WO1998053307A2 - Use of single scatter electron monte carlo transport for medical radiation sciences - Google Patents
Use of single scatter electron monte carlo transport for medical radiation sciences Download PDFInfo
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- WO1998053307A2 WO1998053307A2 PCT/US1998/010589 US9810589W WO9853307A2 WO 1998053307 A2 WO1998053307 A2 WO 1998053307A2 US 9810589 W US9810589 W US 9810589W WO 9853307 A2 WO9853307 A2 WO 9853307A2
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
- G01N23/22—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by measuring secondary emission from the material
Definitions
- the present invention relates to radiation therapy, and more specifically it relates to a method for modeling the precise microscopic interactions of electrons with matter to enhance physical understanding of radiation sciences. Description of Related Art
- the radiation source may be may be in the form of external beams of ionizing particles or radioactive sources internal to the patient. External beams are usually produced by machines acting as particle accelerators.
- the beam delivery system consists of the radiation source, which is mounted on a gantry which can rotate about a 360° arc around the patient. Each beam is shaped by a rotatable collimator. The patient lies on a rotatable table. The gantry and table both rotate about a single isocenter.
- External beam radiation therapy is performed with several types of ionizing radiation. Approximately 80% of patients are treated with photons, ranging in maximum energy from 250 keN to 25 MeV. The balance are treated primarily with electrons with energies from 4 to 25 MeV. In addition, there are several fast neutron and proton therapy facilities which have treated thousands of patients worldwide. Fast neutron therapy is performed with neutron energies up to 70
- Radioactive neutron capture therapy is conducted with thermal and epithermal neutron sources. Most internal radioactive sources irradiate the patient with photons, although some sources emit low energy electrons. The effects of ionizing radiation on the body are quantified as radiation dose. Absorbed radiation dose is defined as the ratio of energy deposited to unit mass of tissue. Because tumors and sensitive structures are often located in close proximity, accuracy in the calculation of dose distributions is critically important. The goal of radiation therapy is to deliver a lethal dose to the tumor while maintaining an acceptable dose level in surrounding sensitive structures. This goal is achieved by computer-aided planning of the radiation treatments to be delivered.
- the treatment planning process consists of characterizing the individual patient's anatomy (most often, this is done using a computed tomography (CT) scan), determining the shape, intensity, and positioning of radiation sources (the subject of the present invention), and calculating the distribution of absorbed radiation dose in the patient. Most current methods used to calculate dose in the body are based on dose measurements made in a water box.
- CT computed tomography
- Monte Carlo transport is the most accurate method of determining dose distributions in heterogeneous media.
- a computer is used to simulate the passage of particles through an object of interest.
- PEREGRINE is an all-particle, first-principles 3D Monte Carlo dose calculation system designed to serve as a dose calculation engine for clinical radiation therapy treatment planning (RTP) systems.
- RTP radiation therapy treatment planning
- PEREGRINE performs high-resolution, high accuracy, Monte Carlo RTP calculations in times that are reasonable for clinical use. Because of its speed and simple interface with conventional treatment planning systems, PEREGRINE brings Monte Carlo radiation transport calculations to the clinical RTP desktop environment.
- PEREGRINE is designed to calculate dose distributions for photon, electron, fast neutron and proton therapy.
- the PEREGRINE Monte Carlo dose calculation process depends on four key elements: complete material composition description of the patient as a transport mesh, accurate characterization of the radiation source , first-principles particle transport algorithms (the subject of the present invention), and reliable, self-consistent particle-interaction databases (also an element of the present invention).
- PEREGRINE uses these elements to provide efficient, accurate Monte Carlo transport calculation for radiation therapy planning.
- the patient transport mesh is a Cartesian map of material composition and density determined from the patient's CT scan.
- CT scan pixel is used to identify the atomic composition and density of a corresponding transport mesh voxel.
- Atomic composition is determined from CT threshold values set by the user or by default values based on user-specified CT numbers for air and water. The user also assigns materials and densities to the interior of contoured structures. If the user specifies a structure as the outer contour of the patient, PEREGRINE constructs a transport mesh that is limited to the maximum extent of that structure, and sets all voxels outside that structure to be air. This provides a simple method of subtracting the CT table from the calculation.
- the default resolution of the transport mesh is 1 x 1 x 3 mm, for small-volume areas such as the head and neck, or 2 x 2 x 10 mm, for large-volume treatment sites such as the chest and pelvis.
- the resolution can also be reduced from the CT scan resolution.
- material composition and density are determined as the average of all CT pixels that fall within the transport mesh voxel.
