Minimal skin dose increase in longitudinal rotating biplanar linac-MR systems: examination of radiation energy and flattening filter design.

The magnetic fields of linac-MR systems modify the path of contaminant electrons in photon beams, which alters patient entrance skin dose. Also, the increased SSD of linac-MR systems reduces the maximum achievable dose rate. To accurately quantify the changes in entrance skin dose, the authors use EGSnrc Monte Carlo calculations that incorporate 3D magnetic field of the Alberta 0.5 T longitudinal linac-MR system. The Varian 600C linac head geometry assembled on the MRI components is used in the BEAMnrc simulations for 6 MV and 10 MV beam models and skin doses are calculated at an average depth of 70 µm using DOSXYZnrc. 3D modeling shows that magnetic fringe fields decay rapidly and are small at the linac head. SSDs between 100 and 120 cm result in skin-dose increases of between ~6%-19% and ~1%-9% for the 6 and 10 MV beams, respectively. For 6 MV, skin dose increases from ~10.5% to ~1.5% for field-size increases of 5 × 5 cm(2) to 20 × 20 cm(2). For 10 MV, skin dose increases by ~6% for a 5 × 5 cm(2) field, and decreases by ~1.5% for a 20 × 20 cm(2) field. Furthermore, the proposed reshaped flattening filter increases the dose rate from the current 355 MU min(-1) to 529 MU min(-1) (6 MV) or 604 MU min(-1) (10 MV), while the skin-dose increases by only an additional ~2.6% (all percent increases in skin dose are relative to D max). This study suggests that there is minimal increase in the entrance skin dose and minimal/no decrease in the dose rate of the Alberta longitudinal linac-MR system. The even lower skin dose increase at 10 MV offers further advantages in future designs of linac-MR prototypes.

FEM design and simulation of a short, 10 MV, S-band Linac with Monte Carlo dose simulations.

PURPOSE: Current commercial 10 MV Linac waveguides are 1.5 m. The authors' current 6 MV linear accelerator-magnetic resonance imager (Linac-MR) system fits in typical radiotherapy vaults. To allow 10 MV treatments with the Linac-MR and still fit within typical vaults, the authors design a 10 MV Linac with an accelerator waveguide of the same length (27.5 cm) as current 6 MV Linacs. METHODS: The first design stage is to design a cavity such that a specific experimental measurement for breakdown is applicable to the cavity. This is accomplished through the use of finite element method (FEM) simulations to match published shunt impedance, Q factor, and ratio of peak to mean-axial electric field strength from an electric breakdown study. A full waveguide is then designed and tuned in FEM simulations based on this cavity design. Electron trajectories are computed through the resulting radio frequency fields, and the waveguide geometry is modified by shifting the first coupling cavity in order to optimize the electron beam properties until the energy spread and mean energy closely match values published for an emulated 10 MV Linac. Finally, Monte Carlo dose simulations are used to compare the resulting photon beam depth dose profile and penumbra with that produced by the emulated 10 MV Linac. RESULTS: The shunt impedance, Q factor, and ratio of peak to mean-axial electric field strength are all matched to within 0.1%. A first coupling cavity shift of 1.45 mm produces an energy spectrum width of 0.347 MeV, very close to the published value for the emulated 10 MV of 0.315 MeV, and a mean energy of 10.53 MeV, nearly identical to the published 10.5 MeV for the emulated 10 MV Linac. The depth dose profile produced by their new Linac is within 1% of that produced by the emulated 10 MV spectrum for all depths greater than 1.5 cm. The penumbra produced is 11% narrower, as measured from 80% to 20% of the central axis dose. CONCLUSIONS: The authors have successfully designed and simulated an S-band waveguide of length of 27.5 cm capable of producing a 10 MV photon beam. This waveguide operates well within the breakdown threshold determined for the cavity geometry used. The designed Linac produces depth dose profiles similar to those of the emulated 10 MV Linac (waveguide-length of 1.5 m) but yields a narrower penumbra.

Feasibility of producing a short, high energy s-band linear accelerator using a klystron power source.

