D Constantin

Rapid Monte Carlo simulation of detector DQE(f)

PURPOSE Performance optimization of indirect x-ray detectors requires proper characterization of both ionizing (gamma) and optical photon transport in a heterogeneous medium. As the tool of choice for modeling detector physics, Monte Carlo methods have failed to gain traction as a design utility, due mostly to excessive simulation times and a lack of convenient simulation packages. The most important figure-of-merit in assessing detector performance is the detective quantum efficiency (DQE), for which most of the computational burden has traditionally been associated with the determination of the noise power spectrum (NPS) from an ensemble of flood images, each conventionally having 10(7) - 10(9) detected gamma photons. In this work, the authors show that the idealized conditions inherent in a numerical simulation allow for a dramatic reduction in the number of gamma and optical photons required to accurately predict the NPS. METHODS The authors derived an expression for the mean squared error (MSE) of a simulated NPS when computed using the International Electrotechnical Commission-recommended technique based on taking the 2D Fourier transform of flood images. It is shown that the MSE is inversely proportional to the number of flood images, and is independent of the input fluence provided that the input fluence is above a minimal value that avoids biasing the estimate. The authors then propose to further lower the input fluence so that each event creates a point-spread function rather than a flood field. The authors use this finding as the foundation for a novel algorithm in which the characteristic MTF(f), NPS(f), and DQE(f) curves are simultaneously generated from the results of a single run. The authors also investigate lowering the number of optical photons used in a scintillator simulation to further increase efficiency. Simulation results are compared with measurements performed on a Varian AS1000 portal imager, and with a previously published simulation performed using clinical fluence levels. RESULTS On the order of only 10-100 gamma photons per flood image were required to be detected to avoid biasing the NPS estimate. This allowed for a factor of 10(7) reduction in fluence compared to clinical levels with no loss of accuracy. An optimal signal-to-noise ratio (SNR) was achieved by increasing the number of flood images from a typical value of 100 up to 500, thereby illustrating the importance of flood image quantity over the number of gammas per flood. For the point-spread ensemble technique, an additional 2× reduction in the number of incident gammas was realized. As a result, when modeling gamma transport in a thick pixelated array, the simulation time was reduced from 2.5 × 10(6) CPU min if using clinical fluence levels to 3.1 CPU min if using optimized fluence levels while also producing a higher SNR. The AS1000 DQE(f) simulation entailing both optical and radiative transport matched experimental results to within 11%, and required 14.5 min to complete on a single CPU. CONCLUSIONS The authors demonstrate the feasibility of accurately modeling x-ray detector DQE(f) with completion times on the order of several minutes using a single CPU. Convenience of simulation can be achieved using GEANT4 which offers both gamma and optical photon transport capabilities.

A study of the effect of in-line and perpendicular magnetic fields on beam characteristics of electron guns in medical linear accelerators

