Phase-space Source Research Articles

SU‐FF‐T‐437: Improving the Efficiency of Monte Carlo by Using Systematic Sampling in Phase Space Reconstruction: Feasibility Study

Purpose: Generalized source model is effective in characterizing the radiation source for the implementation of Monte Carlo dose calculation. The purpose of this research is to investigate a systematic sampling method in source phase space reconstruction to reduce uncertainty of the MC dose calculation. Method and Materials: As a computer random sampling method, MC introduces statistical uncertainties in each random sampling step during simulation. Uncertainty of final result can be reduced by reducing the sampling uncertainty in any of the step. In our study, a systematic sampling method was used to reduce the uncertainty in source phase space reconstruction. The basic idea of the systematic sampling is to divide the sampling range into small intervals and sample from each interval. For a distribution f(x), a⩽x⩽b. instead of sampling x between a and b, we divide the space between a and b into small intervals: d1⩽d2⩽d3⩽……⩽dn, where d1=a, dn=b, and sample x from each interval with a new distribution fi(x). Results: Particle energy and location sampling in phase space reconstruction have been tested using the systematic sampling. For particle location sampling from a 10×10 cm2 field, the field was divided into 100 small units and N/100 particles were uniformly sampled from each unit. The sampling uncertainty was reduced to 1/10 compare to the direct sampling and a more uniform distribution was found in a 2D distribution map. For particle energy sampling, energy distribution was divided to N bins from the minimum energy to the maximum. Number of particles sampled from each bin depends on the probability density. There is no fluctuation in dose contribution if particle energy were sampled using the systematic method. Conclusion: Phase space reconstruction from the source model using the systematic sampling can significantly reduce the sampling uncertainty and therefore can reduce the uncertainty in MC dose calculation.

An efficient framework for photon Monte Carlo treatment planning**This work was presented in part at the First European Workshop on Monte Carlo Treatment Planning (EWG-MCTP) held in Gent, Belgium from 22 to 25 October 2006.

Currently photon Monte Carlo treatment planning (MCTP) for a patient stored in the patient database of a treatment planning system (TPS) can usually only be performed using a cumbersome multi-step procedure where many user interactions are needed. This means automation is needed for usage in clinical routine. In addition, because of the long computing time in MCTP, optimization of the MC calculations is essential. For these purposes a new graphical user interface (GUI)-based photon MC environment has been developed resulting in a very flexible framework. By this means appropriate MC transport methods are assigned to different geometric regions by still benefiting from the features included in the TPS. In order to provide a flexible MC environment, the MC particle transport has been divided into different parts: the source, beam modifiers and the patient. The source part includes the phase-space source, source models and full MC transport through the treatment head. The beam modifier part consists of one module for each beam modifier. To simulate the radiation transport through each individual beam modifier, one out of three full MC transport codes can be selected independently. Additionally, for each beam modifier a simple or an exact geometry can be chosen. Thereby, different complexity levels of radiation transport are applied during the simulation. For the patient dose calculation, two different MC codes are available. A special plug-in in Eclipse providing all necessary information by means of Dicom streams was used to start the developed MC GUI. The implementation of this framework separates the MC transport from the geometry and the modules pass the particles in memory; hence, no files are used as the interface. The implementation is realized for 6 and 15 MV beams of a Varian Clinac 2300 C/D. Several applications demonstrate the usefulness of the framework. Apart from applications dealing with the beam modifiers, two patient cases are shown. Thereby, comparisons are performed between MC calculated dose distributions and those calculated by a pencil beam or the AAA algorithm. Interfacing this flexible and efficient MC environment with Eclipse allows a widespread use for all kinds of investigations from timing and benchmarking studies to clinical patient studies. Additionally, it is possible to add modules keeping the system highly flexible and efficient.

A virtual photon source model of an Elekta linear accelerator with integrated mini MLC for Monte Carlo based IMRT dose calculation