PURPOSE: To use a finite-element method (FEM) model to study the feasibility of producing a short s-band (2.9985 GHz) waveguide capable of producing x-rays energies up to 10 MV, for applications in a linac-MR, as well as conventional radiotherapy. METHODS: An existing waveguide FEM model developed by the authors' group is used to simulate replacing the magnetron power source with a klystron. Peak fields within the waveguide are compared with a published experimental threshold for electric breakdown. The RF fields in the first accelerating cavity are scaled, approximating the effect of modifications to the first coupling cavity. Electron trajectories are calculated within the RF fields, and the energy spectrum, beam current, and focal spot of the electron beam are analyzed. One electron spectrum is selected for Monte Carlo simulations and the resulting PDD compared to measurement. RESULTS: When the first cavity fields are scaled by a factor of 0.475, the peak magnitude of the electric fields within the waveguide are calculated to be 223.1 MV∕m, 29% lower than the published threshold for breakdown at this operating frequency. Maximum electron energy increased from 6.2 to 10.4 MeV, and beam current increased from 134 to 170 mA. The focal spot FWHM is decreased slightly from 0.07 to 0.05 mm, and the width of the energy spectrum increased slightly from 0.44 to 0.70 MeV. Monte Carlo results show dmax is at 2.15 cm for a 10 × 10 cm(2) field, compared with 2.3 cm for a Varian 10 MV linac, while the penumbral widths are 4.8 and 5.6 mm, respectively. CONCLUSIONS: The authors' simulation results show that a short, high-energy, s-band accelerator is feasible and electric breakdown is not expected to interfere with operation at these field strengths. With minor modifications to the first coupling cavity, all electron beam parameters are improved.

2D lag and signal nonlinearity correction in an amorphous silicon EPID and their impact on pretreatment dosimetric verification.

PURPOSE: This investigation provides measurements of signal lag and nonlinearity separately for the Varian aS500 electronic portal imaging device (EPID), and an algorithm to correct for these effects in 2D; their potential impact on intensity modulated radiation therapy (IMRT) verification is also investigated. The authors quantify lag, as a function of both delivered monitor units (MU) and time, by using a range of MUs delivered at a clinically used rate of 400 MU∕min. Explicit cumulative lag curves are thus determined for a range of MUs and times between the end of irradiation and the end of image acquisition. Signal nonlinearity is also investigated as a function of total MUs delivered. The family of cumulative lag curves and signal nonlinearity are then used to determine their effects on dynamic multileaf collimator (MLC) (IMRT) deliveries, and to correct for theses effects in 2D. METHODS: Images acquired with an aS500 EPID and Varis Portal-Vision software were used to quantify detector lag and signal-nonlinearity. For the signal lag investigation, Portal-Vision's service monitor was used to acquire EPID images at a rate of 8 frames/s. The images were acquired during irradiation and 66 s thereafter, by inhibiting the M-holdoff-In signal of the Linac for a range of 4.5-198.5 MUs. Relative cumulative lag was calculated by integrating the EPID signal for a time after beam-off, and normalizing this to the integrated EPID signal accumulated during radiation. Signal nonlinearity was studied by acquiring 10 × 10 cm(2) open-field EPID images in "integrated image" mode for a range of 2-500 MUs, and normalized to the 100 MU case. All data were incorporated into in-house written software to create a 2D correction map for these effects, using the field's MLC file and a field-specific calculated 2D "time-map," which keeps track of the time elapsed from the last fluence delivered at each given point in the image to the end of the beam delivery. RESULTS: Relative cumulative lag curves reveal that the lag alone can deviate the EPID's perceived dose by as large as 6% (1 MU delivery, 60 s postirradiation). For signal nonlinearity relative to 100 MU, EPID signals per MU of 0.84 and 1.01 were observed for 2 and 500 MUs, respectively. Correction maps were applied to a 1 cm sweeping-window 14 × 14 cm(2) field and clinical head-and-neck IMRT field. A mean correction of 1.028 was implemented in the head-and-neck field, which significantly reduced lag-related asymmetries in the EPID images, and restored linearity to the EPID imager's dose response. Corrections made to the sweeping-field showed good agreement with the treatment planning system-predicted field, yielding an average percent difference of 0.05% ± 0.91%, compared to the -1.32% ± 1.02% before corrections, or 1.75% ± 1.04% when only a signal nonlinearity correction is made. CONCLUSIONS: Lag and signal-nonlinearity have been quantified for an aS500 EPID imager, and an effective 2D correction method has been developed which effectively removes nonlinearity and lag effects. Both of these effects were shown to negatively impact IMRT verifications. Especially fields that involve prolonged irradiation and small overall MUs should be corrected for in 2D.

Magnetic shielding investigation for a 6 MV in-line linac within the parallel configuration of a linac-MR system.

