EGS4 Code Research Articles

Influence of voxel size on specific absorbed fractions and S-values in a mouse voxel phantom

Photon and electron specific absorbed fractions (SAFs) and S-values have been evaluated using mouse voxel phantoms. In voxel phantoms, it is important to choose the voxel size carefully since it affects the accuracy of results. In this study, two mouse voxel phantoms were constructed, with cubic voxels, one with 0.1-mm sides and the other with 0.4-mm sides. The sources were considered to be distributed uniformly in the main organs and the radiation transport was simulated using the Monte Carlo code EGS4. It was found that the effect of voxel size on SAFs for self-irradiation was not high (<10 %) for electrons and photons. However, it was appreciable for cross-irradiation especially for electrons. The effect of voxel size was investigated on S-values for some beta emitters such as (131)I, (153)Sm, (188)Re and (90)Y.

SU‐GG‐T‐412: PyEGS5 ‐ a Python Programming Interface for EGS5

INTRODUCTION: EGS5 is the state‐of‐the‐art implementation of the Electron Gamma Shower (EGS) Monte Carlo code system, which is written purely in Fortran language. The Fortran language is an efficient computer language for numerical calculation, however, it is much limited in functionalities like GUI, plotting. In order to make EGS5 an easy‐to‐use tool, a Python programming interface to EGS5 and a pythonic Monte Carlo simulation environment for radiation transport calculations has been developed, referred to as PyEGS5. METHOD AND MATERIALS: With binding to Python, PyEGS5 can be used in a scripting and interpreted way, while maintaining the efficiency of the Fortran binary code. As a powerful modern programming language, Python brings to EGS5 many advanced programming features and resources, with which users can create full‐featured Monte Carlo simulation applications with much less efforts than using Fortran language. RESULT: PyEGS5 is a very useful tool in medical physics research, fast‐prototyping of medical physics devices and teaching Monte Carlo simulation to medical physics students. PyEGS5 will be provided as an open source code to anybody who requires it, under the general GNU license (GNL). CONCLUSION: PyEGS5 provides the medical physics community with an easy‐to‐use programming interface to EGS5 code system which combines the advantages of the Python language and the Fortran language.

Incorporation of Landau–Pomeranchuk–Migdal effect and dielectric suppression effect in EGS5 code

Abstract We incorporated the Landau–Pomeranchuk–Migdal (LPM) effect and the dielectric suppression effect for bremsstrahlung and the LPM effect for pair production in the EGS5 code. To verify the validity of the EGS5 code with the LPM plus dielectric cross section for bremsstrahlung, we compared the bremsstrahlung spectrum calculated using this code with that obtained considering the LPM effect. The values calculated using the EGS5 code reproduced the experimental value of the LPM effect well.

Feasibility study of ultra-short gamma ray pulse generation by laser Compton scattering in an electron storage ring

We are developing an ultra-short gamma ray pulse source based on laser Compton scattering technology. Ultra-short gamma ray pulses can be generated by injecting femtosecond laser pulses into the electron beam circulating in an electron storage ring from the direction perpendicular to the orbital plane. The energy, intensity, and pulse width of the gamma rays are estimated to be 6.6 MeV, 2.4×10 6 photons s −1, and 288 fs, respectively, for the case of a 750 MeV electron storage ring with a commercially available femtosecond laser. A preliminary head-on collision experiment was conducted. The measured spectral shape agreed well with the simulation, including the detector response calculated by the EGS5 code. The results implied that the generation of gamma rays occurred by laser Compton scattering and confirmed the validity of the estimation of the gamma ray intensity in the case of 90° collisions.