- the PEREGRINE source model designed to provide a compact, accurate representation of the radiation source, divides the beam-delivery system into two parts: an accelerator-specific upper portion and a treatment-specific lower part.
- the accelerator-specific upper portion consisting of the electron target, flattening filter, primary collimator and monitor chamber is precharacterized based on the machine vendor's model-specific information.
- These precharacterized sources are derived from Monte Carlo simulations from off-line Monte Carlo simulations using BEAM and MCNP4A, as described in copending U.S. Patent Application Serial No. 08/610,971, which is fully incorporated herein by reference.
- Particle histories from off-line simulations are cast into multidimensional probability distributions, which are sampled during the PEREGRINE calculation.
- the photon beam is divided into three subsources: primary, scattered, and contaminant. Separating the source into subsources facilitates investigation of the contributions of each individual component.
- the installation procedures will consist of a limited number of beam description parameter adjustments, based on simple beam characterization measurements.
- the lower portion of the radiation source consists of treatment-specific beam modifiers such as collimators, apertures, blocks, and wedges. This portion is modeled explicitly during each PEREGRINE calculation. Particles are transported through this portion of the source using a pared-down transport scheme. Photons intersecting the collimator jaws are absorbed. Photons intersecting the block or wedge are tracked through the material using the same physical database and methods described below for patient transport. However, all electrons set in motion by photon interactions in the block or wedge are immediately absorbed.
- PEREGRINE tracks all photons, electrons, positrons and their daughter products through the transport mesh until they reach a specified minimum tracking energy or leave the patient transport mesh. Developing good statistics requires tracking millions of representative particles (histories) through the patient transport mesh.
- PEREGRINE records energy deposited at each interaction, which builds up a map of energy deposited in the transport mesh.
- a dose map is created by dividing the total energy deposited in each voxel by its material mass.
- PEREGRINE transports photons through the body using the standard analog method. Woodcock or delta-scattering is used to efficiently track particles through the transport mesh.
- PEREGRINE transports electrons and positrons using a class II condensed-history scheme. This procedure groups soft collisions with small energy losses or deflections, but simulates directly those major or catastrophic events in which the energy or deflection angle is changed by more than a preset threshold. Delta-ray and bremsstrahlung production are modeled discretely for energy transfers >200 keV. PEREGRINE uses Moliere's theory of multiple elastic electron/positron scattering . Pathlength corrections described are used to account for the effect of multiple scattering on the actual distance traveled by the electron or positron. A minimum electron/positron transport energy is assigned to each transport voxel based on range rejection.
- the range-rejection minimum energy corresponds to the minimum electrort/positron range required to traverse 20% of the minimum zone dimension, with range determined as the minimum range calculated for that zone plus all directly adjacent zones.
- Two 511 keV photons are created at the end of each positron range. The direction of the first photon is chosen randomly, while the second is set to 180° opposed to the first.
- CREEP uses the LLNL Evaluated Electron Data Library, which is described in further detail below.
- PEREGRINE accounts for photon interactions via the photoelectric effect, incoherent /coherent photon scattering, and pair production. All photon cross sections used by PEREGRINE are derived from the Lawrence Livernore National Laboratory Evaluated Photon Data Library (EPDL). EPDL data are taken from a variety of sources that have been selected for accuracy and consistency over a wide range of photon energies ( 10-eV-lOO- MeV) for all elements.
- PEREGRINE At low incident photon energies ( ⁇ 0.1 MeV for tissue components, ⁇ 1 MeV for high-Z materials such as lead and tungsten), the photoelectric effect is the dominant absorption mechanism.
- the cross sections contained in PEREGRINE were obtained by direct evaluation of the relativistic S-matrix in a screened central potential. These cross sections accurately describe ionization from electrons bound in isolated atoms and provide predictions at the percent level for compounds where the K and L shells are well-represented by atomic orbitals. For most elements, at energies typical of those encountered for clinical photon beams, Compton scattering is the most important process in the photon-atom interaction.
- the Compton scattering cross sections used in PEREGRINE are obtained in the incoherent scattering factor approximation.
- This approximation includes screening effects.
- Relativistic effects enter through use of the Klein-Nishina cross section. Coherent scattering does not contribute significantly to the total photon-atom interaction cross section for most radiation therapy applications. However, these cross sections are still modeled, and were obtained under similar assumptions to those for incoherent scattering. At very high incident photon energies (> 30 MeV for tissue components, > 5 MeV for high-Z materials such as lead and tungsten), the dominant photon interaction mechanism is pair production.
- the cross sections for pair and triplet production used by PEREGRINE include Coulomb and screening effects and radiative corrections.