PURPOSE: In our current linac-magnetic resonance (MR) design, a 6 MV in-line linac is placed along the central axis of the MR's magnet where the MR's fringe magnetic fields are parallel to the overall electron trajectories in the linac waveguide. Our previous study of this configuration comprising a linac-MR SAD of 100 cm and a 0.5 T superconducting (open, split) MR imager. It showed the presence of longitudinal magnetic fields of 0.011 T at the electron gun, which caused a reduction in target current to 84% of nominal. In this study, passive and active magnetic shielding was investigated to recover the linac output losses caused by magnetic deflections of electron trajectories in the linac within a parallel linac-MR configuration. METHODS: Magnetic materials and complex shield structures were used in a 3D finite element method (FEM) magnetic field model, which emulated the fringe magnetic fields of the MR imagers. The effects of passive magnetic shielding was studied by surrounding the electron gun and its casing with a series of capped steel cylinders of various inner lengths (26.5-306.5 mm) and thicknesses (0.75-15 mm) in the presence of the fringe magnetic fields from a commercial MR imager. In addition, the effects of a shield of fixed length (146.5 mm) with varying thicknesses were studied against a series of larger homogeneous magnetic fields (0-0.2 T). The effects of active magnetic shielding were studied by adding current loops around the electron gun and its casing. The loop currents, separation, and location were optimized to minimize the 0.011 T longitudinal magnetic fields in the electron gun. The magnetic field solutions from the FEM model were added to a validated linac simulation, consisting of a 3D electron gun (using OPERA-3d/scala) and 3D waveguide (using comsol Multiphysics and PARMELA) simulations. PARMELA's target current and output phase-space were analyzed to study the linac's output performance within the magnetic shields. RESULTS: The FEM model above agreed within 1.5% with the manufacturer supplied fringe magnetic field isoline data. When passive magnetic shields are used, the target current is recoverable to greater than 99% of nominal for shield thicknesses greater than 0.75 mm. The optimized active shield which resulted in 100% target current recovery consists of two thin current rings 110 mm in diameter with 625 and 430 A-turns in each ring. With the length of the passive shield kept constant, the thickness of the shield had to be increased to achieve the same target current within the increased longitudinal magnetic fields. CONCLUSIONS: A ≥99% original target current is recovered with passive shield thicknesses >0.75 mm. An active shield consisting of two current rings of diameter of 110 mm with 625 and 430 A-turns fully recovers the loss that would have been caused by the magnetic fields. The minimal passive or active shielding requirements to essentially fully recover the current output of the linac in our parallel-configured linac-MR system have been determined and are easily achieved for practical implementation of the system.

MO-A-213AB-02: Redesign of a 6 MV Linear Accelerator Waveguide to Produce Energies Up to 10 MV.

PURPOSE: To investigate the feasibility of producing a short, high-energy linear accelerator for use in a proposed hybrid linear accelerator magnetic resonance imager (linac-MRI). METHODS: A short 6MV waveguide was previously simulated in COMSOL and benchmarked against experiment. The simulated input power is increased from 2.5 to 7.5 MW to reflect replacing the magnetron power source with a commercially available klystron, and the RF fields within the waveguide are calculated. The RF solution is used as an input into PARMELA, an electron-tracking software, to calculate the electron energy and spatial distribution exiting the waveguide. The electric fields within the waveguide are compared with experimental thresholds for electric breakdown within the waveguide to determine the possibility of operation at increased input power. The energy spectrum of the electron beam incident on the target is analyzed for suitability for radiotherapy. Finally, some potential modifications to the simulated cavity dimensions and positioning are discussed, and a preliminary estimate of the effects on the electron distributions are analyzed. RESULTS: When the input power is increased, peak surface electric fields within the waveguide of 215 MV/m are calculated, below the threshold determined by experiment of 240 - 300 MV/m for similar resonant structures. The FWHM of the electron focal spot is shown to be 1.5 times larger than the focal spot from the unmodified waveguide. The maximum electron energy increases from 6.1 to 10.6 MeV and the spread of electron energies is 5 times larger than the original. The modifications to the first cavity are shown to reduce the focal spot and energy spread to be comparable to the unmodified waveguide. CONCLUSIONS: It is feasible to produce a high-energy waveguide that is short enough for use in our linac-MRI. Slight modifications to the existing waveguide design will be required to optimize beam parameters for treatment. ACF Graduate Studentship.

Magnetic decoupling of the linac in a low field biplanar linac-MR system.