PURPOSE Using magnetic resonance imaging (MRI) for real-time guidance during radiotherapy is an active area of research and development. One aspect of the problem is the influence of the MRI scanner, modeled here as an external magnetic field, on the medical linear accelerator (linac) components. The present work characterizes the behavior of two medical linac electron guns with external magnetic fields for in-line and perpendicular orientations of the linac with respect to the MRI scanner. METHODS Two electron guns, Litton L-2087 and Varian VTC6364, are considered as representative models for this study. Emphasis was placed on the in-line design approach in which case the MRI scanner and the linac axes of symmetry coincide and assumes no magnetic shielding of the linac. For the in-line case, the magnetic field from a 0.5 T open MRI (GE Signa SP) magnet with a 60 cm gap between its poles was computed and used in full three dimensional (3D) space charge simulations, whereas for the perpendicular case the magnetic field was constant. RESULTS For the in-line configuration, it is shown that the electron beam is not deflected from the axis of symmetry of the gun and the primary beam current does not vanish even at very high values of the magnetic field, e.g., 0.16 T. As the field strength increases, the primary beam current has an initial plateau of constant value after which its value decreases to a minimum corresponding to a field strength of approximately 0.06 T. After the minimum is reached, the current starts to increase slowly. For the case when the beam current computation is performed at the beam waist position the initial plateau ends at 0.016 T for Litton L-2087 and at 0.012 T for Varian VTC6364. The minimum value of the primary beam current is 27.5% of the initial value for Litton L-2087 and 22.9% of the initial value for Varian VTC6364. The minimum current is reached at 0.06 and 0.062 T for Litton L-2087 and Varian VTC6364, respectively. At 0.16 T the beam current increases to 40.2 and 31.4% from the original value of the current for Litton L-2087 and Varian VTC6364, respectively. In contrast, for the case when the electron gun is perpendicular to the magnetic field, the electron beam is deflected from the axis of symmetry even at small values of the magnetic field. As the strength of the magnetic field increases, so does the beam deflection, leading to a sharp decrease of the primary beam current which vanishes at about 0.007 T for Litton L-2087 and at 0.006 T for Varian VTC6364, respectively. At zero external field, the beam rms emittance computed at beam waist is 1.54 and 1.29n-mm-mrad for Litton L-2087 and Varian VTC6364, respectively. For the inline configuration, there are two particular values of the external field where the beam rms emittance reaches a minimum. Litton L-2087 rms emittance reaches a minimum of 0.72n and 2.01 n-mm-mrad at 0.026 and 0.132 T, respectively. Varian VTC6364 rms emittance reaches a minimum of 0.34n and 0.35n-mm-mrad at 0.028 and 0.14 T, respectively. Beam radius dependence on the external field is shown for the in-line configuration for both electron guns. CONCLUSIONS 3D space charge simulation of two electron guns, Litton L-2087 and Varian VTC6364, were performed for in-line and perpendicular external magnetic fields. A consistent behavior of Pierce guns in external magnetic fields was proven. For the in-line configuration, the primary beam current does not vanish but a large reduction of beam current (up to 77.1%) is observed at higher field strengths; the beam directionality remains unchanged. It was shown that for a perpendicular configuration the current vanishes due to beam bending under the action of the Lorentz force. For in-line configuration it was determined that the rms beam emittance reaches two minima for relatively high values of the external magnetic field.

Linking computer-aided design (CAD) to Geant4-based Monte Carlo simulations for precise implementation of complex treatment head geometries

Most of the treatment head components of medical linear accelerators used in radiation therapy have complex geometrical shapes. They are typically designed using computer-aided design (CAD) applications. In Monte Carlo simulations of radiotherapy beam transport through the treatment head components, the relevant beam-generating and beam-modifying devices are inserted in the simulation toolkit using geometrical approximations of these components. Depending on their complexity, such approximations may introduce errors that can be propagated throughout the simulation. This drawback can be minimized by exporting a more precise geometry of the linac components from CAD and importing it into the Monte Carlo simulation environment. We present a technique that links three-dimensional CAD drawings of the treatment head components to Geant4 Monte Carlo simulations of dose deposition.