A dedicated, efficient Monte Carlo (MC) accelerator head model for intensity modulated stereotactic radiosurgery treatment planning is needed to afford a highly accurate simulation of tiny IMRT fields. A virtual source model (VSM) of a mini multi-leaf collimator (MLC) (the Elekta Beam Modulator (EBM)) is presented, allowing efficient generation of particles even for small fields. The VSM of the EBM is based on a previously published virtual photon energy fluence model (VEF) (Fippel et al 2003 Med. Phys. 30 301) commissioned with large field measurements in air and in water. The original commissioning procedure of the VEF, based on large field measurements only, leads to inaccuracies for small fields. In order to improve the VSM, it was necessary to change the VEF model by developing (1) a method to determine the primary photon source diameter, relevant for output factor calculations, (2) a model of the influence of the flattening filter on the secondary photon spectrum and (3) a more realistic primary photon spectrum. The VSM model is used to generate the source phase space data above the mini-MLC. Later the particles are transmitted through the mini-MLC by a passive filter function which significantly speeds up the time of generation of the phase space data after the mini-MLC, used for calculation of the dose distribution in the patient. The improved VSM model was commissioned for 6 and 15 MV beams. The results of MC simulation are in very good agreement with measurements. Less than 2% of local difference between the MC simulation and the diamond detector measurement of the output factors in water was achieved. The X, Y and Z profiles measured in water with an ion chamber (V = 0.125 cm3) and a diamond detector were used to validate the models. An overall agreement of 2%/2 mm for high dose regions and 3%/2 mm in low dose regions between measurement and MC simulation for field sizes from 0.8 × 0.8 cm2 to 16 × 21 cm2 was achieved. An IMRT plan film verification was performed for two cases: 6 MV head&neck and 15 MV prostate. The simulation is in agreement with film measurements within 2%/2 mm in the high dose regions (⩾0.1 Gy = 5% Dmax) and 5%/2 mm in low dose regions (<0.1 Gy).

SU‐FF‐T‐246: Implementation of Helical CT Source in DOSXYZnrc/EGSnrc Monte Carlo Code

Purpose: To implement and verify a new source type — phase‐space source in helical mode in DOSXYZnrc/EGSnrc Monte Carlo (MC) code. Method and Materials: To implement a new source type — “phase space source in helical mode” in DOSXYZnrc/EGSnrc, we modified the mortran and macro files related with DOSXYZnrc user code; dosxyznrc.mortan, srcxyznrc.mortran and srcxyznrc.macro. This source was implemented based on the source type 8 in DOSXYZnrc (isource=8: phase‐space source from multiple directions). Different with other researcher's approach (i.e. random sampling of possible particle locations with coordinate transformation in a spiral band) we introduced a step method which calculates the isocenter displacement in each step with a new parameter of table travel distance into srcxyznrc.mortran file. Conceptually this is easier to be comprehended and implementation is simpler. Accompanying with modifying the DOSXYZnrc source code, we also changed the GUI tcl/tk files of DOSXYZnrc to make users easy to build input files for MC simulations. To validate this new source geometrically, we established the Monte Carlo model of GE lightspeed RT using BEAMnrc/EGSnrc and simulated the helical beams to PMMA cylinder phantom using DOSXYZnrc. We have tested our source model with various pitch sizes and we have used DOSXYZ_SHOW to check whether the helical irradiation field is correct or not. Results: By performing the distance measurements of isocenters in an axial irradiation field using DOSXYZ_SHOW, we found that the source model worked well without any geometrical errors. Conclusion: In this study, we have implemented a new source model — phase‐space source in helical mode in DOSXYZnrc/EGSnrc Monte Carlo (MC) code and verified the model using DOSXYZ_SHOW. In future, we will perform the dosimetric evaluations of this source with physical measurements.

Efficient photon treatment planning by the use of Swiss Monte Carlo Plan

Currently photon Monte Carlo treatment planning (MCTP) for a patient stored in the patient database of a treatment planning system (TPS) usually can only be performed using a cumbersome multi-step procedure where many user interactions are needed. Automation is needed for usage in clinical routine. In addition, because of the long computing time in MCTP, optimization of the MC calculations is essential. For these purposes a new GUI-based photon MC environment has been developed resulting in a very flexible framework, namely the Swiss Monte Carlo Plan (SMCP). Appropriate MC transport methods are assigned to different geometric regions by still benefiting from the features included in the TPS. In order to provide a flexible MC environment the MC particle transport has been divided into different parts: source, beam modifiers, and patient. The source part includes: Phase space-source, source models, and full MC transport through the treatment head. The beam modifier part consists of one module for each beam modifier. To simulate the radiation transport through each individual beam modifier, one out of three full MC transport codes can be selected independently. Additionally, for each beam modifier a simple or an exact geometry can be chosen. Thereby, different complexity levels of radiation transport are applied during the simulation. For the patient dose calculation two different MC codes are available. A special plug-in in Eclipse providing all necessary information by means of Dicom streams was used to start the developed MC GUI. The implementation of this framework separates the MC transport from the geometry and the modules pass the particles in memory, hence no files are used as interface. The implementation is realized for 6 and 15 MV beams of a Varian Clinac 2300 C/D. Several applications demonstrate the usefulness of the framework. Apart from applications dealing with the beam modifiers, three patient cases are shown. Thereby, comparisons between MC calculated dose distributions and those calculated by a pencil beam or the AAA algorithm. Interfacing this flexible and efficient MC environment with Eclipse allows a widespread use for all kinds of investigations from timing and benchmarking studies to clinical patient studies. Additionally, it is possible to add modules keeping the system highly flexible and efficient.

