Linear Accelerator Treatment Head Research Articles

Validation of Monte Carlo multithreading TOpas 3.6 software in order to simulate the 6 MV Elekta Synergy MLCi2 platform linac.

Monte Carlo simulation is generally appreciated as an extraordinary technique to investigate particle physics processes and interactions in nuclear medicine and Radiation Therapy. The present task validates a new methodology of Monte Carlo simulation based on the Multithreading technique to reduce CPU time to simulate a 6 MV photon beam provided by the Elekta Synergy MLCi2 platform medical linear accelerator treatment head utilizing TOpas version 3.6 Monte Carlo software and the Slurm Marwan cluster. The simulation includes the linear accelerator (LINAC) major components. Calculations are performed for the photon beam with several treatment field sizes varying from 3 × 3 to 10 × 10 cm2 at a 100 cm of distance from the source to the surface of the IBA dosimetry water box. The simulation was wholly approved by comparison with experimental distributions. To evaluate simulation accuracy, gamma index formalism for (2%/2mm) and (3%/2mm) criteria, Distance To Agreement DTA, and the estimator standard error ɛ and ɛ max are used. Good agreement between simulations and measurements was observed for depth doses and lateral dose profiles, respectively. The gamma index comparisons also highlighted this agreement; more than 97% of the points for all simulations satisfy the quality assurance criteria of (2%/2mm). Regarding calculation performance, the event processing speed is faster using Gate-[mp] compared to TOpas-[mt] mode when running the identical simulation code for both. Consequently, according to the achieved results, the proposed methodology shows the first validation of TOpas in radiotherapy linacs simulations and a reduction in calculation time, capping simulation accuracy as much as possible. For this reason, this software is recommended to be serviceable for Treatment Planning Systems (TPS) purposes.

Monte Carlo calculated detector-specific correction factors for Elekta radiosurgery cones

A radiation field is considered small if its dimension is lower than the range of secondary electrons and the collimating devices partially occlude the source. Different detector types, such as unshielded diodes, diamond detectors, and small-volume ion chambers, are used for small-field measurements. Although the active volumes of these detectors are small, their non-water equivalent materials cause response variations. Herein, we aim to calculate the correction factors for our clinical detectors, EDGE detector (Sun Nuclear), 60017 diode (PTW), and CC01 ion chamber (IBA), for stereotactic radiosurgery cones of diameters of 5–15 mm in an Elekta Synergy linear accelerator using a Monte Carlo simulation. An Elekta Synergy linear accelerator treatment head was simulated using BEAMnrc Monte Carlo code as per the manufacturer specification. All three detectors were simulated as per the manufacturer specification. Three EGSnrc user codes were used for the detector simulation based on the detector geometry. The Monte Carlo model of the treatment head was validated against the measured data for a standard field size of 10 × 10 cm2. The off-axis profile, percentage depth dose, and tissue phantom ratio were verified in the validation procedure. The measured and Monte Carlo calculated relative output factors (ROFs) were not consistent. In a 5 mm field size, EDGE diode overestimated the ROF by 7.06%, and 60017 diode to 4.611%. In a 7.5 mm field size, the variations were 4.295% and 3.691% for EDGE and 60017 diodes, respectively. CC01 ion chamber under-responded up to 10% because of its low-density active volume. The maximum corrections were obtained in the smallest field size, which were 0.939(0.007), 0.962(0.006), and 1.117(0.008) for EDGE, PTW T60017, and CC01 detectors, respectively. After applying the Monte Carlo calculated correction factor to the measured ROF, it became consistent with the Monte Carlo calculated ROF.

A Novel Approach to Contamination Suppression in Transmission Detectors for Radiotherapy

The current trend in X-ray radiotherapy is to treat cancers that are in difficult locations in the body using beams with a complex intensity profile. Intensity-modulated radiotherapy (IMRT) is a treatment which improves the dose distribution to the tumor whilst reducing the dose to healthy tissue. Such treatments administer a larger dose per treatment fraction and hence require more complex methods to verify the accuracy of the treatment delivery. Measuring beam intensity fluctuations is difficult as the beam is heavily distorted after leaving the patient and transmission detectors will attenuate the beam and change the energy spectrum of the beam. Monolithic active pixel sensors (MAPSs) are ideal solid-state detectors to measure the 2-D beam profile of a radiotherapy beam upstream of the patient. MAPS sensors can be made very thin (~30 μm) with still very good signal-to-noise performance. This means that the beam would pass through the sensor virtually undisturbed (<; 1% attenuation). Pixel pitches of between 2 μm to 100 μm are commercially available. Large area devices (~15×15 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) have been produced. MAPS can be made radiation hard enough to be fully functional after a large number of fractions. All this makes MAPS a very realistic transmission detector candidate for beam monitoring upstream of the patient. A remaining challenge for thin, upstream sensors is that the detectors are sensitive to the signal of both therapeutic photons and electron contamination. Here, a method is presented to distinguish between the signal due to electrons and photons and thus provide real-time dosimetric information in very thin sensors that does not require Monte Carlo simulation of each linear accelerator treatment head.