A novel electron gun for inline MRI‐linac configurations

PURPOSE This work introduces a new electron gun geometry capable of robust functioning in the presence of a high strength external magnetic field for axisymmetric magnetic resonance imaging (MRI)-linac configurations. This allows an inline MRI-linac to operate without the need to isolate the linear accelerator (linac) using a magnetic shield. This MRI-linac integration approach not only leaves the magnet homogeneity unchanged but also provides the linac flexibility to move along the magnet axis of symmetry if the source to target distance needs to be adjusted. METHODS Simple electron gun geometry modifications of a Varian 600 C electron gun are considered and solved in the presence of an external magnetic field in order to determine a set of design principles for the new geometry. Based on these results, a new gun geometry is proposed and optimized in the fringe field of a 0.5 T open bore MRI magnet (GE Signa SP). A computer model for the 6 MeV Varian 600 C linac is used to determine the capture efficiency of the new electron gun-linac system in the presence of the fringe field of the same MRI scanner. The behavior of the new electron gun plus the linac system is also studied in the fringe fields of two other magnets, a 1.0 T prototype open bore magnet and a 1.5 T GE Conquest scanner. RESULTS Simple geometrical modifications of the original electron gun geometry do not provide feasible solutions. However, these tests show that a smaller transverse cathode diameter with a flat surface and a slightly larger anode diameter could alleviate the current loss due to beam interactions with the anode in the presence of magnetic fields. Based on these findings, an initial geometry resembling a parallel plate capacitor with a hole in the anode is proposed. The optimization procedure finds a cathode-anode distance of 5 mm, a focusing electrode angle of 5°, and an anode drift tube length of 17.1 mm. Also, the linac can be displaced with ± 15 cm along the axis of the 0.5 T magnet without capture efficiency reduction below the experimental value in zero field. In this range of linac displacements, the electron beam generated by the new gun geometry is more effectively injected into the linac in the presence of an external magnetic field, resulting in approximately 20% increase of the target current compared to the original gun geometry behavior at zero field. The new gun geometry can generate and accelerate electron beams in external magnetic fields without current loss for fields higher than 0.11 T. The new electron-gun geometry is robust enough to function in the fringe fields of the other two magnets with a target current loss of no more than 16% with respect to the current obtained with no external magnetic fields. CONCLUSIONS In this work, a specially designed electron gun was presented which can operate in the presence of axisymmetric strong magnetic fringe fields of MRI magnets. Computer simulations show that the electron gun can produce high quality beams which can be injected into a straight through linac such as Varian 600 C and accelerated with more efficiency in the presence of the external magnetic fields. Also, the new configuration allows linac displacements along the magnet axis in a range equal to the diameter of the imaging spherical volume of the magnet under consideration. The new electron gun-linac system can function in the fringe field of a MRI magnet if the field strength at the cathode position is higher than 0.11 T. The capture efficiency of the linac depends on the magnetic field strength and the field gradient. The higher the gradient the better the capture efficiency. The capture efficiency does not degrade more than 16%.

Performance of a clinical gridded electron gun in magnetic fields: Implications for MRI‐linac therapy

PURPOSE MRI-linac therapy is a rapidly growing field, and requires that conventional linear accelerators are operated with the fringe field of MRI magnets. One of the most sensitive accelerator components is the electron gun, which serves as the source of the beam. The purpose of this work was to develop a validated finite element model (FEM) model of a clinical triode (or gridded) electron gun, based on accurate geometric and electrical measurements, and to characterize the performance of this gun in magnetic fields. METHODS The geometry of a Varian electron gun was measured using 3D laser scanning and digital calipers. The electric potentials and emission current of these guns were measured directly from six dose matched true beam linacs for the 6X, 10X, and 15X modes of operation. Based on these measurements, a finite element model (FEM) of the gun was developed using the commercial software opera/scala. The performance of the FEM model in magnetic fields was characterized using parallel fields ranging from 0 to 200 G in the in-line direction, and 0-35 G in the perpendicular direction. RESULTS The FEM model matched the average measured emission current to within 5% across all three modes of operation. Different high voltage settings are used for the different modes; the 6X, 10X, and 15X modes have an average high voltage setting of 15, 10, and 11 kV. Due to these differences, different operating modes show different sensitivities in magnetic fields. For in line fields, the first current loss occurs at 40, 20, and 30 G for each mode. This is a much greater sensitivity than has previously been observed. For perpendicular fields, first beam loss occurred at 8, 5, and 5 G and total beam loss at 27, 22, and 20 G. CONCLUSIONS A validated FEM model of a clinical triode electron gun has been developed based on accurate geometric and electrical measurements. Three different operating modes were simulated, with a maximum mean error of 5%. This gun shows greater sensitivity to in-line magnetic fields than previously presented models, and different operating modes show different sensitivity.

SU‐D‐103‐02: Image Quality Assurance Study of a Cone‐Beam C‐Arm CT with Automatic Exposure Control for Body Applications