Efficient photon beam dose calculations using DOSXYZnrc with BEAMnrc

This study examines the efficiencies of doses calculated using DOSXYZnrc for 18 MV (10 X 10 cm2 field size) and 6 MV (10 X 10 cm2 and 20 X 20 cm2 field sizes) photon beams simulated using BEAMnrc. Both phase-space sources and full BEAMnrc simulation sources are used in the DOSXYZnrc calculations. BEAMnrc simulation sources consist of a BEAMnrc accelerator simulation compiled as a shared library and run by the user code (DOSXYZnrc in this case) to generate source particles. Their main advantage is in eliminating the need to store intermediate phase-space files. In addition, the efficiency improvements due to photon splitting and particle recycling in the DOSXYZnrc simulation are examined. It is found that photon splitting increases dose calculation efficiency by a factor of up to 6.5, depending on beam energy, field size, voxel size, and the type of secondary collimation used in the BEAMnrc simulation (multileaf collimator vs photon jaws). The optimum efficiency with photon splitting is approximately 55% higher than that with particle recycling, indicating that, while most of the gain is due to time saved by reusing source particle data, there is significant gain due to the uniform distribution of interaction sites and faster DOSXYZnrc simulation time when photon splitting is employed. Use of optimized directional bremsstrahlung splitting in the BEAMnrc simulation sources increases the efficiency of photon beam simulations sufficiently that the peak efficiencies (i.e., with optimum setting of the photon splitting number) of DOSXYZnrc simulations using these sources are only 3-13% lower than those with phase-space file sources. This points towards eliminating the need for storing intermediate phase-space files.

Using a photon phase-space source for convolution/superposition dose calculations in radiation therapy

For a given linac design, the dosimetric characteristics of a photon beam are determined uniquely by the energy and radial distributions of the electron beam striking the x-ray target. However, in the usual commissioning of a beam from measured data, a large number of variables can be independently tuned, making it difficult to derive a unique and self-consistent beam model. For example, the measured dosimetric penumbra in water may be attributed in various proportions to the lateral secondary electron range, the focal spot size and the transmission through the tips of a non-divergent collimator; the head-scatter component in the tails of the transverse profiles may not be easy to resolve from phantom scatter and head leakage; and the head-scatter tails corresponding to a certain extra-focal source model may not agree self-consistently with in-air output factors measured on the central axis. To reduce the number of adjustable variables in beam modelling, we replace the focal and extra-focal sources with a single phase-space plane scored just above the highest adjustable collimator in a EGS/BEAM simulation of the linac. The phase-space plane is then used as photon source in a stochastic convolution/superposition dose engine. A photon sampled from the uncollimated phase-space plane is first propagated through an arbitrary collimator arrangement and then interacted in the simulation phantom. Energy deposition kernel rays are then randomly issued from the interaction points and dose is deposited along these rays. The electrons in the phase-space file are used to account for electron contamination. 6 MV and 18 MV photon beams from an Elekta SL linac are used as representative examples. Except for small corrections for monitor backscatter and collimator forward scatter for large field sizes (<0.5% with <20 × 20 cm2 field size), we found that the use of a single phase-space photon source provides accurate and self-consistent results for both relative and absolute dose calculations.