Characterisation of a megavolt X-Ray therapeutic beam using non-orthogonal strip detectors for future enhanced primary standard dosimetry

The National Physical Laboratory (NPL) provides traceable high accuracy measurements for absorbed dose via primary standard graphite calorimeters. Patients undergoing radiotherapy for cancer treatments in the U.K. receive a dose that is traceable to these devices. In the case of hadron radiotherapy, reducing the uncertainty of these measurements requires the addition of spatial information to understand not only the integral dose delivered, but where the energy is deposited locally. The PRaVDA consortium developed strip detectors able to track individual protons at low rates for proton computed tomography. For calorimetry at clinical dose rates only real-time spot location is required, which could be projected to reconstruct a 2D visualisation from which the beam centre can be determined. This work reports on a proof-of-concept trial carried out using 6 MV X-rays using an Elekta Synergy linear accelerator. We show how the beam positions can be determined within ± 0.5 mm of the expected radiation beam position, which is similar to the tolerances of the field defining multileaf collimator within the linear accelerator treatment head. The trial will be extended to perform measurements in a clinical proton beam alongside the NPL graphite proton calorimeter.

SU‐E‐T‐465: Implementation of An Automated Collision Detection Program Using Open Source Software for the Pinnacle Treatment Planning System

Purpose:Potential collisions between the gantry head and the patient or table assembly are difficult to detect in most treatment planning systems. We have developed and implemented a novel software package for the representation of potential gantry collisions with the couch assembly at the time of treatment planning.Methods:Physical dimensions of the Varian Edge linear accelerator treatment head were measured and reproduced using the Visual Python display package. A script was developed for the Pinnacle treatment planning system to generate a file with the relevant couch, gantry, and isocenter positions for each beam in a planning trial. A python program was developed to parse the information from the TPS and produce a representative model of the couch/gantry system. Using the model and the Visual Python libraries, a rendering window is generated for each beam that allows the planner to evaluate the possibility of a collision.Results:Comparison against heuristic methods and direct verification on the machine validated the collision model generated by the software. Encounters of &lt;1 cm between the gantry treatment head and table were visualized as collisions in our virtual model. Visual windows were created depicting the angle of collision for each beam, including the anticipated table coordinates. Visual rendering of a 6 arc trial with multiple couch positions was completed in under 1 minute, with network bandwidth being the primary bottleneck.Conclusion:The developed software allows for quick examination of possible collisions during the treatment planning process and helps to prevent major collisions prior to plan approval. The software can easily be implemented on future planning systems due to the versatility and platform independence of the Python programming language. Further integration of the software with the treatment planning system will allow the possibility of patient‐gantry collision detection for a range of treatment machines.

Simulation of the 6 MV Elekta Synergy Platform linac photon beam using Geant4 Application for Tomographic Emission.

The present work validates the Geant4 Application for Tomographic Emission Monte Carlo software for the simulation of a 6 MV photon beam given by Elekta Synergy Platform medical linear accelerator treatment head. The simulation includes the major components of the linear accelerator (LINAC) with multi-leaf collimator and a homogeneous water phantom. Calculations were performed for the photon beam with several treatment field sizes ranging from 5 cm × 5 cm to 30 cm × 30 cm at 100 cm distance from the source. The simulation was successfully validated by comparison with experimental distributions. Good agreement between simulations and measurements was observed, with dose differences of about 0.02% and 2.5% for depth doses and lateral dose profiles, respectively. This agreement was also emphasized by the Kolmogorov–Smirnov goodness-of-fit test and by the gamma-index comparisons where more than 99% of the points for all simulations fulfill the quality assurance criteria of 2 mm/2%.

