Electron Beam Output Research Articles

SU‐C‐BRD‐07: The Radiological Physics Center (RPC): 45 Years of Improving Radiotherapy Dosimetry

Purpose:The RPC, established in 1968 has contributed to the development, conduct, and QA of NCI funded multi‐institutional cooperative group clinical trials and institutions, primarily in the USA/Canada and 242 other countries, participating in trials.Methods:The RPC QA program components were designed to audit the radiation dose calculation chain from the NIST traceable reference beam calibration, to inclusion of dosimetry parameters used to calculate tumor doses, to the delivery of the radiation dose. The QA program included: 1) remote TLD/OSLD audit of machine output, 2) on‐site dosimetry review visits, 3) credentialing for advanced technologies, and 4) review of patient treatment records. The RPC presented and published their findings to the radiation oncology community.Results:The number of institutions monitored by the RPC increased from around 1200 in the late 90s, to ∼2000 in 2013. There were over 4000 megavoltage therapy machines and ∼28,000 therapy beams in the 1991 institutions monitored by the RPC by the end of 2013. Within the 14,000 photon, electron and proton beam outputs remotely monitored with TLD/OSLD annually, between 10‐20% of the institutions have one or more beams outside the RPC 5% criterion. Dosimetry site visits to photon and proton centers continue to result in 2‐4 recommendations affecting key dosimetry parameters that impact patient treatment times. One in four patient treatment records reviewed by the RPC have their dose data corrected by >5% before trial groups use them for outcomes analysis. Twelve of fourteen clinically active proton centers are approved to participate in NCI funded clinical trials. The RPC published 222 peer reviewed articles since 1972.Conclusion:Findings from the RPC suggest that human errors continue to play a role in radiotherapy discrepancies and without the RPC independent QA program, the number of undetected errors and time elapsed before their discovery would have been greater.Work supported by MGH C06 CA059267 and grants CA10953, CA081647 awarded by NCI, DHHS.

Analysis of electron beam output factors by Monte Carlo method

Objective To investigate the application of the Monte Carlo dose calculation of output factors for electron beams in radiotherapy.Methods The code EGS4/MCTP was used to simulate the head of a medical linear accelerator (Varian 23EX) to calculate the output factors for 6 MeV,9 MeV and 18 MeV electron beams.The source-to-surface distance used was 100 cm.The field sizes ranged from 2 cm × 2 cm to 25 cm × 25 cm.The calculated output factors agreed with the corresponding measured factors which were measured by the IBA Phantom system to within 2％.Then,the output factors of direct articles and indirect articles which were under different energy and various cone-insert combinations were calculated by the code EGS4/MCTP.Results The calculated output factors agreement with the measurements is found to be mostly under the 1％ level.The variation of output factors depends on the characteristics of the beams and the modifications that the various cone-insert combinations introduce to these characteristics.Conclusions Monte Carlo dose calculations for electron beams in homogeneous water phantoms have been demonstrated to be accurate under the 1％ level in comparison with measurements.The output factors were influenced by energy and cone-insert combinations in complex ways. Key words: Electronbeam irradiation; Output factor; Monte Carlo method

Control of electron-seeding phase in a cascaded laser wakefield accelerator

Two segments of plasmas with different densities, which are operated as the electron injector and accelerator, respectively, are designed to realize a cascaded laser wakefield accelerator. Particle-in-cell simulations indicate that the further acceleration of the electrons in the second uniform-density plasma relies on the injection and acceleration in the first stage. It is found that electrons trapped in the second wakefield period in the first stage can be seeded into the next stage with an optimized phase for efficient acceleration and reducing in the relative energy spread. And finally a 700 MeV electron beam with a relative rms energy spread about 0.6% and the normalized transverse emittance of 1.4π mm mrad was obtained after a 5.5-mm-long acceleration in a dark-current free cascaded laser wakefield accelerator. Our results demonstrate that, for a given laser energy, choices in laser and plasma parameters strongly affect the output electron beam energy and quality, and that all of these parameters can be controlled.