Monte Carlo simulation of Tabata’s electron backscattering experiments

Abstract Electron backscattering coefficients, η , obtained from several targets in the MeV range were calculated by using electron–photon Monte Carlo transport calculation codes, i.e., EGS5 and ITS 3.0. These calculated values were compared with those obtained from the electron backscattering experiment performed by Tabata using an ionization chamber [15] . We found that Tabata’s estimation of the multiplication factor of the ionization chamber, f, had a non-negligible error. Then, we calculated the ionization chamber output, I, which is a product of η and f. The ratios of I between the experimental and the calculated values were within 1.5 and 1.3 for the EGS5 code and the ITS 3.0 code, respectively. The ratios of η between the experimental and the calculated values were within 2.4 and 1.5 for the EGS5 code and the ITS 3.0 code, respectively. The differences between the experimental and the calculated values of I and η are large for low-Z targets (Be and C). Here, the ratios obtained by using the ITS 3.0 code are closer to unity than those obtained by using the EGS5 code. The reason of this is the fact that the calculated value obtained by using the ITS 3.0 code is underestimated for low-Z targets; this underestimation can, in turn, be attributed to the use of the default value of the number of steps in the electron transport algorithm in the ITS 3.0 code.

Generation and Application of Bremsstrahlung Production Data Calculated by EGS4 Code

Bremsstrahlung radiation (hereinafter referred to as bremsstrahlung) production data are needed in the calculation of buildup factors, including the contribution of secondary photons by the photon transport codes, which do not handle electron transport. The emission of bremsstrahlung is treated as exactly as possible by the introduction of EGS4 results. The bremsstrahlung production data by pair-created electrons per pair creation reaction and Compton scattered electrons per Compton scattering are evaluated for 26 elements from hydrogen to uranium and four compounds and mixtures of water, concrete, air, and lead glass. The error estimation of bremsstrahlung contribution to buildup factors by the invariant embedding (IE) method coupled with these bremsstrahlung data is coincident with fully transported results by the EGS4 code within ˜5%. By the introduction of bremsstrahlung production data into IE methods, we can calculate buildup factors included by the contribution of those with good accuracy up to deep penetration. By the interpolation and mixture of bremsstrahlung production data for each element, we can evaluate the data of the element or mixture whose data are not evaluated by the EGS4 code.

Data Library and Four-Parameter Empirical Formula for the Response Functions for Gamma-Ray Skyshine Dose

Line-beam response function (LBRF) and conical-beam response function (CBRF) data for gamma-ray skyshine are generated using the EGS4 code in an arbitrary geometry for energies ranging from 0.1 to 10 MeV at all the emitted angles up to a distance of 2000 m from the source. The skyshine dose is calculated for the air kerma, exposure, ambient dose equivalent H*(10), and effective dose E with anterior-posterior and isotropic irradiation geometries.A response function with a four-parameter empirical formula,R(E,Φ,x) = ℜEκ(ρ/ρ0)2 exp(a + cxρ/ρ0)xb+dxρ/ρ0,can be used to approximate the LBRF and CBRF with good accuracy. The values of the four parameters a, b, c, and d are determined for a given beam energy and direction by fitting the four-parameter function such that the maximum fractional deviation of the LBRF and CBRF values is minimized for a set of discrete source-to-detector distances. The parameter set is selected to realize the interpolation of LBRF and CBRF in relation to the energy and direction by the interpolation of these parameters. Consequently, discrete LBRF and CBRF data are converted to continuous data with regard to both energy and direction.The evaluation of gamma-ray skyshine dose analyses can be accomplished easily and quickly by using the four-parameter formula.These data can be downloaded in Excel format from http://rcwww.kek.jp/rc_en.html as “Data Library of Line- and Conical-Beam Response Functions and Four-Parameter Empirical Formula in Approximating Response Functions for Gamma-Ray Skyshine Dose Analyses.”

Simulation of the Carrier Trapping Effect in a Schottky CdTe Detector by Considering a Nonuniform Electric Field

In order to simulate the low-energy tail in the response function of a Schottky CdTe detector, which is a high-resolution room temperature X- or γ-ray spectrometer, we introduced the effect of a nonuniform electric field within the detector to the response function by a Monte Carlo calculation using the EGS5 code. The simulated response functions were compared with measurements for γ-rays from 109Cd, 241Am, 133Ba, and 57Co sources. The present method successfully reproduced the low-energy tail due to the charge carrier trapping effect, while the conventional model assuming a uniform electric field underestimated the low-energy tail significantly, particularly for higher energy γ-rays.