Measurement-based Monte Carlo simulation of high definition dose evaluation for nasopharyngeal cancer patients treated by using intensity modulated radiation therapy

Measurement-based Monte Carlo (MBMC) simulation using a high definition (HD) phantom was used to evaluate the dose distribution in nasopharyngeal cancer (NPC) patients treated with intensity modulated radiation therapy (IMRT). Around nasopharyngeal cavity, there exists many small volume organs-at-risk (OARs) such as the optic nerves, auditory nerves, cochlea, and semicircular canal which necessitate the use of a high definition phantom for accurate and correct dose evaluation. The aim of this research was to study the advantages of using an HD phantom for MBMC simulation in NPC patients treated with IMRT. The MBMC simulation in this study was based on the IMRT treatment plan of three NPC patients generated by the anisotropic analytical algorithm (AAA) of the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA, USA) using a calculation grid of 2 mm2. The NPC tumor was treated to a cumulative dose of 7000 cGy in 35 fractions using the shrinking-field sequential IMRT (SIMRT) method. The BEAMnrc MC Code was used to simulate a Varian EX21 linear accelerator treatment head. The HD phantom contained 0.5 × 0.5 × 1 mm3 voxels for the nasopharyngeal area and 0.5 × 0.5 × 3 mm3 for the rest of the head area. An efficiency map was obtained for the amorphous silicon aS1000 electronic portal imaging device (EPID) to adjust the weighting of each particle in the phase-space file for each IMRT beam. Our analysis revealed that small volume organs such as the eighth cranial nerve, semicircular canal, cochlea and external auditory canal showed an absolute dose difference of ≥200 cGy, while the dose difference for larger organs such as the parotid glands and tumor was negligible for the MBMC simulation using the HD phantom. The HD phantom was found to be suitable for Monte Carlo dose volume analysis of small volume organs.

MODELING OF PHOTON AND ELECTRON BEAMS FOR MONTE CARLO SIMULATION BASED ON TREATMENT PLANNING SYSTEMS

Monte Carlo simulation (MC simulation) is considered the golden standard for radiation transport. In order to calculate the dose by Monte Carlo simulation based on treatment planning systems (TPSs), the detailed geometry of the medical linear accelerator (linac) treatment head components must be accurately modeled. Thus in the present work, photon and electron beams from Varian trilogy machine were simulated using Monte Carlo method. Simulated Percentage depth doses (PDDs) and profiles were compared to measurement for both photon and electron beams. We have presented simulated PDDs and profiles for 6 MV photon beams with 4 x 4 cm2, 6 x 6 cm2, 10 x 10 cm2, and 20 x 20 cm2 field areas, we also presented simulated and measured 6 MeV electron beams for 6 x 6 cm2, and 10 x 10 cm2 field areas. Analysis showed good matching between the simulation and measurement. It can be concluded that Monte Carlo code was capable of modeling both photon and electron beams. It was shown that Monte Carlo simulation is a powerful tool in radiotherapy (RT) research.

Development of a Geant4 based Monte Carlo Algorithm to evaluate the MONACO VMAT treatment accuracy

A method to evaluate the dosimetric accuracy of volumetric modulated arc therapy (VMAT) treatment plans, generated with the MONACOTM (version 3.0) treatment planning system in realistic CT-data with an independent Geant4 based dose calculation algorithm is presented.Therefore a model of an Elekta Synergy linear accelerator treatment head with an MLCi2 multileaf collimator was implemented in Geant4. The time dependent linear accelerator components were modeled by importing either logfiles of an actual plan delivery or a DICOM-RT plan sequence. Absolute dose calibration, depending on a reference measurement, was applied. The MONACO as well as the Geant4 treatment head model was commissioned with lateral profiles and depth dose curves of square fields in water and with film measurements in inhomogeneous phantoms. A VMAT treatment plan for a patient with a thoracic tumor and a VMAT treatment plan of a patient, who received treatment in the thoracic spine region including metallic implants, were used for evaluation.MONACO, as well as Geant4, depth dose curves and lateral profiles of square fields had a mean local gamma (2%, 2mm) tolerance criteria agreement of more than 95% for all fields. Film measurements in inhomogeneous phantoms with a global gamma of (3%, 3mm) showed a pass rate above 95% in all voxels receiving more than 25% of the maximum dose. A dose-volume-histogram comparison of the VMAT patient treatment plans showed mean deviations between Geant4 and MONACO of -0.2% (first patient) and 2.0% (second patient) for the PTVs and (0.5±1.0)% and (1.4±1.1)% for the organs at risk in relation to the prescription dose.The presented method can be used to validate VMAT dose distributions generated by a large number of small segments in regions with high electron density gradients. The MONACO dose distributions showed good agreement with Geant4 and film measurements within the simulation and measurement errors.