SU‐E‐T‐126: Analysis of Uncertainties for the RPC Remote Dosimetry Using Optically Stimulated Light Dosimetry (OSDL)

Purpose: Determine the uncertainty of the Radiological Physics Centerˈs (RPC) remote audit dose measurement of photon and electron beam outputs using Optically Stimulated Luminescence Dosimeters (OSLD). Methods: In June 2010 the RPC implemented a switch from TLD to an OSLD mailed dosimetry audit using Landauerˈs InLight nanoDot™ OSL dosimeters and microStar reader System™. The steps and factors for the calculation of dose with OSLD were designed to follow those already in use with TLD including the use of the same acrylic mini‐phantoms and irradiation instructions. The OSLD sensitivity was referenced to Co‐60, and corrections for linearity, individual dosimeter response, fading, use of mini‐phantoms and energy were determined for the OSLD. An analysis of the uncertainty of each of the correction factors was determined to give an overall uncertainty of the OSLD dose measurement under Co‐60 reference conditions and megavoltage beam audits. Results: The expectation was that with the physical processes for OSLD, being so closely resembling those of TLD, the uncertainties of OSLD would be comparable to those of TLD. A combined uncertainty of 0.8% for irradiations with Co‐60 under controlled reference conditions and 1.8% for irradiations with high‐energy X‐rays was determined as compared to 1% and 2% for the previously used TLD, respectively. The OSLD energy block correction factor (KE) is the primary source of the overall OSLD dose measurement uncertainty. To date, the OSLD audit results after a year are essentially identical to the previous 5 years of TLD results both showing a mean TLD/Inst. Ratio of 1.00 ± 0.018.Conclusion: The uncertainty of the RPCˈs new OSLD remote dosimetry audit programˈs dose measurement is equal to if not less than that for the historical TLD program.Work supported by PHS CA010953, awarded by NCI, DHHS.

Exploring laser-wakefield-accelerator regimes for near-term lasers using particle-in-cell simulation in Lorentz-boosted frames

Plasma-based acceleration offers compact accelerators with potential applications for high-energy physics and photon sources. The past five years have seen an explosion of experimental results with monoenergetic electron beams up to 1 GeV on a centimetre-scale, using plasma waves driven by intense lasers. The next decade will see tremendous increases in laser power and energy, permitting beam energies beyond 10 GeV. Leveraging on the Lorentz transformations to bring the laser and plasma spatial scales together, we have reduced the computational time for modelling laser–plasma accelerators by several orders of magnitude, including all the relevant physics. This scheme enables the first one-to-one particle-in-cell simulations of the next generation of accelerators at the energy frontier. Our results demonstrate that, for a given laser energy, choices in laser and plasma parameters strongly affect the output electron beam energy, charge and quality, and that all of these parameters can be optimized. Modelling the interaction of an intense laser with a plasma in an optimal ‘Lorentz boosted’ frame of reference decreases by many orders of magnitude the computation time needed to simulate a laser-driven particle accelerator. This provides a powerful tool for optimizing the characteristics of accelerators driven by the next generation of high-intensity lasers, which will be able to deliver powers beyond a petawatt.

Laser wakefield plasma accelerators

AbstractOver the past several years there have been significant advances in research towards generating compact relativistic electron beams using high power short pulse laser produced plasmas (i.e., laser wakefield accelerators). In particular, an explosion of interest was generated in this field following the discovery in 2004 of a method to create such beams with low energy spread using a “plasma bubble” shaped wake. Recent work has increased the energy of these beams to the GeV range by extending the acceleration distance from a few millimetres to several centimeters. From both experimental and theoretical work, a more complete understanding of this “plasma bubble” regime for electron acceleration has also been obtained, enabling a significant improvement in the output electron beam quality and stability. There is ongoing work to further improve the parameters and stability of these beams with the goal of constructing “table‐top” 4th generation sources of coherent x‐ray radiation.