Discrete beta dose kernel matrices for nuclides applied in targeted radionuclide therapy (TRT) calculated with MCNP5

Radiopharmaceuticals administered in targeted radionuclide therapy (TRT) rely to a great extent not only on beta-emitting nuclides but also on emitters of monoenergetic electrons. Recent advances like combined PET/CT devices, the consequential coregistration of both data, the concept of using beta couples for diagnosis and therapy, respectively, as well as the development of voxel models offer a great potential for developing TRT dose calculation systems similar to those available for external beam treatment planning. The deterministic algorithms in question for this task are based on the convolution of three-dimensional matrices, one representing the activity distribution and the other the dose point kernel. This study aims to report on three-dimensional kernel matrices for various nuclides used in TRT. The Monte Carlo code MCNP5 was used to calculate discrete dose kernels of beta particles including the contributions from their respective secondary radiation in soft tissue for the following nuclides: 32P, 33P, 67Cu, 89Sr, 90Y, 103Rh9m), 131I, 177Lu, 186Re, and 188Re. For each nuclide a kernel cube of 10 x 10 x 10 mm3 was calculated, the dimensions of a voxel being 1 mm3. Additional kernels with voxel sizes of 3 x 3 x 3 mm3 were simulated. Comparison with the S-value data regarding 32P, 89Sr, 90Y, and 131I of the MIRD committee which were calculated with the EGS4 code showed a very good agreement, the secondary particle transport of 90Y being the only exception. Documented analytical kernels on the other side show deviations very close and very far to the source. The good accordance with the only discrete dose kernels published up to date justifies the method chosen. Together with the additional six nuclides, this report provides a considerable database for three-dimensional kernel matrices with regard to beta radionuclides applied in TRT. In contrast to analytical dose point kernels, the discrete kernels elude the problem of overestimation near the source and take energy depositions into account, which occur beyond the range of the continuous-slowing-down approximation (csda range). Recalculation of the 1 x 1 x 1 mm3 kernels to other dose kernels with varying voxel dimensions, cubic or noncubic, is shown to be easily manageable and thereby provides a resolution-independent system of dose calculation.

MONTE CARLO CHARACTERIZATION OF SMALL BEAMS FROM A NEW IRISTM COLLIMATOR WITH THE CYBERKNIFE®

Purpose: To calculate total scatter factors (sc,p), profiles and Tissue Phantom Ratios (TPRs) of small beams from a new Iris Variable Aperture Collimator (IRIS) with the Cyberknife R © Robotic Radiosurgery System. Materials: A method based on Monte Carlo simulations is proposed to estimate sc,p, horizontal profiles, and TPRs of small fields obtained with the IRIS. The simulated aperture openings correspond to the fixed cone sizes of 5, 7.5, 10, 12.5, 15, 25, and 60 mm. Two diodes (PTW diode 60012 and Sun Nuclear EdgeDetector), the PTW PinPoint chamber and the PTW microLion liquid chamber are included in the investigation. Detector doses are obtained with the EGSnrc user code egs_chamber. The treatment head is simulated using BEAMnrc. Values of the full width at half maximum (FWHM) in the range 2.0 2.5 mm, as well as energies of 6.5, 7.0 and 7.5 MeV, for the electron beam incident on the bremsstrahlung target are studied. The average dose calculated in the sensitive volume of the detectors is compared with measurements. The correction factors to be applied to experimental sc,p values are provided. Results: The best estimate of the source parameters is a FWHM of 2.3 mm and energy of 7.0 MeV. For these values, the Monte Carlo calculated sc,p in water (in a voxel of 0.5x0.5x0.5 mm) are: 0.537, 0.778, 0.866, 0.911, 0.938, 0.974, and 1, respectively, for the above mentioned apertures. Preliminary results for the PTW diode 60012 show that corrected measured sc,p agree with the Monte Carlo calculated sc,p in water. The simulated profiles with the PTW diode displaced in different horizontal positions in water are coincident with the experimental profiles. Conclusions: The Monte Carlo method is able to determine the sc,p in water of small beams from the IRIS collimator, to simulate the detector response, and to calculate the correction factors for each detector in a very accurate way. The correction factors for diodes and microchambers show different behaviours: diodes tend to overestimate sc,p, while the PinPoint underestimates sc,p. The correction factors for the microLion are very close to unity for almost all fields, except the smallest one (2.5% underestimation). The proposed approach allows the accurate estimation of the source parameters of the incident electron beam. The agreement between experimental and simulated profiles with diodes is a further demonstration of the efficacy of the proposed method. The results for the remaining detectors will be shown.