MO-E-T-618-01: An Integrated CT-Based Monte Carlo Dose-Evaluation System for Brachytherapy and Its Application to Permanent Prostate Implant Postprocedure Dosimetric Analysis

Purpose: To develop a novel integrated CT-based Monte Carlo (MC) dose-evaluation system and use it to investigate the effect of tissue heterogeneities and interseed attenuation on brachytherapy dose distributions. Method and Materials: A MC photon-transport (MCPT) brachytherapy dose-calculation engine, PTRAN_CT, which uses x-ray CT to estimate tissue cross-sections, is developed based on PTRAN_CCG version 7.44. PTRAN_CT uses fast ray tracing that combines patient anatomy modeling via 3D voxel arrays with modeling of sources and applicators via a general combinatorial geometry code. A generalized phase-space source is used for further increasing efficiency. PTRAN_CT uses cross-section data derived from a single-energy CT image. PTRAN_CT was incorporated into an integrated system for performing postimplant dosimetric analysis of permanent prostate implants. A clinical implant consisting of 78 Model-6711 I-125 seeds was investigated to quantify interseed and tissue heterogeneity effects. Three scenarios were simulated a) comprehensive MC simulation including tissue heterogeneities and complete seed geometry b) complete seed geometry in homogeneous water and c) homogeneous water using 2D TG-43 source superposition. Results: Compared to case b), tissue heterogeneities (case a)) reduce doses by 5.8% on average in the target volume, after excluding artifacts from CT data. Interseed effects (case b) vs c)), reduce doses by 3% on average. The seed streaking artifact in CT data can affect MC result as large as 6%. The efficiency of PTRAN_CT relative to the predecessor code PTRAN_CCG is increased by factors of two and five, respectively, with and without enabling the phase-space option. In case b), PTRAN_CT yields the same dose distribution as benchmarked PTRAN_744. Conclusion: A novel post-implant dose evaluation system, based upon an accelerated CT-based Monte Carlo brachytherapy dose-calculation engine, has been developed and its accuracy and efficiency demonstrated. Preliminary results suggest that tissue heterogeneity effects can be incorporated into the clinical planning process.

A Monte Carlo multiple source model applied to radiosurgery narrow photon beams.

Monte Carlo (MC) methods are nowadays often used in the field of radiotherapy. Through successive steps, radiation fields are simulated, producing source Phase Space Data (PSD) that enable a dose calculation with good accuracy. Narrow photon beams used in radiosurgery can also be simulated by MC codes. However, the poor efficiency in simulating these narrow photon beams produces PSD whose quality prevents calculating dose with the required accuracy. To overcome this difficulty, a multiple source model was developed that enhances the quality of the reconstructed PSD, reducing also the time and storage capacities. This multiple source model was based on the full MC simulation, performed with the MC code MCNP4C, of the Siemens Mevatron KD2 (6 MV mode) linear accelerator head and additional collimators. The full simulation allowed the characterization of the particles coming from the accelerator head and from the additional collimators that shape the narrow photon beams used in radiosurgery treatments. Eight relevant photon virtual sources were identified from the full characterization analysis. Spatial and energy distributions were stored in histograms for the virtual sources representing the accelerator head components and the additional collimators. The photon directions were calculated for virtual sources representing the accelerator head components whereas, for the virtual sources representing the additional collimators, they were recorded into histograms. All these histograms were included in the MC code, DPM code and using a sampling procedure that reconstructed the PSDs, dose distributions were calculated in a water phantom divided in 20000 voxels of 1 x 1 x 5 mm3. The model accurately calculates dose distributions in the water phantom for all the additional collimators; for depth dose curves, associated errors at 2sigma were lower than 2.5% until a depth of 202.5 mm for all the additional collimators and for profiles at various depths, deviations between measured and calculated values were less than 2.5% or 1 mm.

History by history statistical estimators in the BEAM code system.

A history by history method for estimating uncertainties has been implemented in the BEAMnrc and DOSXYznrc codes replacing the method of statistical batches. This method groups scored quantities (e.g., dose) by primary history. When phase-space sources are used, this method groups incident particles according to the primary histories that generated them. This necessitated adding markers (negative energy) to phase-space files to indicate the first particle generated by a new primary history. The new method greatly reduces the uncertainty in the uncertainty estimate. The new method eliminates one dimension (which kept the results for each batch) from all scoring arrays, resulting in memory requirement being decreased by a factor of 2. Correlations between particles in phase-space sources are taken into account. The only correlations with any significant impact on uncertainty are those introduced by particle recycling. Failure to account for these correlations can result in a significant underestimate of the uncertainty. The previous method of accounting for correlations due to recycling by placing all recycled particles in the same batch did work. Neither the new method nor the batch method take into account correlations between incident particles when a phase-space source is restarted so one must avoid restarts.

Muon accelerator for the neutrino factory

A concept for a neutrino factory muon accelerator driver is presented. Acceleration of a muon beam is a challenging task because of a large source phase space and short species lifetime. In the design concept presented here, acceleration starts after ionization cooling at 190MeV/c and proceeds to 50GeV where the beam is injected into a neutrino factory storage ring. The proposed driver consists of a 3GeV linac and two consecutive recirculating linear accelerators. A choice of accelerator technology, the machine layout and its main parameters are discussed.