PURPOSE: The integration of a low field biplanar magnetic resonance (MR) imager and linear accelerator (linac) causes magnetic interference at the linac due to the MR fringe fields. In order to eliminate this interference, passive and active magnetic shielding designs are investigated. METHODS: The optimized design of passive magnetic shielding was performed using the finite element method. The design was required to achieve no greater than a 20% electron beam loss within the linac waveguide and electron gun, no greater than 0.06 T at the multileaf collimator (MLC) motors, and generate a distortion of the main MR imaging volume of no greater than 300 ppm. Through the superposition of the analytical solution for a single current carrying wire loop, active shielding designs in the form of three and four sets of coil pairs surrounding the linac waveguide and electron gun were also investigated. The optimized current and coil center locations that yielded the best cancellation of the MR fringe fields at the linac were determined using sequential quadratic programming. RESULTS: Optimized passive shielding in the form of two steel cylinders was designed to meet the required constraints. When shielding the MLC motors along with the waveguide and electron gun, the thickness of the cylinders was less than 1 mm. If magnetically insensitive MLC motors are used, no MLC shielding would be required and the waveguide shield (shielding the waveguide and electron gun) became 1.58 mm thick. In addition, the optimized current and coil spacing for active shielding was determined for both three and four coil pair configurations. The results of the active shielding optimization produced no beam loss within the waveguide and electron gun and a maximum MR field distortion of 91 ppm over a 30 cm diameter spherical volume. CONCLUSIONS: Very simple passive and active shielding designs have been shown to magnetically decouple the linac from the MR imager in a low field biplanar linac-MR system. The MLC passive shielding produced the largest distortion of the MR field over the imaging volume. With the use of magnetically insensitive motors, the MR field distortion drops substantially since no MLC shield is required. The active shielding designs yielded no electron beam loss within the linac.

Waveguide detuning caused by transverse magnetic fields on a simulated in-line 6 MV linac.

PURPOSE: Due to the close proximity of the linear accelerator (linac) to the magnetic resonance (MR) imager in linac-MR systems, it will be subjected to magnet fringe fields larger than the Earth's magnetic field of 5 x 10(-5) T. Even with passive or active shielding designed to reduce these fields, some magnitude of the magnetic field is still expected to intersect the linac, causing electron deflection and beam loss. This beam loss, resulting from magnetic fields that cannot be eliminated with shielding, can cause a detuning of the waveguide due to excessive heating. The detuning, if significant, could lead to an even further decrease in output above what would be expected strictly from electron deflections caused by an external magnetic field. Thus an investigation of detuning was performed through various simulations. METHODS: According to the Lorentz force, the electrons will be deflected away from their straight course to the target, depositing energy as they impact the linac copper waveguide. The deposited energy would lead to a heating and deformation of the copper structure resulting in resonant frequency changes. PARMELA was used to determine the mean energy and fraction of total beam lost in each linac cavity. The energy deposited into the copper waveguide from the beam losses caused by transverse magnetic fields was calculated using the Monte Carlo program DOSRZnrc. From the total energy deposited, the rise in temperature and ultimately the deformation of the structure was estimated. The deformed structure was modeled using the finite element method program COMSOL MULTIPHYSICS to determine the change in cavity resonant frequency. RESULTS: The largest changes in resonant frequency were found in the first two accelerating cavities for each field strength investigated. This was caused by a high electron fluence impacting the waveguide inner structures coupled with their low kinetic energies. At each field strength investigated, the total change in accelerator frequency was less than a manufacturing tolerance of 10 kHz and is thus not expected to have a noticeable effect on accelerator performance. CONCLUSIONS: The amount of beam loss caused by magnetic fringe fields for a linac in a linac-MR system depends on the effectiveness of its magnetic shielding. Despite the best efforts to shield the linac from the magnetic fringe fields, some persistent magnetic field is expected which would result in electron beam loss. This investigation showed that the detuning of the waveguide caused by additional electron beam loss in persistent magnetic fields is not a concern.

Effect of longitudinal magnetic fields on a simulated in-line 6 MV linac.