SU‐FF‐I‐55: Reconstruction of Mammography X‐Ray Spectrum Using Rayleigh and Compton Scattering Corrections

Purpose: An analysis of the x‐ray spectrum is important for quality assurance (QA) and quality control (QC) of a radiographic system. In the case of mammography, the direct measurement of the primary x‐ray spectra under clinical conditions is very difficult and time‐consuming. The method of obtaining the primary x‐ray spectrum by the Compton scattering correction has been established for relatively high x‐ray tube voltages. However, the influence of Rayleigh scattering can not be neglected for mammography. We have developed a new method of reconstructing the primary x‐ray spectrum from the scattered x‐ray spectrum taking into account both Rayleigh and Compton scattering. Method and Materials: The primary x‐ray beam from a 28 kV mammography x‐ray unit was incident on a PMMA (polymethyl methacrylate) sphere scatterer with a diameter of 6 mm. The 90‐degree scattered x‐ray spectrum was measured by using a CdTe semiconductor detector. The measured x‐ray spectrum was separated into three energy regions, and the characteristic x‐ray peaks and bremsstrahlung x‐rays were fitted by Gaussian and quadratic functions. The energy shift corrections of the Compton and Rayleigh components were made at each photon energy channel by splitting the number of photons at each channel into the two scattering components according to the corresponding cross sections. The Monte Carlo simulation of the 90‐degree scattered x‐ray spectra were also performed using the EGS5 code. Results: The reconstructed spectrum agreed fairly well with a directly measured primary x‐ray spectrum. In the Monte Carlo simulation, the scattered x‐ray spectrum calculated for the incidence of the reconstructed x‐ray spectrum showed a very good agreement to the measured scattered x‐ray spectrum. Conclusions: The Rayleigh and Compton scattering correction method could be suitable for measuring the mammography x‐ray spectra under clinical conditions and useful for QA and QC of the mammography x‐ray units.

Reply to: Determinants of 131I radiation dose to thyroid follicular cells

We thank Dr. Eterovic and colleagues for their interest and comments. Our work was aimed at showing the ability of the Monte Carlo code “CELLDOSE” to assess electron dose to specific cells in a complex environment [1]. This approach can be of considerable interest for comparing different radiopharmaceuticals as therapy agents in oncology. Eterovic et al. used the EGS5 code, with a predefined energy cut-off of 1 keV [2]. Our track structure code follows all primary and secondary electrons down to 7.4 eV (i.e. below the first ionisation potential of the water molecule ≅ 12.6 eV) [3]. This makes “CELLDOSE” a valuable tool to calculate energy deposited at the tissue, cellular as well as sub-cellular scale. We used a model of normal thyroid tissue as an example. The geometrical model assumed spherical thyroid follicles of 170 μm diameter with a cell height of 10 μm. Neighbouring follicles are 4 μm apart. Based on the specific geometry presented, it is found that when the average I electron dose to thyroid tissue is 1 Gy, the dose to the colloidal matter is 1.168 Gy, the dose specifically received by thyroid cells is 0.982 Gy and the dose to the inter-follicular tissue is 0.895 Gy [1]. Eterovic and colleagues suggest that the thyroid cell dose we found is overestimated because of small follicle diameter and a packing of spheres based on the simple cubic model [4]. We fully agree that thyroid follicles may vary in size according to age and pathological conditions. The follicle size we used is compatible with the average for normal thyroid [5]. In a recent study, Faggiano et al. found an average diameter for thyroid follicles of 171 μm in adults and smaller values in children <12 years [6]. Of course, average diameter as seen on a single cut is an underestimation and data from full 3-D evaluation of thyroid tissue are lacking. Both our simulation and that of Eterovic have considered thyroid follicles as spherical [1, 2]. Thyroid follicles might not be truly spherical. However, it should not be forgotten that many of the aspects seen in histology can be influenced by the fixation and water extraction procedure during sample preparation. The amount of connective and vascular inter-follicular tissue should also be variable and should probably increase in pathological thyroid states in which inflammation plays an important role. We would like to point out that our model for follicles’ arrangement (follicles of 170 μm diameter in a cube with a side of 174 μm) leads to a volume occupied by follicles of 48.8% and not 42% as stated by Eterovic and colleagues. This suggests that new calculations based on the right volumes should allow the authors to obtain results even closer to ours. Eterovic et al. propose a model where follicles would occupy 90% of the tissue space, yet composed of spherical follicles of identical size [2, 4]. However, this assumption is somewhat surprising since it is well known from crystallography data and from mathematical approaches that, whatever the arrangement, the maximum density of spheres which can be achieved is 74% or d 1⁄4 p 3 ffiffi