Validation of the final aperture superposition technique to calculate electron output factors and depth dose curves

The final aperture superposition technique (FAST) is a method to reproduce rapidly the electron-beam depth dose curves and output factors that would be calculated by a full Monte Carlo simulation. FAST uses precalculated Monte Carlo-based differential dose arrays and performs a superposition of open and shielded contributions to account for arbitrarily shaped insert openings. The objective of this work was to refine and validate the accuracy of the FAST method for a full range of treatment parameters. Compared to full simulations, raw FAST calculations tended to underestimate dose near the surface deposited by particles that crossed the shield-opening interface of the insert. In this study, a set of empirical correction curves was derived to reduce the errors from this "collimator effect." FAST and full simulation calculations were compared for every combination of six beam energies (6-21 MeV), four applicator sizes (10-25 cm), and two source-to-surface distances (SSDs) (100 and 110 cm). Validation tests were performed for a total of 192 fields using four sample insert openings: an open insert and 2, 3, and 5 cm diameter circular openings. Calculations were also performed for four patient inserts with irregularly shaped openings. Using the empirical correction curves, systematic errors were reduced, resulting in mean dose differences of less than 1% of the maximum full simulation dose. FAST relative output factors reproduced full simulation output factors to within 3% for all configurations except for the 2 and 3 cm diameter openings for the 6 and 9 MeV beams at 110 cm SSD. The maximum shift between the FAST and full simulation depth dose curves in the 90%-80% fall-off region was less than 3 mm for 97% of the fields. For the patient insert calculations, differences in output factors and mean differences in depth dose curves were within 1.5% with maximum shifts of 1.5 mm in the 90%-80% fall-off region. A small set measurements also demonstrated 3% accuracy in FAST output factors except for a 5% deviation for a 2 cm diameter insert for the 6 MeV beam at 110 cm SSD. These results demonstrate that FAST can be used to provide output factors and depth dose curves for most clinical cases.

SU‐GG‐T‐266: Implementation of Total Skin Electron Irradiation (TSEI) Using High Dose Rate Mode at Siriraj Hospital

Purpose: To implement and commissioning of applied Stanford total skin electron irradiation (TSEI) technique using standard linear accelerator at our institute. Method and Materials: Patient was treated in six different upright patient positions using dual angle fields. This study was carried out on Varian 23EX linear accelerator with 6 MeV, 36×36 cm2 field size at collimator in HDTSe mode at a SSD of 400 cm behind a beam degrader. The dosimetric characteristics were verified experimentally using a parallel plate ion chamber, electron diodes, and TLD‐100 chip and rod types. The delivered dose is 36 Gy in 9 weeks. The basic dosimetric parameter consists of beam profile, depth dose curve and electron beam output in the treatment plane. The output and depth dose curve was measured by PTW23343 Markus chamber in solid phantom. Delivered dose from six dual fields technique in treatment position was verified on round phantom by TLD‐100. Results: The uniform vertical profile was obtained for gantry angulations of ± 17.5 degree above and below horizontal within 10% of the prescribed dose. The depth dose curve shows maximum dose at skin surface. The mean energy is 2.09 MeV with beam degrader 1 cm thick. The dose uniformity from six dual fields on round phantom is within 10%. The average measured skin dose in the anterior and posterior at the level of head, chest and pelvis on rando phantom is within 10% of prescribed point. Total X‐ray contamination in the rando phantom is about 1.35% of the delivered dose. Conclusion: The TSEI technique using six dual fields applied Stanford technique produced a uniformity at the whole patient's surface cannot be better than 15% of the prescribed dose because of variable skin distance, self‐shielding, and differences in curvature.

The root percent depth dose method for calculating monitor units for irregularly shaped electron fields