Generation of a novel phase‐space‐based cylindrical dose kernel for IMRT optimization

Improving dose calculation accuracy is crucial in intensity-modulated radiation therapy (IMRT). We have developed a method for generating a phase-space-based dose kernel for IMRT planning of lung cancer patients. Particle transport in the linear accelerator treatment head of a 21EX, 6 MV photon beam (Varian Medical Systems, Palo Alto, CA) was simulated using the EGSnrc/BEAMnrc code system. The phase space information was recorded under the secondary jaws. Each particle in the phase space file was associated with a beamlet whose index was calculated and saved in the particle's LATCH variable. The DOSXYZnrc code was modified to accumulate the energy deposited by each particle based on its beamlet index. Furthermore, the central axis of each beamlet was calculated from the orientation of all the particles in this beamlet. A cylinder was then defined around the central axis so that only the energy deposited within the cylinder was counted. A look-up table was established for each cylinder during the tallying process. The efficiency and accuracy of the cylindrical beamlet energy deposition approach was evaluated using a treatment plan developed on a simulated lung phantom. Profile and percentage depth doses computed in a water phantom for an open, square field size were within 1.5% of measurements. Dose optimized with the cylindrical dose kernel was found to be within 0.6% of that computed with the nontruncated 3D kernel. The cylindrical truncation reduced optimization time by approximately 80%. A method for generating a phase-space-based dose kernel, using a truncated cylinder for scoring dose, in beamlet-based optimization of lung treatment planning was developed and found to be in good agreement with the standard, nontruncated scoring approach. Compared to previous techniques, our method significantly reduces computational time and memory requirements, which may be useful for Monte-Carlo-based 4D IMRT or IMAT treatment planning.

Monte Carlo simulation of small field electron beams for small animal irradiation

Abstract The volume effect of detectors in the dosimetry of small fields for photon beams has been well studied due to interests in radiosurgery and small beamlets used in IMRT treatments; but there is still an unexplored research field for small electron beams used in small animal irradiation. This study proposes to use the BEAM Monte Carlo (MC) simulation to assess characteristics of small electron beams (4, 6, 14, 30 mm in diameter) with the kinetic energies of 6 and 18 MeV. Three factors influencing beam characteristics were studied (1) AE and ECUT settings, (2) photon jaw settings and (3) simulation pixel sizes. Study results reveal that AE/ECUT settings at 0.7 MeV are adequate for linear accelerator treatment head simulation, while 0.521 MeV is more favorable to be used for the phantom study. It is also demonstrated that voxel size setting at 1/4 of the simulation field width in all directions is sufficient to achieve accurate results. As for the photon jaw setting, it has great impact on the absolute output of different field size setting (i.e. output factor) but with minimum effect on the relative lateral distribution.

Monte Carlo commissioning of clinical electron beams using large field measurements

Monte Carlo simulation can accurately calculate electron fluence at the patient surface and the resultant dose deposition if the initial source electron beam and linear accelerator treatment head geometry parameters are well characterized. A recent approach used large electron fields to extract these simulation parameters. This method took advantage of the absence of lower energy, widely scattered electrons from the applicator resulting in more accurate data. It is important to validate these simulation parameters for clinically relevant fields. In the current study, these simulation parameters are applied to fields collimated by applicators and inserts to perform a comprehensive validation. Measurements were performed on a Siemens Oncor linear accelerator for 6 MeV, 9 MeV, 12 MeV, 15 MeV, 18 MeV and 21 MeV electron beams and collimators ranging from an open 25 × 25 cm2 applicator to a 10 × 10 cm2 applicator with a 1 cm diameter cerrobend insert. Data were collected for inserts placed in four square applicators. Monte Carlo simulations were performed using EGSnrc/BEAMnrc. Source and geometry parameters were obtained from previous measurements and simulations with the maximum field size (40 × 40 cm2). The applicators were modelled using manufacturer specifications, confirmed by direct measurements. Cerrobend inserts were modelled based on calliper measurements. Monte Carlo-calculated percentage depth dose and off-axis profiles agreed with measurements to within the least restrictive of 2%/1 mm in most cases. For the largest applicator (25 × 25 cm2), and 18 MeV and 21 MeV beams, differences in dose profiles of 3% were observed. Calculated relative output factors were within 2% of those measured with an electron diode for fields 1.5 cm in diameter or larger. The disagreement for 1 cm diameter fields was up to 5%. For open applicators, simulations agreed with parallel plate chamber-measured relative output factors to 1%. This work has validated a recent methodology used to extract data on the electron source and treatment head from large electron fields, resulting in a reduction in the number of unknown parameters in treatment head simulation. Applicator and insert collimated electron fields were accurately simulated without adjusting these parameters. Results demonstrate that commissioning of electron beams based on large electron field measurements is a viable option.