Differences among Monte Carlo codes in the calculations of voxel values for radionuclide targeted therapy and analysis of their impact on absorbed dose evaluations

Several updated Monte Carlo (MC) codes are available to perform calculations of voxel S values for radionuclide targeted therapy. The aim of this work is to analyze the differences in the calculations obtained by different MC codes and their impact on absorbed dose evaluations performed by voxel dosimetry. Voxel S values for monoenergetic sources (electrons and photons) and different radionuclides (90Y, 131I, and 188Re) were calculated. Simulations were performed in soft tissue. Three general-purpose MC codes were employed for simulating radiation transport: MCNP4C, EGSnrc, and GEANT4. The data published by the MIRD Committee in Pamphlet No. 17, obtained with the EGS4 MC code, were also included in the comparisons. The impact of the differences (in terms of voxel S values) among the MC codes was also studied by convolution calculations of the absorbed dose in a volume of interest. For uniform activity distribution of a given radionuclide, dose calculations were performed on spherical and elliptical volumes, varying the mass from 1 to 500 g. For simulations with monochromatic sources, differences for self-irradiation voxel S values were mostly confined within 10% for both photons and electrons, but with electron energy less than 500 keV, the voxel S values referred to the first neighbor voxels showed large differences (up to 130%, with respect to EGSnrc) among the updated MC codes. For radionuclide simulations, noticeable differences arose in voxel S values, especially in the bremsstrahlung tails, or when a high contribution from electrons with energy of less than 500 keV is involved. In particular, for 90Y the updated codes showed a remarkable divergence in the bremsstrahlung region (up to about 90% in terms of voxel S values) with respect to the EGS4 code. Further, variations were observed up to about 30%, for small source-target voxel distances, when low-energy electrons cover an important part of the emission spectrum of the radionuclide (in our case, for 131I). For 90Y and 188Re, the differences among the various codes have a negligible impact (within few percents) on convolution calculations of the absorbed dose; thus either one of the MC programs is suitable to produce voxel S values for radionuclide targeted therapy dosimetry. However, if a low-energy beta-emitting radionuclide is considered, these differences can affect also dose depositions at small source-target voxel distances, leading to more conspicuous variations (about 9% for 1311) when calculating the absorbed dose in the volume of interest.

Novel approach for the Monte Carlo calculation of free-air chamber correction factors

A self-consistent approach for the Monte Carlo calculation of free-air chamber (FAC) correction factors needed to convert the chamber reading into the quantity air-kerma at the point of measurement is introduced, and its implementation in the new EGSnrc user code egs_fac is discussed. To validate the method, comparisons between computed and measured FAC correction factors for attenuation Ax, scatter (Ascat), and electron loss (Aeloss) are performed in the medium energy range where the experimental determination is believed to be accurate. The Monte Carlo calculations utilize a full simulation of the x-ray tube with BEAMnrc and a detailed model of the parallel-plate FAC. Excellent agreement between the computed Ascat and Aeloss and the measured values for these correction factors currently used in the National Research Council (NRC) of Canada primary FAC standard is observed. Our simulations also agree with previous Monte Carlo results for Ascat and Aeloss for the 135 and 250 kVp Consultative Committee for Ionizing Radiation reference beam qualities. The computed attenuation correction agrees with the measured Aatt within the stated uncertainties, although the authors' simulations demonstrate that the evacuated-tube technique employed at NRC to measure the attenuation correction slightly overestimates Aatt in the medium energy range. The newly introduced corrections for backscatter, beam geometry, and lack of charged particle equilibrium along the beam axis are found to be negligible. On the other hand, the correction for photons leaking through the FAC aperture, currently ignored in the NRC standard, is shown to be significant.