PURPOSE To evaluate the influence of peak x-ray tube voltage and the size of the Z field of view (FOV) on body image quality of a cone-beam C-arm CT system with automatic exposure control. METHODS We measured dose accumulated in an elliptical-shaped body phantom with tissue equivalent density using a small ion chamber at 23 distributed points following the AAPM TG111 approach at two tube voltage requests (109kVp, 125kVp), 4 detector dose requests (0.17, 0.36, 0.54, and 0.81μGy/frame at the detector), and 3 FOVs (small, medium, and large in Z). For dose efficiency analysis, we scanned the same phantom again after replacing the central cylindrical part with the QRM cone-beam phantom which has 20 inserts of various diameters and contrast steps. Six experienced observers were asked to count the number of visible circles in slices reconstructed with 1 or 5mm thickness, 0.5 isotropic in-plane pixel size, and Siemens medium smooth convolution kernel. RESULTS After dose normalization, fifty percent of objects with a diameter of 6.3, 4.4, 4.2, 2.2 mm at 109kVp and 6.5, 5.1, 3.8, and 3.0 mm at 125kVp having a nominal contrast of 2.0, 2.5, 3.0, and 4.5%, respectively were detectable at a diagnostic reference dose level for routine abdomen of 35mGy. Small, medium, and large FOVs at a 125kVp and 0.36 μGy/frame setting showed 46.3, 41.5, and 37.3% detectability with a mean dose of 43.5, 48.4, and 50.4 mGy, respectively. CONCLUSION The detectability of the C-arm CT images improved significantly with z-direction collimation, and the lower kVp protocol setting of 109 kVp provided improved detectability over 125 kVp after dose normalization even for this relatively large body phantom. This work was supported by National Institutes of Health (NIH SIG S10 RR026714-01), by Siemens Medical Solutions, AX, and by the Lucas Foundation.

WE-G-214-05: Robotic Linac Adaptation (RLA) with a Novel Electron Gun Design for the In-Line MRI-Linac Configuration

Purpose: The emergence of MRI based guidance systems in radiation therapy has the potential for real‐time volumetric imaging and targeting. This work investigates a continuous delivery technique for an in‐line MRI‐linac system with a non‐MR‐shielded linac. Similar to the CyberKnife(TM) system the linac tracks the tumor motion and adapts its orientation in the fringe field of the magnet. Methods: An electron gun model resembling a plane capacitor is proposed to function with an unmodified Varian 600C linac. Space charge and electron transport simulations were used to characterize the electron gun‐linac system in the fringe field of a 0.5T open bore MRI scanner (GE Signa SP) at 0.18T. The assembly was displaced off the axis of symmetry and moved along the magnetic field lines to account for tumor motion and the beam characteristics of the systems were determined. Results: The minimum transverse rms emittance of the beam is ∼6.4 pi‐mm‐mrad 4.4mm away from the electron gun cathode with a current of 360mA. This compares favorably with the performance of a standard Pierce‐type gun placed at the same location in the magnet, which demonstrates significant reduction of current and/or emittance as high as 25 pi‐mm‐mrad. We show that the linac can be displaced off axis with minimal reduction in capture efficiency while maintaining low emittance if the linac is aligned with the field lines. The linac capture efficiency, the beam current at the gun exit, and the beam current at the linac exit were computed within the domain of linac motion. Conclusions: A specially designed electron gun‐linac assembly can function in the fringe field of a MRImagnet for various relative orientations of the linac with respect to the MR scanner. This technique allows the linac to simultaneously deliver dose and continuously track and adapt its position based on tumor motion.

SU‐D‐103‐07: Dose Measurement and Estimation Methods in An Elliptical Body Phantom for a Conebeam CT System

PURPOSE To propose new metrics to estimate a mean dose of an automatic exposure control-enabled angiographic C-arm system over a noncircular large body-shaped phantom based on multi-point dose measurements. METHODS Dose was measured at 9 points in 2 central subregions (C1, C2) and 14 points in 2 peripheral subregions (P1, P2) of the body phantom using a small 0.6cc ion chamber (IC) while operating the system at 16 different combinations of tube voltage, detector dose request, and vertical collimation. In order to acquire complete 2D dose profiles in an axial direction, we carried out Monte Carlo (MC) simulations. After validating the MC model by comparing it to chamber values, the mean dose from MC simulations was used as a ground truth for our mean dose metrics. Mean dose was estimated in 3 ways: 1) Area ratio for each point weights the contribution of the point. 2) Point dose surface fitting method using biharmonic interpolation model. 3) The acquired MC dose profile-based MC template method. We investigated how each methods performance varies as a function of the number of measured data points (7∼23 points). RESULTS The error of MC compared to chamber readings was 0.9mGy (+/- 0.03 mGy) per point. The relative errors of 1), 2), and 3) methods with 23 IC points in comparison with the MC mean dose were 0.6, 3.37, and 1.9%, respectively. Method 1 performed best for 6 different cases of number of points measured. However, its performance fluctuated compared to methods 2 and 3. Method 3 remained within 3% error with 23∼8 points and showed the most stable performance. Method 2 performed worst with 23∼11 points, with constant error of ∼ 5%. CONCLUSION The 3 metrics estimated a mean dose accumulated in a body phantom with about 5% relative error using only 11 points. This work was supported by National Institutes of Health (NIH SIG S10 RR026714-01), by Siemens Medical Solutions, AX, and by the Lucas Foundation. There is no conflict of interest to disclose.