- PEREGRINE accounts for the effects of large-angle elastic scattering (delta-ray production) and bremsstrahlung production on an event-by-event basis. All other energy-loss mechanisms are accounted for through continuous-slowing-down-approximation (CSDA) energy loss.
- CSDA continuous-slowing-down-approximation
- Moller (Bhabha) scattering is the ionization of an atom by an electron (positron). Moller and Bhabha cross sections and sampling methods follow those given by Messel and Crawford.
- the threshold for these processes in PEREGRINE is set so that the ejected electron kinetic energy is >200 keV.
- Bremsstrahlung cross sections contained in PEREGRINE are derived from the LLNL Evaluated Electron Data Library (EEDL). These cross sections were determined by interpolating between the relativistic S-matrix data available up to 2 MeV, and the Bethe-Heitler result, expected to be valid above 50 MeV. Bremsstrahlung cross sections are processed to reflect a bremsstrahlung photon energy cutoff of 200 keV.
- PEREGRINE uses restricted collision and radiative stopping powers, which exclude energy lost due to Moller /Bhabha events with energy transfers > 200 keV and bremsstrahhrng events with energy transfers > 200 keV. Restricted collision stopping powers are calculated.
- Restricted radiative stopping powers are calculated by subtracting the total energy transferred to the bremsstrahlung photon per distance, as determined from the bremsstrahlung cross section data.
- the accuracy of PEREGRINE transport calculations has been demonstrated by benchmarking PEREGRINE against a wide range of measurements and well-established Monte Carlo codes such as EGS4 and MCNP.
- the accuracy of the CREEP-based electron transport package has been demonstrated by comparing PEREGRINE with and without this feature enabled, as well as comparing CREEP results to calorimetric experiments directly, independent of the PEREGRINE code.
- the impact ionization cross sections are based on the Moller formalism with other corrections to accurately model small energy loss collisions.
- Energy loss spectra are available at a number of incident energies for individual ionization and bremsstrahlung events, as well as the spectral average energy loss.
- the bremsstrahlung cross sections were determined by Seltzer and Berger by interpolating between the relativistic data from the code of Tseng and Pratt available up to 2 MeV, and the results of Bethe-Heitler, expected to be valid above 50 MeV.
- the excitation database contains cross sections and the average energy loss to excitation as a function of incident energy. There are no spectral data for excitation energy loss in EEDL at this time. A summary of the database features that were used at some point in CREEP can be seen in Table 1.
- the CREEP code is written in FORTRAN and C, in a very simple style with the intent of being extremely portable. Since this code is intended primarily as a means to explore basic physical properties of the medium, the present incarnation assumes only simple geometries: either spherical (user specifies radius) or slab (user specifies x, y, z), consisting of one type of material. Several slabs may be pieced together to simulate a layered geometry, since the output of one slab may be used as spectral input into a distal slab, and the backscattered energy spectrum from each interface can be transported backwards in the prior medium.
- Figures 1A and IB show two examples of backscatter information generated by CREEP compared to experimental values.
- Figures 2A and 2B strikingly illustrate both the strengths and the limitations of the present version of CREEP.
- Figures 3A-G show comparisons of the CREEP single scatter Monte Carlo (SSMC) code with experiment.
- Table 1 shows some relevant contents of the EEDL database for an element.
- Table 2 shows how the run time scales with different media.
- DETAILED DESCRIPTION OF THE INVENTION CREEP is able to obtain its accuracy by simulating electron events in an "analog” or “single scatter” fashion.
- CREEP is actually a family of four codes, having a similar ancestor code, but they have evolved separately to fill specific niches.
- SlabcreepI was written for the purpose of benchmarking with slab and foil experiments.
- SlabcreepII does the same but for media that are not comprised of a single element; it handles compounds and mixtures and was primarily designed as a means to compare the CREEP method with other codes and experiments for generating depth-dose curves in water, which is the most important medium for radiotherapy applications.
- the ultimate application for CREEP was to generate a library for the Macro Response Method (MRMC), for which probability distribution functions arising from transport through a sphere were required.
- MRMC Macro Response Method
- the slab-geometry code and the spherical-geometry kugel code there are two types of input files.
- the first is a very simple user-generated file explaining the Monte Carlo tracking parameters, the medium, and the output information desired.
- the second type of information files required is the EEDL datafiles for each element in the medium.
- the CREEP code deviates from the ideal single scatter algorithm in that (for most applications) it does not simulate every excitation event individually. Instead, it subtracts off the expected excitation loss after each of the other events, as described in the excitation section below.