Basic Study on the Estimation of Medical Exposure Dose Using Monte Carlo Simulation

In order to make dose estimation in FDG-PET which is a very useful technique for whole-body cancer screening, we developed a computation system by employing a Monte Calro photon transport program UCBEAM based on the EGS4 code and the voxel phantom OTOKO. The code simulates the dose in each organ on the basis of source (FDG) distribution using the Monte Carlo simulation of annihilation photons transport.By using the experimental data for the distribution of cumulative activity by H M Deloar et al which was obtained using TLD during PET, the present approach provides dose distribution generally consistent with measurement. It is promising for estimation of exposure rate in each organ during medical screening.

Efficiency improvements for ion chamber calculations in high energy photon beams

This article presents the implementation of several variance reduction techniques that dramatically improve the simulation efficiency of ion chamber dose and perturbation factor calculations. The cavity user code for the EGSnrc Monte Carlo code system is extended by photon cross-section enhancement (XCSE), an intermediate phase-space storage (IPSS) technique, and a correlated sampling (CS) scheme. XCSE increases the density of photon interaction sites inside and in the vicinity of the chamber and results-in combination with a Russian Roulette game for electrons that cannot reach the cavity volume-in an increased efficiency of up to a factor of 350 for calculating dose in a Farmer type chamber placed at 10 cm depth in a water phantom. In combination with the IPSS and CS techniques, the efficiency for the calculation of the central electrode perturbation factor Pcel can be increased by up to three orders of magnitude for a single chamber location and by nearly four orders of magnitude when considering the Pcel variation with depth or with distance from the central axis in a large field photon beam. The intermediate storage of the phase-space properties of particles entering a volume that contains many possible chamber locations leads to efficiency improvements by a factor larger than 500 when computing a profile of chamber doses in the field of a linear accelerator photon beam. All techniques are combined in a new EGSnrc user code egs_chamber. Optimum settings for the variance reduction parameters are investigated and are reported for a Farmer type ion chamber. A few example calculations illustrating the capabilities of the egs_chamber code are presented.

Effects of Positron Range and Annihilation Photon Acolinearity on Image Resolution of a Compton PET

In order to assess the effects of positron range and acolinearity on image resolution of a Compton PET scanner designed for sub-millimeter resolution in mice, Monte Carlo simulations were performed using EGS4 code. The PET device consists of a BGO ring (17.6 cm dia., 16 cm length, and 2 cm thickness segmented into 3 mm times 3 mm times20 mm crystals) surrounding a concentric position-sensitive silicon ring (4 cm dia. times4 cm long times1.6 cm thick segmented into 16 layers of 0.3 mm times0.3 mm times1 mm elements) for the scatter detector. For each detected event, interaction position was obtained from the silicon pad in which a single Compton scatter occurs and the BGO crystal which has maximum energy deposited. Acolinearity was considered in sampling the direction vector of annihilation photon pairs. The cusp-like distribution of positron range in water was added in sinogram. An intrinsic detector resolution of 230 mum FWHM is obtained. By considering positron range and acolinearity, the overall image resolutions of 18F, 11C, 13N, and 15O point sources in water are 360, 440, 490, and 550 mum FWHM, respectively. Image quality was evaluated with 2-D images of cylindrical source for each positron emitter in a water cylinder reconstructed with filtered back projection (FBP). Monte Carlo simulation indicates the blurring effect from positron range of various isotopes dominates the image resolution of the Compton PET instrument. A negligibly small effect on resolution was introduced from acolinearity due to the small diameter of the silicon detector.