Radiotherapy treatment planning based on Monte Carlo techniques

At the present, treatment planning systems (TPS) used in radiotherapy treatments use determinist correlations based on measurements in water to evaluate doses in the volume of interest and dose distributions around it. Nevertheless, it is well known that doses assigned with this type of planner can be problematic, especially in the presence of heterogeneities. The present work has developed a computational model using the Monte Carlo (MC) code MCNP5 (Monte Carlo N-Particle) for the simulation of a 6 MeV photon beam emitted by Elekta Precise medical linear accelerator treatment head. The model includes the major components of the accelerator head and the cube-shaped heterogeneous water tank “ RFA-300”. A low-density heterogeneity has been placed inside this water tank. It consists of a extruded polystyrene piece (97% air and 3% polystyrene) whose dimensions are 30 cm×10 cm×8 cm and with a density of 0.0311 g/cm 3. Calculations were performed for a photon beam setting 10 cm×10 cm and 20 cm×20 cm irradiation field sizes at 100 cm distance from source. The MC simulation is able to predict the absorbed dose distribution within the water tank using the *F8 or FMESH4 tally. These results have been compared with experimental values measured at the Hospital Clínic Universitari de Valencia. Dosimetric parameters calculated by simulation at the water tank and the experimental measures agreed, with an average deviation inside the heterogeneity region of 3%. Simulation results have been also compared with dose curves predicted by a commercial TPS in the same irradiation conditions, focusing attention on the accuracy that both systems reach in the dose calculation at the interphase zone and inside the heterogeneity. In contrast, TPS results overestimate the dose inside the heterogeneity and after it, with a maximum deviation of 7% for the 6 MeV photon beam and a field size of 20 cm×20 cm. We can conclude that the algorithms of computation of the TPS are not able to predict the variation of dose in the heterogeneous zones with the same accuracy as MC methods, since MCNP5 provides more trustworthy results and fits the experimental values better in the heterogeneous region.

Electron beam therapy at extended source‐to‐surface distance: a Monte Carlo investigation

Electron‐beam therapy is used to treat superficial tumors at a standard 100 cm source‐to‐surface distance (SSD). However, certain clinical situations require the use of an extended SSD. In the present study, Monte Carlo methods were used to investigate clinical electron beams, at standard and non‐standard SSDs, from a Siemens Oncor Avant Garde (Siemens Healthcare, Erlangen, Germany) linear accelerator (LINAC). The LINAC treatment head was modeled in BEAMnrc for electron fields 5 cm in diameter and 10×10 cm, 15×15 cm, and 20×20 cm; for 6 MeV, 9 MeV, and 12 MeV; and for 100 cm, 110 cm, and 120 cm SSD. The DOSXYZnrc code was used to calculate extended SSD factors and dose contributions from various parts of the treatment head.The main effects of extended SSD on water phantom dose distributions were verified by Monte Carlo methods. Monte Carlo–calculated and measured extended SSD factors showed an average difference of ±1.8%. For the field 5 cm in diameter, the relative output at extended SSD declined more rapidly than it did for the larger fields. An investigation of output contributions showed this decline was mainly a result of a rapid loss of scatter dose reaching the dmax point from the lower scrapers of the electron applicator. The field 5 cm in diameter showed a reduction in dose contributions; the larger fields generally showed an increased contribution from the scrapers with increase in SSD. Angular distributions of applicator‐scattered electrons have shown a large number of acute‐angle electron tracks contributing to the output for larger field sizes, explaining the shallow output reduction.PACS numbers: 87.53.Wz, 87.53.Vb, 87.53.Hv

