaptive planning in proton therapy.Partly supported by Varian Medical Systems.

SU‐E‐J‐247: Time Evolution of Radiation‐Induced Lung Injury After Stereotactic Proton Therapy

Purpose:Quantitative metrics to assess patient‐specific radiation‐induced lung injury have the potential to guide individualization of therapy and be early indicators of recurrence. Here we investigate computed tomography (CT) density changes in normal lung after stereotactic Proton Therapy.Methods:Participants in a phase‐I clinical trial for stereotactic body radiation therapy (SBRT) with protons are analyzed on a rolling basis. The dataset includes 9 patients with 34 CT images to date. Follow‐up images are registered to the planning CT using deformable image registration and the change in CT density is correlated to the dose to examine the time‐evolution of Hounsfield Unit (HU) changes after large doses of proton radiation.Results:The lung density observed on the follow‐up images increases significantly with dose for all dose levels above 5 Gy(RBE) (p<0.001) for 8/9 patients. The change per unit dose [HU/Gy] varies significantly among the patients, from 0.1 (for the one patient without significant correlation) to 5.7 ΔHU/Gy(RBE). The current population average of ΔHU/Gy(RBE) is 2.1, i.e. a 1 Gy(RBE) increase in dose leads on average to a 2.1 HU increase in CT density. The slope of the dose‐response curve is constant for all timepoints investigated (from 3–24+ months). Additionally a pronounced non‐linearity in the dose response curve is noted for long follow‐up times (>18 months).Conclusion:CT density changes have a robust correlation with proton dose, quantitatively similar to photon dose, and may allow estimation of a patient's intrinsic radiosensitivity after proton therapy. The stability of the correlation with time however diverges from what is known about CT response after photon irradiation. This could have important implications for clinical decision‐making during proton therapy for lung cancer, especially for scheduling of follow‐up CT/PET imaging and diagnosis of recurrence.

SU‐F‐BRD‐09: A Random Walk Model Algorithm for Proton Dose Calculation

Purpose:To develop a random walk model algorithm for calculating proton dose with balanced computation burden and accuracy.Methods:Random walk (RW) model is sometimes referred to as a density Monte Carlo (MC) simulation. In MC proton dose calculation, the use of Gaussian angular distribution of protons due to multiple Coulomb scatter (MCS) is convenient, but in RW the use of Gaussian angular distribution requires an extremely large computation and memory. Thus, our RW model adopts spatial distribution from the angular one to accelerate the computation and to decrease the memory usage. From the physics and comparison with the MC simulations, we have determined and analytically expressed those critical variables affecting the dose accuracy in our RW model.Results:Besides those variables such as MCS, stopping power, energy spectrum after energy absorption etc., which have been extensively discussed in literature, the following variables were found to be critical in our RW model: (1) inverse squared law that can significantly reduce the computation burden and memory, (2) non‐Gaussian spatial distribution after MCS, and (3) the mean direction of scatters at each voxel. In comparison to MC results, taken as reference, for a water phantom irradiated by mono‐energetic proton beams from 75 MeV to 221.28 MeV, the gamma test pass rate was 100% for the 2%/2mm/10% criterion. For a highly heterogeneous phantom consisting of water embedded by a 10 cm cortical bone and a 10 cm lung in the Bragg peak region of the proton beam, the gamma test pass rate was greater than 98% for the 3%/3mm/10% criterion.Conclusion:We have determined key variables in our RW model for proton dose calculation. Compared with commercial pencil beam algorithms, our RW model much improves the dose accuracy in heterogeneous regions, and is about 10 times faster than MC simulations.

SU‐E‐T‐210: Comparison of Proton with Electron Boost in Breast Cancer Treatment

Purpose:Electron beams are commonly used for boost radiation following whole breast irradiation (WBI) to improve the in‐breast local control. Proton beams have a finite range and a sharper distal dose falloff compared to electron beams, thus potentially sparing more heart and lung in breast treatment. The purpose of the study is to compare protons with electrons for boost breast treatment in terms of target coverage and normal tissue sparing.Methods:Six breast cancer patients were included in this study. All women received WBI to 45–50 Gy, followed by a 10–16.2 Gy boost with standard fractionation. If proton beams were used for the boost treatment, an electron plan was retrospectively generated for comparison using the same CT set and structures, and vice versa if electron beams were used for treatment. Proton plans were generated using the treatment planning system (TPS) with two to three uniform scanning proton beams. Electron plans were generated using the Pinnacle TPS with one single en face beam. Dose‐volume histograms (DVH) were calculated and compared between proton and electron boost plans.Results:Proton plans show a similar boost target coverage, similar skin dose, and much better heart and lung sparing. For an example patient, V95% for PTV was 99.98% and skin (5 mm shell) received a max dose close to the prescription dose for both protons and electrons; however, V2 and V5 for the ipsilateral lung and heart were 37.5%, 17.9% and 19.9%, 4.9% respectively for electrons, but were essentially 0 for protons.Conclusions:This dosimetric comparison demonstrates that while both proton therapy and electron therapy provided similar coverage and skin dose, proton therapy could largely reduce the dose to lung and heart, thus leading to potential less side effects.