- knock-on electron For incident electrons, the interaction is often pictured as a "black box” in the vicinity of an atomic electron, where two electrons exit. Because electrons are indistinguishable from each other, it is simply assumed that the electron with the higher exit energy was the primary electron, making the remaining electron the "knock-on”. With this definition, the maximum energy a knock-on electron can have is given by
- the knock-on electron energy is sampled from a spectrum.
- the ⁇ DL database has a number of spectra tallied for various incident energies; statistical interpolation is used to choose between them.
- 2-body kinematics are used to update the primary electron's trajectory. If T 0 is the kinetic energy of the electron in electron mass units, and the ratio ⁇ is defined by
- CREEP handles secondary electrons by putting the primary on a memory "stack" and tracking the knock-on immediately, until they fall below the energy cutoff or escape the volume, at which point the primary history is continued.
- a special energy cutoff parameter is used for knock-ons, so the user can readily imitate class II condensed history codes, which only simulate ionization events if the knock-on is above a particular energy.
- the incident charged particle will experience an acceleration due to this force of magnitude
- Equation [2.7] illustrates several important concepts governing bremsstrahlung emission.
- bremsstrahlung is much more important in high atomic number materials (due to the Z 2 ) than in low atomic number materials.
- N is the number of electrons per gram
- T is the kinetic energy of the electron
- the important physics revealed by this equation is that energy loss increases directly with atomic number of the material, and the loss increases to a somewhat greater extent with the energy of the electron.
- charged particles, especially electrons have a small probability of losing almost all of their energy in a single interaction, however, this only occurs at extreme relativistic energies.
- both the photon and the scattered charged particle advance preferentially in the forward direction. For moderate energy charged particles, however, the photon carries only a very small momentum and can be emitted in any direction.
- the denominator represents the total energy of the electron before the event.
- CREEP itself does not track the bremsstrahlung photons that are created; they are tallied on the spot so that their phase space can be banked and passed off to another Monte Carlo code with photon tracking capabilities, such as PEREGRINE. Note that any additional electrons the bremsstrahlung photons would have generated are
- CREEP cannot assume any energy deposits arising from photons. If, however, CREEP is coupled with a photon MC code in a way that allows that code to pass back further secondary electrons these can be restored. It should be noted that the bremsstrahlung photon is much more penetrating than the charged particle that caused it, and therefore carries its energy far from the original charged particle track. Monte Carlo codes that neglect bremsstrahlung interactions thus fail to model this energy deposition pattern accurately.
- the primary charged particle can excite an atom even thought it does not impart enough energy to the atomic electron to free it. Instead, the energy transferred to the atom causes the orbital electron to be promoted to a higher electronic state.
- the promoted orbital electron relaxes either by producing characteristic (photon) radiation of energy hv; producing Auger electrons of energy hv - E bmdmg ; or some combination of both. Since the energies involved are typically very low compared to the energy range of interest, the individual events are often not modeled and the energy that is given to them is instead considered to be locally deposited. In fact, these events may be lumped together and assumed to cause a uniform energy loss per unit distance. This is an "excitation-only" stopping power.
- N A is Avogadro's number
- A is the atomic weight
- p is the density
- ⁇ ⁇ is the total excitation cross section (summed over all subshells)
- ⁇ a is the mean energy loss due to excitation at a given primary energy. This stopping power is multiplied by the distance between the last event and the present event to get the energy lost to excitations in transit, which is subtracted from the electron's energy before calculating the distance to the next event.
- excitation cross section is summed into the cumulative event probability function and chosen accordingly. There is no deflection angle, and, rather than sampling from a spectrum of possible energy losses, the average energy loss per event (for an electron of the current energy) is used. As can be seen in Table 1 the spectral energy loss distributions for individual excitation events is not available at this time.
- a special version of the CREEP code handles all compounds and mixtures by combining the EEDL element data using Bragg additivity. The user must enter mass fractions of each element in the compound or mixture. The density used is that for the compound as a whole.
- Figure 1A shows CREEP backscatter percentage (including backscattered secondary electrons) compared to the experiments of Darlington et al. and Neubert et al..
- Figure IB shows the backscattered energy spectrum resulting from a 10 keV electron impinging on the surface of an aluminum slab that is large in x, y, and z compared to the mean free path of the incident electron.
- FIGS 2A and 2B strikingly illustrate both the strengths and the limitations of the present version of CREEP.
- an electron of relatively low energy (not more than 500 times the binding energy) is incident on a thin slab (not more than 20 mean free paths) and the amount of energy each electron lost after having traversed the slab is tallied.
- a Landau energy loss distribution (the basis for energy loss in some condensed history codes) would predict a wide, smooth distribution, SSMC gives a highly structured, asymmetric distribution, having the same mean.