SU‐GG‐T‐407: Modeling a New Varian Linac Using a CAD to Geant4 Geometry Implementation: Dose and IAEA‐Compliant Phase Space Calculations

Purpose: To (1) use accurate CAD linac treatment head geometry for Geant4 Monte Carlo calculations to minimize the errors due to approximations of the linac treatment head components used in previous studies, (2) investigate the CAD to Geant4 linking procedure through 6MV dose calculations, and (3) provide IAEA‐compliant patient‐independent phase space files. Method and Materials: Geant4 v4.9.2.p01 was employed to simulate the treatment head geometry of the newest Varian linac for a 6MV beam. The geometry components are tessellated solids included in Geant4 as GDML (Generalized Dynamic Markup Language) files obtained via STEP (STandard for the Exchange of Product) export from Pro/Engineering, followed by STEP import in Fastrad, a STEP‐GDML converter. An IAEA‐compliant phase space writer was implemented between the shielding collimator and the upper jaws on a cylindrical geometry due to the compact treatment head and the divergent arc of the jaws. The simulation was run in parallel on the SLAC cluster. The dose calculations were based on a combined total of 26 billion histories. The voxel size for the 60×60×40cm3 water phantom was 4×4×4 mm3. Results: For the percent depth dose profiles, the agreement between experiment and Monte Carlo is within 2% for 4×4, 10×10, 20×20, 30×30 and 40×40 cm2field sizes. For the lateral dose profiles, the agreement is within 3%. The fractional uncertainty of the average dose for the voxels with dose larger than 0.5 of the maximum dose decreased from ∼4% to ∼2% for field sizes increasing from 4×4 to 40×40 cm2. Conclusions: We have developed and validated the Geant4 simulated IAEA‐compliant phase space of the new Varian linac for the 6MV beam obtained using a high precision geometry implementation from CAD. These files will be made publicly available and could be used for further dose calculations. Supported in part by Varian Medical Systems

TH‐D‐BRB‐03: Monte Carlo Simulations of Beam Characteristics for a Compact Plasma Proton Accelerator

Purpose: To (1) develop Monte Carlo applications for the geometry and particle transport in the treatment head of a compact coaxial plasmaproton accelerator and (2) quantify the sensitivity of the output particle flux and dose distributions on the energy and angular spread of the input proton phase space. Method and Materials: Geant4 v4.9.3 was employed to simulate a plasmaprotontreatment head geometry including the beam collimator the magnetic field region and the energy selector. The energy spectrum assumed a Gaussian distribution centered at Emax=200MeV and the full width at half maximum (FWHM) was 0% 12.5% 25% 37.5% and 50% of Emax. The angular spread of the input proton beam was considered to be 0° 15° and 20° respectively. Flux and dose calculations were performed using a phase space implemented at the energy selector exit plane and a voxelized water phantom respectively. Results: For a fixed angular spread the output particle flux is fairly constant as the energy spectrum becomes broader while for a fixed energy spread the flux decreases significantly with increasing angular spread. The decrease is 10 times for 15° and 40 times for 20° angular spread. Dose measurements show that 200MeV protons even with a broad energy spectrum are preferable to photons when low entrance dose and conformal distal edge falloff are desired. The energy spread translates into a spread out Bragg peak used clinically without the introduction of any scattering devices in the beam path. Conclusions: This study shows that the delivered proton flux is highly sensitive to the angular spectrum of the input plasma accelerated protons and dose is sensitive to the spread of the energy spectrum. Monte Carlo simulations of beam characteristics for compact plasmaproton accelerators provide important design parameters for the development of the next generation proton therapy units.