This study outlines an improved method for calculating dose per monitor unit values for irregularly shaped electron fields using the nth root percent depth dose method. This method calculates the percent depth dose and output factors for an irregularly shaped electron field directly from the measured electron beam percent depth dose curves and output factors for circular fields. The percent depth dose curves and output factors for circular fields are normalized and measured at a fixed depth of maximum dose for a reference field, respectively. When compared with the sector integral lateral buildup ratio method, the percent depth dose data calculated using the nth root method accounts more accurately for the change in lateral scatter with decreasing field size. Therefore, it provides more accurate values of dose per monitor unit at different depths for all type of field shapes and beam energies. For beam energies in the range of 6-21 MeV, the differences between measured and calculated dose per monitor unit values, at different depths, were found to be within +/- 1.0% when the nth root percent depth dose method was used for calculation and 12.6% when the sector integral lateral buildup ratio method was used. The nth root percent depth dose method was tested and compared with the sector integral lateral buildup ratio method for ten clinically used irregularly shaped inserts (cutouts). For small irregularly shaped fields, a maximum difference of 2% was found between calculated dose per monitor unit values and measurements when the nth root percent depth dose method was used; this difference changed to 7% when comparisons were made between measurements and calculations based on the sector integral lateral buildup ration method. For large irregular fields this difference was found to be within 1.5% and 3.5%, respectively.

Characterization and Stability of Electron Beam Output Factors for an Elekta Precise Linac

To measure and characterize electron beam output factors as a function of beam energy and field size for an Elekta Precise linac and to investigate its variation over time.

Generating multi-GeV electron bunches using single stage laser wakefield acceleration in a 3D nonlinear regime

The extraordinary ability of space-charge waves in plasmas to accelerate charged particles at gradients that are orders of magnitude greater than in current accelerators has been well documented. We develop a phenomenological framework for laser wakefield acceleration (LWFA) in the 3D nonlinear regime, in which the plasma electrons are expelled by the radiation pressure of a short pulse laser, leading to nearly complete blowout. Our theory provides a recipe for designing a LWFA for given laser and plasma parameters and estimates the number and the energy of the accelerated electrons whether self-injected or externally injected. These formulas apply for self-guided as well as externally guided pulses (e.g. by plasma channels). We demonstrate our results by presenting a sample particle-in-cell (PIC) simulation of a $30\text{ }\mathrm{fs}$, 200 TW laser interacting with a 0.75 cm long plasma with density $1.5\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ to produce an ultrashort (10 fs) monoenergetic bunch of self-injected electrons at 1.5 GeV with 0.3 nC of charge. For future higher-energy accelerator applications, we propose a parameter space, which is distinct from that described by Gordienko and Pukhov [Phys. Plasmas 12, 043109 (2005)] in that it involves lower plasma densities and wider spot sizes while keeping the intensity relatively constant. We find that this helps increase the output electron beam energy while keeping the efficiency high.

Phase motion of accelerated electrons in vacuum laser acceleration

The phase stability in the capture and acceleration scenario (CAS) is studied and compared with that of conventional linear electron accelerators (CLEAs). For the CAS case, it has been found that a slow phase slippage occurs due to the difference between the electron velocity and the phase velocity of the longitudinal accelerating electric field. Thus, CAS electrons cannot remain in a fixed small phase region of the accelerating field to obtain a quasimonoenergy gain in contrast to the stability of phase oscillation in CLEAs. Also, the energy spread of the output electron beam for the CAS case cannot be kept as small as the CLEA because there is no good phase bunching phenomenon generated by phase oscillation.

SU‐FF‐T‐103: Application of the Strydom Analytical Model to the Remote Monitoring of Electron Beam Dosimetry Parameters by Thermoluminescent Dosimetry

Purpose: To provide a practical methodology and a reference percentage depth dose (PDD) data set for streamlining the Radiation Dosimetry Services (RDS) TLD verification of electron beam output and PDD. Methods and Materials: PDD data from 29 Siemens Primus and 19 Varian Clinac 21EX accelerators were retrieved from the databases of RDS and the Radiological Physics Center (RPC) and were simultaneously fitted to the nonlinear Strydom analytical model. The NLIN (SAS 9.1) procedure was used to fit the models. The PDD data sets used in the models were independently measured by many physicists from many institutions and verified by TLD and RPC onsite measurements. The resulting reference PDD data sets were incorporated into the RDS TLD analysis software for electron beam monitoring. Results: The parameters of the model were estimated for 5 to 21 MeV electron beams for Siemens and Varian machines. Individual PDD data fell within ± 1.5 mm of the reference PDD data for all depths for all beams analyzed. The curves exhihited similar shapes and were found to agree within ± 1.0 mm at all depths when all the curves for a particular energy were made to coincide at the depth of 50%. The validated reference model's predictions shows good agreement with other published results. Conclusions: The shape of the PDD curves was found to depend only on beam energy, and was independent of model for newer machines. These results allowed RDS to streamline its process of evaluating TLD results for electron beams and to acquire additional information about an institution's PDD data to facilitate the identification of problems. The reference PDD data sets can serve as a resource to the medical physics community. This work was supported by PHS grant CA 10953 awarded by the NCI, DHHS and Radiation Dosimetry Services.