Partons in phase space

Within QED, we examine several issues related to constructing a parton-model-based QCD transport theory. We rewrite the QED analog of the parton model, the Weizsaecker-Williams Approximation, entirely in terms of phase-space quantities and we study the phase-space photon and electron densities created by a classical point charge. We find that the densities take a distinctive ``source-propagator'' form. This form does not arise in a conventional derivation of the semiclassical transport equations because of the overuse of the gradient approximation. We do not apply the gradient approximation and so derive the phase-space analog of the Generalized Fluctuation-Dissipation Theorem. Together, this theorem and the expression for the phase-space particle self-energies give a set of coupled phase-space evolution equations. We illustrate how these evolution equations can be used perturbatively or to derive semiclassical transport equations. Our work relies on phase-space propagators and sources, so we describe them in detail when calculating the photon and electron phase-space densities. We use these tools to discuss the shape of a nucleon's parton cloud.

Flux, irradiance, and transmission calculations for the ALS wiggler beamline 5.0

A protein crystallography facility is being constructed for the Advanced Light Source (ALS) wiggler beamline 5.0. The radiation source is a 38 pole, 2.0 T wiggler. Calculations have been performed to determine the source phase space characteristics and the power loading on and transmission of various beamline elements. A set of computer codes have been developed for this purpose. The wiggler horizontal and vertical phase space flux density is calculated by phasex and phasey, respectively. WrFlux calculates the spectral flux density along the principal ray of the optical system. WrPwr calculates the power impinging on a target. If a filter function is specified, the transmitted, or reflected, power is calculated. The theory and operation of the codes will be presented as well as several results of calculations.

Near‐specular reflection of ions at quasi‐parallel shocks

Previous numerical simulations have shown that ions are reflected at quasi‐parallel shocks in a bursty manner. At times, relatively cold and dense beams of ions that appear to be specularly reflected are seen, consistent with observations at the quasi‐parallel portions of the bow shock. The intermittent reflection of ions has been shown to be connected to the periodic disruption and re‐formation of the shock front. Though the connection of the reflected ions to the re‐formation cycle has been studied extensively, the reflection process itself has received little attention. Using one‐dimensional hybrid simulations and a semianalytic model of the shock front, the reflection process is investigated to determine the source regions in incident ion phase space of reflected and transmitted ions, the relative importance of the electric and magnetic forces in the reflection of incident ions, and how these characteristics change in time. We find that the phase space origin of reflected particles and the fraction of incident ions that reflect depend upon the electromagnetic field structure of the shock front at the time the ions encounter it. The reflection fraction is maximized when the electric field along the shock normal (Ex) and the noncoplanar magnetic field (By) are at their maximum values. This happens because the phase space source region of reflected particles lies closer to the center of the incident ion distribution function when those two field components are large than it does when they are small. Also, when Ex and By are large, the reflection process produces a beam that is cooler, more dense, and closer to specular than when they are small. We also find that, in general, the electric field provides most of the force involved in the reflection of incident ions, although the magnetic field is also needed to reflect all but a few ions. When Ex is near zero, almost no ions reflect in the shock ramp.

Renormalized plasma turbulence theory: A quasiparticle picture

A general renormalized statistical theory of Vlasov turbulence is given which proceeds directly from the Vlasov equation and does not assume prior knowledge of sophisticated field-theoretic techniques. Quasiparticles are the linear excitations of the turbulent system away from its instantaneous mean (ensemble-averaged) state or background; the properties of this background state ''dress'' or renormalize the quasiparticle responses. It is shown that all two-point responses (including the dielectric) and all two-point correlation functions can be completely described by the mean distribution function and three fundamental quantities. Two of these are the quasiparticle responses: the propagator and the potential source: which measure, respectively, the separate responses of the mean distribution function and the mean electrostatic potential to functional changes in an external phase-space source added to Vlasov's equation. The third quantity is the two-point correlation function of the incoherent part of the phase-space density which acts as a self-consistent source of quasiparticle and potential fluctuations. This theory explicitly takes into account the self-consistent nature of the electrostatic-field fluctuations which introduces new effects not found in the usual ''test-particle'' theories. Explicit equations for the fundamental quantities are derived in the direct interaction approximation. Special attention is paid to the two-point correlations and themore » relation to theories of phase-space granulation.« less