- the first distinctive feature of these curves is a zero- amplitude region in the low energy loss region, implying that no electrons escape without losing at least some energy. This region ends abruptly at the energy loss that corresponds to the excitation-only stopping power times the thickness of the slab, where a sharp peak is seen.
- the peaks are due to electrons that escape the slab without undergoing any ionization (or bremsstrahlung) events. The sharpness of these peaks is therefore an artifact resulting from not modeling individual excitation events.
- CREEP also calculates analog stopping powers (the amount of energy lost per unit distance for both radiative and collisional events), energy deposits due to individual interaction types, and "real" pathlength (cumulative distance between events) which can be used to calculate detour factors (the ratio to the real range compared to the CSDA range).
- analog stopping powers the amount of energy lost per unit distance for both radiative and collisional events
- energy deposits due to individual interaction types the amount of energy lost per unit distance for both radiative and collisional events
- detour factors the ratio to the real range compared to the CSDA range
- the version of the code which includes compounds and mixtures is also notably slower than the element versions, due to the need to find cross sections in each element for every step, and then compare them to decide in which element the interaction will take place.
- Table 2 gives some feel for how the run time scales with different media. It is clear that these times are not acceptable for clinical radiotherapy calculations. For this reason, one important application of this code is to compile the results of detailed runs in small geometry elements of homogenous materials. The results are stored in a library of probability distribution functions, which can later be used to represent the net effect of many individual interactions in a single step.
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AU86565/98A AU8656598A (en) | 1997-05-22 | 1998-05-22 | Use of single scatter electron monte carlo transport for medical radiation sciences |
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Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US6714620B2 (en) | 2000-09-22 | 2004-03-30 | Numerix, Llc | Radiation therapy treatment method |
US6735556B2 (en) | 2001-06-15 | 2004-05-11 | International Business Machines Corporation | Real-time model evaluation |
CN111814334A (en) * | 2020-07-08 | 2020-10-23 | 湘潭大学 | Method and device for analyzing damage of heavy ion latent track of semiconductor device |
Families Citing this family (1)
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US4882313A (en) * | 1987-07-31 | 1989-11-21 | Smithkline Beckman Corporation | Carboxamide derivatives of glycopeptides |
Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
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US5341292A (en) * | 1992-06-04 | 1994-08-23 | New England Medical Center Hospitals, Inc. | Monte Carlo based treatment planning for neutron capture therapy |
WO1997032630A1 (en) * | 1996-03-05 | 1997-09-12 | The Regents Of The University Of California | Calculation of radiation therapy dose using all particle monte carlo transport |
-
1998
- 1998-05-22 AU AU86565/98A patent/AU8656598A/en not_active Abandoned
- 1998-05-22 WO PCT/US1998/010589 patent/WO1998053307A2/en active Application Filing
Patent Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5341292A (en) * | 1992-06-04 | 1994-08-23 | New England Medical Center Hospitals, Inc. | Monte Carlo based treatment planning for neutron capture therapy |
WO1997032630A1 (en) * | 1996-03-05 | 1997-09-12 | The Regents Of The University Of California | Calculation of radiation therapy dose using all particle monte carlo transport |
Non-Patent Citations (2)
Title |
---|
HARTMANN ET AL.: "Peregrine: an all-particle monte carlo code for radiation therapy." PROCEEDINGS OF THE INTERNATIONAL CONFERENCE ON MATHEMATICS AND COMPUTATIONS, REACTOR PHYSICS AND ENVIRONMENTAL ANALYSES., vol. 2, 30 April 1995 - 4 May 1995, pages 857-865, XP002083804 * |
SVATOS ET AL.: "Electron transport in radiotherapy using local-to-local monte carlo." PROCEEDINGS OF THE INTERNATIONAL CONFERENCE ON MATHEMATICS AND COMPUTATIONS, REACTOR PHYSICS AND ENVIRONMENTAL ANALYSES.,30 April 1995 - 4 May 1995, pages 867-875, XP002083805 * |
Cited By (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US6714620B2 (en) | 2000-09-22 | 2004-03-30 | Numerix, Llc | Radiation therapy treatment method |
US6735556B2 (en) | 2001-06-15 | 2004-05-11 | International Business Machines Corporation | Real-time model evaluation |
CN111814334A (en) * | 2020-07-08 | 2020-10-23 | 湘潭大学 | Method and device for analyzing damage of heavy ion latent track of semiconductor device |
CN111814334B (en) * | 2020-07-08 | 2024-04-09 | 湘潭大学 | Method and apparatus for heavy ion potential track damage analysis of semiconductor device |
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WO1998053307A3 (en) | 1999-02-25 |
AU8656598A (en) | 1998-12-11 |
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