SU‐FF‐T‐438: Using Diode Dosimeters to Characterize Dose in the Buildup Region of High‐Energy Photon Beams

Purpose: For external photon beams, the depth dependence of the stopping power ratio between air and water in the buildup region is difficult for accurate quantitative characterization. Qualitatively the depth dependence increases as the photon energy increases above the cobalt‐60 energy. As a result, the depth dependence is often ignored and the “uncorrected” percentage depth dose curves are being used in many clinics. However, accurate knowledge on dose in the buildup region is important for applications such as the tangential beam setup. This work is to look for a dosimeter that measures with ease the correct photon beam PDD curves for all depth in phantom including the buildup region and beyond. Method and Materials: The PDDs of a 10 MV photon beam at field size of 10×10 cm2 and 100 cm SSD was scanned in a water‐tank with a cylindrical (Scanditronix, RK) ion chamber, a parallel‐plane (NACP) ion chamber, and a diode dosimeter (Scanditronix, designed for photon field with back‐scatter absorber). The raw scan readings were corrected only for the effective point shifts as defined for depth beyond dmax. The Monte Carlo code EGS was used to generate the “true” PDD curve. Results: The ion chambers, diode and EGS show good agreement beyond the dmax region. In the buildup region, the diode matches the EGS curve within 3% while the two ion chamber curves differ from the EGS curve by 5% to 20%. Conclusion: A scanning diode dosimeter can be designed to generate true PDD curves, with a simple effective point correction, for both the buildup region and beyond for high energy photon beams. The ease of use afforded by such diode dosimeters would mean more availability of true PDD curves at clinic, including the dose buildup region.

WE‐C‐AUD‐03: Investigation of Fast Monte Carlo Dose Calculation for CyberKnife SRS/SRT Treatment Planning

Purpose: Advanced stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) treatments require accurate dose calculation for treatment planning especially for treatment sites involving heterogeneous patient anatomy. In this work, we have implemented a fast Monte Carlo dose calculation algorithm for SRS/SRT treatment planning with the CyberKnife® system. Methods and Materials: Our system employs a superposition Monte Carlo algorithm. Photon mean free paths and interaction types for different materials and energies as well as the tracks of secondary electrons are pre‐simulated using the EGS4 code system. Photon interaction forcing and splitting are applied to the source photons in a patient calculation and the pre‐simulated tracks are repeated with proper corrections based on the tissue density and electron stopping powers. Electron energy is deposited along the tracks and accumulated in every voxel of the simulation geometry. Scattered and bremsstrahlung photons are transported, after applying the Russian Roulette technique, in the same way as the primary photons. Dose calculations are compared with full Monte Carlo simulations and the CyberKnife treatment planning system (TPS) for lung and head &amp; neck treatments. Results: Comparisons with full Monte Carlo simulations show excellent agreement (within 0.5%). Significant differences in the target dose are found between Monte Carlo simulations and the CyberKnife TPS for SRS lung treatment. The calculation time using our superposition Monte Carlo algorithm is reduced up to 62 times (46 times on average for 10 typical clinical cases) compared to full Monte Carlo simulations. Conclusions: SRS/SRT dose distributions calculated by simple dose algorithms may be significantly overestimated for small lung target volumes, which can be improved by accurate Monte Carlo dose calculations. Properly implemented fast Monte Carlo algorithms can improve dosimetric accuracy with little or no compromise to computational efficiency.

Monte Carlo assessment of peak-to-valley dose ratio for MRT

Small angle X-ray scattering experiments were conducted at the ID02 beamline of the ESRF to collect the form factors (FFs) of several biological tissues, among which were dry and fresh (wet) bone, as well as brain tissue. From the experimental data, the elastic diffusion cross-sections could be worked out and used for upgrading the cross-section libraries of the EGS4 code. A more accurate description of the radiation transport across biological samples is anticipated from the inclusion of the new values. The effect on the radiological dose deposited by arrays of planar microbeams to be used for therapeutic purposes has been evaluated. This technique is known as microbeam radiation therapy (MRT). In particular, we have focused on the dose profile which features valleys (between two consecutive peaks generated by the incoming microbeams) and peaks. The peak-to-valley dose ratio values obtained by using experimental data and via the independent atoms model (IAM) have been compared and critically discussed.