Spectral study of the electron beam emitted from a 3 kJ plasma focus

In a 3 kJ Mather-type plasma focus device operated in neon, the electron beam emission is investigated using both a magnetic electron energy analyser (in the 30–660 keV range) and a Rogowski coil (coupled with an appropriate RC passive integrator). Several electron emission features are identified and correlated with the x-ray emission in different energy ranges. The electron beam output shows very strong correlation with the general plasma dynamics (breakdown, axial and radial acceleration, pinch and post-pinch phases). The electrons' energy spectra showed most of the electron emission concentrating below 200 keV and negligible emission with energy above 350 keV. At 4 mbar neon, the electron emission, as well as the beam energy, is the highest and has a good shot-to-shot reproducibility.

Evaluation of two water-equivalent phantom materials for output calibration of photon and electron beams

Two commercially available water-equivalent solid phantom materials were evaluated for output calibration in both photon (6–15 MV) and electron (6–20 MeV) beams. The solid water 457 and virtual water materials have the same chemical composition but differ in manufacturing process and density. A Farmer-type ionization chamber was used for measuring the output of the photon beams at 5- and 10-cm depth and electron beams at maximum buildup depth in the solid phantoms and in natural water. The water-equivalency correction factor for the solid materials is defined as the ratio of the chamber reading in natural water to that in the solid at the same linear depth. For photon beams, the correction factor was found to be independent of depth and was 0.987 and 0.993 for 6- and 15-MV beams, respectively, for solid water. For virtual water, the corresponding correction factors were 0.993 and 0.998 for 6- and 15-MV beams, respectively. For electron beams, the correction factors ranged from 1.013 to 1.007 for energies of 6 to 20 MeV for both solid materials. This indicated that the water-equivalency of these materials is within ± 1.3%, making them suitable substitutes for natural water in both photon and electron beam output measurements over a wide energy range. These correction factors are slightly larger than the manufacturers' advertised values (± 1.0% for solid water and ± 0.5% for virtual water). We suggest that these corrections are large enough in most cases and should be applied in the calculation of beam outputs.

Characterization of sheet electron beams from planar crossed-field secondary emission diodes

Sheet electron beams of 9 cm width having linear current densities of 1.5–3.0 A/cm have been generated from a planar crossed-field secondary emission (CFSE) electron source operated at ∼20 kV diode voltage. The output electron beam consists of two parallel closely spaced sheets of electron flow originated from each side of the cathode. The full width at half maximum of each electron sheet is 1–2 mm depending on the magnetic field strength. The output current depends strongly on the diode voltage as I∝Un where n=5.0±0.5. At a given magnetic field strength B, there is a minimal diode voltage Umin at which the diode is able to maintain a self-sustained operation. The values of Umin for a range of magnetic fields B have been linked to the potential of the diode electron flow V0=0.42 kV which is approximately ten times greater than the first cross-over voltage in the secondary emission coefficient function. The planar CFSE electron source is capable of operating in a long ∼2 ms regime. The generation of low-voltage (<10 kV) electron beams at an elevated background gas pressure of 10−2 mbar has been also demonstrated.