MO‐A‐AUD A‐01: Review of AAPM Task Group Report No. 105

A special class of Monte Carlo (MC)‐based dose calculation codes optimized for photon and electron beams in patient‐specific geometries has invigorated interest in the use of MC‐based dose calculations for radiotherapy treatment planning. In general, this class of “second generation” codes, including VMC++, XVMC, and DPM, among others, employ electron‐step algorithms that converge faster, i.e. fewer condensed‐history steps are required for the same precision versus “first‐generation” codes (such as EGS and MCNP). These advances, coupled with the use of sophisticated variance reduction techniques (e.g. directional bremsstrahlung splitting), have made it possible to perform MC‐based photon beam dose calculations, in some instances, of the entire linear accelerator treatment head and patient‐geometry, within minutes on a single processor. Consequently, several commercial vendors have released or are currently in the process of releasing MC algorithms for photon and/or electron beam treatment planning. With the impending availability of MC‐based dose calculation algorithms for routine clinical treatment planning, it is important that strategies and paradigms for clinical commissioning and implementation of these systems be formulated and discussed. We provide a review of AAPM Task Group Report No. 105 (Med. Phys. 34 (2007) 4818–4853), a document which outlines the important aspects of a MC‐based dose calculation algorithm, from the basic aspects of the use of the MC method for radiation transport to the application of this approach in routine clinical photon and electron beam treatment planning.Educational Objectives:1. To provide an educational review of the physics of the MC method including discussion of the approaches used for coupled photon and electron transport.2. To review the methods used to improve the MC simulation efficiency.3. To briefly review the vendor transport codes currently used for clinical treatment planning.4. To describe the development of beam models for clinical treatment planning.5. To discuss the factors associated with MC dose calculation within the patient‐specific geometry, such as statistical uncertainties, CT‐number to material density assignments, and reporting of dose‐to‐medium versus dose‐to‐water.6. To discuss the issues associated with experimental verification of MC algorithms.7. To briefly review the potential clinical implications of MC calculated dose distributions.8. To provide example timing comparisons of the major vendor MC codes in the clinical setting.

Clinical implications of the implementation of advanced treatment planning algorithms for thoracic treatments

Background and Purpose Radiotherapy treatment planning algorithms continue to develop and current planning systems typically offer simpler, but faster, algorithms, which may be 2, 2.5 or 3D in modelling scatter, but which do not model electron transport ( type a) and more accurate algorithms which aim to be fully 3D, i.e. which model 3D scatter and also model electron transport ( type b). A range of comparative planning studies and experiments indicate that the main situation where the changes are significant between the two types of algorithm is where lung tissue is involved. However, more generally, interface areas between materials of different electron density and composition are expected to show differences between the two types of algorithms. These are likely to pose potentially significant clinical consequences when a centre changes from using older simpler algorithms to more accurate fully 3D ones and require careful consideration. Materials and Methods Some modelling is presented using the different type algorithms for a recently available novel design of linear accelerator treatment head, as part of the commissioning of that machine and in preparing for a change in TPS algorithm. The TPS data are compared to measurements and to Monte Carlo calculations. Results and Discussion The results add to the evidence of other studies that 3D planning techniques and type b dose calculation algorithms lead to systematic changes in computation and delivery of radiotherapy dose and in dose distributions, as compared to simpler methods, and that these changes are more pronounced in treatments involving lung tissue. The type b algorithms agree well with Monte Carlo modelling. Conclusions Careful analysis of the changes is required before adopting new algorithms into clinical treatment planning practice. Discussion is needed between physicists and oncologists to fully understand the effects and potential consequences. These include changes in delivered dose to the reference point, to coverage of the PTV and to the dose distribution and also to dosimetric parameters used to constrain toxicity for lung, e.g. V20, and other tissues. There are consequences for assessment of dose–effect relationships and of parameters used in treatment planning decisions and this is an opportune time to re-evaluate this information.

SU-FF-T-357: Modeling Thick-Target Bremsstrahlung Production by Compound-Poisson-Process Electron Multiple Scatter

Purpose: To develop an accurate analytic algorithm to model bremsstrahlung production in a linac target and to calculate the angular distribution of the primary photon energy spectrum. This provides an alternative to Monte Carlo calculation of the primary photon spectrum. Method and Materials: The Compound-Poisson-Process (CPP) method is used to calculate the directional distribution of electrons in the target at each depth, explicitly including multiple scatters at large angles. This CPP method is modified to include the electron loss of energy at depth in the thick target. The Schiff integral formula is used to compute the bremsstrahlung spectrum produced at each angle and depth, and the contributions are integrated over depth to yield the photon energy fluence spectrum as a function of polar angle. This formula makes no small-angle approximations, so it should be accurate at all angles. Results: The primary (and attenuated first-scatter) photon energy fluence distribution was computed for a 15 MeV electron beam incident on a thick lead target and compared to a measurement in the same geometry. Conclusion: It is hoped that an accurate analytic calculation of the bremsstrahlung energy fluence distribution emerging from a linear accelerator treatment head will provide a quick and useful alternative to Monte Carlo calculations of this same distribution.

Influence of initial electron beam characteristics on Monte Carlo calculated absorbed dose distributions for linear accelerator electron beams

The least known parameters in a Monte Carlo simulation of a linear accelerator treatment head are often the properties of the initial electron beam directed onto the exit vacuum window. Several initial beams with different spatial fluence distributions, angular divergences and energy spectra have been transported through the geometry of a scattering foil accelerator. The electron beam characteristics (energy spectrum and angular distribution) at the phantom surface and the subsequent relative absorbed dose distribution in a water phantom were calculated. The dose distribution was found to be insensitive to the geometrical properties of the initial beam. Furthermore, the lateral dose profiles are unaffected by the energy spectrum of the initial beam. The effect on the depth–dose curve is negligible if the initial energy spectrum is symmetric (e.g., Gaussian shaped) and its full width at half maximum (FWHM) is less than approximately 10% of the most probable energy. A larger FWHM will decrease the normalized dose gradient, but will not affect the dose in the build-up region. An asymmetric wedge shaped spectrum with a low-energy extension simultaneously increases the dose in the build-up region and decreases the dose gradient. The relationship between the energy spectral width and the normalized dose gradient is, however, smaller than published analytical expressions indicate. Some well-established energy-range relationships were shown to be accurate for most of the initial beams studied. The energy spectrum at the phantom surface was also derived from a measured depth–dose curve through different methods. The extracted spectrum depends on the beam model and the spectral reconstruction algorithm. Even though the depth–dose curve is fairly independent of initial beam characteristics, a correct description of the low-energy tail of the energy spectrum is important to obtain good agreement between measured and Monte Carlo calculated doses in the build-up region.

A virtual source model for Monte Carlo modeling of arbitrary intensity distributions.

A photon virtual source model was developed for simulating arbitrary, external beam, intensity distributions using the Monte Carlo method. The source model consists of a photon fluence grid composed of a matrix of square elements, located 25-cm downstream from the linear accelerator target. Each particle originating from the fluence map is characterized by the seven phase space parameters, position (x, y, z), direction (u, v, w), and energy. The map was reconstructed from fluence and energy spectra acquired by modeling components of the linear accelerator treatment head using the Monte Carlo code MCNP4B. The effect of contaminant electrons is accounted for by the use of a sub-source derived from a phase-space simulation of a 25-MV linac treatment head using the code BEAM. The BEAM sub-source was incorporated into the MCNP4B phase-space model and is sampled using a field-size dependent sampling ratio. A Gaussian blurring kernel is convolved with the photon fluence map to account for the finite focal spot size and scattering effects from structures such as the flattening filter and MLC leaves. Depth dose and profile source calculations for 6-MV and 25-MV photon beams, for 5 x 5 cm2, 10 x 10 cm2, and 15 x 15 cm2 field sizes, are in good agreement with measurement and are well within acceptability criteria suggested by the AAPM Task Group Report No. 53. Irregular field calculations compared with film measurement and with a 3-D pencil beam algorithm show that the source model is capable of accurately simulating arbitrary MLC fields.

An equivalent square field formula for determining head scatter factors of rectangular fields

A simple formula is derived for the calculation of an equivalent square field that gives the same head scatter factor as a given rectangular field. This formula is based strictly on the configuration of a medical linear accelerator treatment head. The geometric parameters used are the distances between the target and the top of each field-defining aperture. The formula accounts for both the effect of field elongation and the collimator exchange effect. This method predicts the output to within 1% accuracy for both open and wedged fields and does not require any new measured data other than the field size dependence of head scatter for a range of square field sizes. Interestingly, the formula we derived has the same format as the formula that was empirically obtained by Vadash and Bjärngard [Med. Phys. 20, 733-734 (1993)].