The clinical implementation of a method for calculating the output factor and per cent depth dose for an electron beam

A modified sector-integration method has been developed that predicts electron beam output factor at any point on the beam central axis, for a given source to surface distance (SSD), as a function of the geometry of the irradiated field. The main concept of this method is that with the arbitrary field shape divided into small sectors, the individual contributions from each sector can be calculated based on the sector radius, using a dataset consisting of circular inserts of standard radii. A computer program was developed based on this algorithm. The program interfaces to a digital camera that is used to capture the shape of the electron insert. We compared the calculated and the measured output factors and per cent depth doses (PDDs) at different SSDs for various rectangular inserts and a typical irregularly shaped insert used in our clinic. To determine the geometric limitations of this algorithm, a series of rectangular inserts were designed with the long-to-short axis ratio between 1:1 and 7:1. The agreement between calculation and measurement for the electron output and PDD was generally within 2% (or 2 mm) for energies from 6 to 20 MeV.

Electron beam therapy at extended SSDs: an analysis of output correction factors for a Mitsubishi linear accelerator

The effects of extended source-to-surface distance (SSD) on the electron beam dose profiles were evaluated for various electron beam energies—6, 9, 12, 15 and 20 MeV—and the accuracy of various output correction methods was analysed on a Mitsubishi linear accelerator using a radiation field analyser (RFA). The dose fall-off region of the central axis depth–dose curves was nearly independent for SSDs up to 120 cm where as in the build-up region, a marginal reduction of surface dose was observed, particularly for lower energies and for smaller field sizes. Effective SSDs and virtual source distances were evaluated for field sizes ranging from 5 × 5 to 15 × 15 cm2 for various energies. Curve fitting was done with the measured outputs with various equations and coefficients were evaluated. The accuracy of the derived output correction factors by effective SSD, virtual source distance and curve-fit methods was assessed by evaluating correlation coefficients between the calculated and the measured values. The correlation coefficient was best with the linear-quadratic equation followed by the effective SSD method and the virtual source method. The output correction based on the linear-quadratic equation showed the best estimate of electron beam output at extended SSDs with an accuracy well within ±1%. The rapid reduction of dose due to the applicator-scattered component at dmax point with an extended SSD was significant for the 5 × 5 cm2 applicator and lower energies.

Two-dimensional simulation and modeling in scanning electron microscope imaging and metrology research.

Traditional Monte Carlo modeling of the electron beam-specimen interactions in a scanning electron microscope (SEM) produces information about electron beam penetration and output signal generation at either a single beam-landing location, or multiple landing positions. If the multiple landings lie on a line, the results can be graphed in a line scan-like format. Monte Carlo results formatted as line scans have proven useful in providing one-dimensional information about the sample (e.g., linewidth). When used this way, this process is called forward line scan modeling. In the present work, the concept of image simulation (or the first step in the inverse modeling of images) is introduced where the forward-modeled line scan data are carried one step further to construct theoretical two-dimensional (2-D) micrographs (i.e., theoretical SEM images) for comparison with similar experimentally obtained micrographs. This provides an ability to mimic and closely match theory and experiment using SEM images. Calculated and/or measured libraries of simulated images can be developed with this technique. The library concept will prove to be very useful in the determination of dimensional and other properties of simple structures, such as integrated circuit parts, where the shape of the features is preferably measured from a single top-down image or a line scan. This paper presents one approach to the generation of 2-D simulated images and presents some suggestions as to their application to critical dimension metrology.

Low emittance electron beam formation with a 17 GHz RF gun

We report on electron beam quality measurement results from the Massachusetts Institute of Technology 17 GHz RF gun experiment. The 1.5 cell RF gun uses a solenoid for emittance compensation. It has produced bunch charges up to 0.1 nC with beam energies up to 1 MeV. The normalized rms emittance of the beam after 35 cm of transport from the gun has been measured by a slit technique to be $3\ensuremath{\pi}\mathrm{mm}\mathrm{mrad}$ for a 50 pC bunch. This agrees well with PARMELA simulations at these beam energies. At the exit of the electron gun, we estimate the emittance to be about $1\ensuremath{\pi}\mathrm{mm}\mathrm{mrad}$, which corresponds to a beam brightness of about $80\mathrm{A}/(\ensuremath{\pi}\mathrm{mm}\mathrm{mrad}{)}^{2}$. Improved beam quality should be possible with a higher energy output electron beam from the gun.