PURPOSE: Linac-magnetic resonance (MR) systems have been proposed in order to achieve realtime image guided radiotherapy. The design of a new linac-MR system with the in-line 6 MV linac generating x-rays along the symmetry axis of an open MR imager is outlined. This new design allows for a greater MR field strength to achieve better quality images while reducing hot and cold spots in treatment planning. An investigation of linac's performance in the longitudinal fringe magnetic fields of the MR imager is given. METHODS: The open MR imager fringe magnetic field was modeled using the analytic solution of the magnetic field generated from current carrying loops. The derived solution was matched to the magnetic fringe field isolines provided for a 0.5 T open MR imager through Monte Carlo optimization. The optimized field solution was then added to the previously validated 6 MV linac simulation to quantify linac's performance in the fringe magnetic field of a 0.5 T MR imager. To further the investigation, linac's performance in large fringe fields expected from other imagers was investigated through the addition of homogeneous longitudinal fields. RESULTS: The Monte Carlo optimization of the analytic current loop solution provided good agreement with the magnetic fringe field isolines supplied by the manufacturer. The range of magnetic fields the linac is expected to experience when coupled to the 0.5 T MR imager was determined to be from 0.0022 to 0.011 T (as calculated at the electron gun cathode). The effect of the longitudinal magnetic field on the electron beam was observed to be only in the electron gun. The longitudinal field changed the electron gun optics, affecting beam characteristics, such as a slight increase in the injection current and beam diameter, and an increasingly nonlaminar transverse phase space. Although the target phase space showed little change in its energy spectrum from the altered injection phase space, a reduction in the target current and spatial distribution peak intensity was observed. Despite these changes, the target phase space had little effect on the depth dose curves or dose profiles calculated for a 40 x 40 cm2 field at 1.5 cm depth. At longitudinal fields larger than 0.012 T, a drastic reduction in the injection current from the electron gun was observed due to a large fraction of electrons striking the anode. This further reduced the target current, which reached a minimum of 28 +/- 2 mA at 0.06 T. A slow increase in the injection and target currents was observed at fields larger than 0.06 T due to greater beam collimation in the anode beam tube. CONCLUSIONS: In an effort to achieve higher quality images and a reduction in hot and cold spots in the treatment plan, a parallel configuration linac-MR system is presented. The longitudinal magnetic fields of the MR imager caused large beam losses within the electron gun. These losses may be eliminated through a redesign of the electron gun optics incorporating a longitudinal magnetic field, or through magnetic shielding, which has already been proven successful for the transverse configuration.

Effect of transverse magnetic fields on a simulated in-line 6 MV linac.

The effects of a transverse magnetic field on an in-line side-coupled 6 MV linear accelerator are given. The results are directly applicable to a linac-MR system used for real-time image guided adaptive radiotherapy. Our previously designed end-to-end linac simulation incorporated the results from the axisymmetric 2D electron gun program EGN2w. However, since the magnetic fields being investigated are non-axisymmetric in nature for the work presented here, the electron gun simulation was performed using OPERA-3d/SCALA. The simulation results from OPERA-3d/SCALA showed excellent agreement with previous results. Upon the addition of external magnetic fields to our fully 3D linac simulation, it was found that a transverse magnetic field of 6 G resulted in a 45 +/- 1% beam loss, and by 14 G, no electrons were incident on the target. Transverse magnetic fields on the linac simulation produced a highly asymmetric focal spot at the target, which translated into a 13% profile asymmetry at 6 G. Upon translating the focal spot with respect to the target coordinates, profile symmetry was regained at the expense of a lateral shift in the dose profiles. It was found that all points in the penumbra failed a 1%/1 mm acceptance criterion for fields between 4 and 6 G. However, it was also found that the lateral profile shifts were corrected by adjusting the jaw positions asymmetrically.

An integrated 6 MV linear accelerator model from electron gun to dose in a water tank.

PURPOSE: The details of a full simulation of an inline side-coupled 6 MV linear accelerator (linac) from the electron gun to the target are presented. Commissioning of the above simulation was performed by using the derived electron phase space at the target as an input into Monte Carlo studies of dose distributions within a water tank and matching the simulation results to measurement data. This work is motivated by linac-MR studies, where a validated full linac simulation is first required in order to perform future studies on linac performance in the presence of an external magnetic field. METHODS: An electron gun was initially designed and optimized with a 2D finite difference program using Child's law. The electron gun simulation served as an input to a 6 MV linac waveguide simulation, which consisted of a 3D finite element radio-frequency field solution within the waveguide and electron trajectories determined from particle dynamics modeling. The electron gun design was constrained to match the cathode potential and electron gun current of a Varian 600C, while the linac waveguide was optimized to match the measured target current. Commissioning of the full simulation was performed by matching the simulated Monte Carlo dose distributions in a water tank to measured distributions. RESULTS: The full linac simulation matched all the electrical measurements taken from a Varian 600C and the commissioning process lead to excellent agreements in the dose profile measurements. Greater than 99% of all points met a 1%/1mm acceptance criterion for all field sizes analyzed, with the exception of the largest 40 x 40 cm2 field for which 98% of all points met the 1%/1mm acceptance criterion and the depth dose curves matched measurement to within 1% deeper than 1.5 cm depth. The optimized energy and spatial intensity distributions, as given by the commissioning process, were determined to be non-Gaussian in form for the inline side-coupled 6 MV linac simulated. CONCLUSIONS: An integrated simulation of an inline side-coupled 6 MV linac has been completed and benchmarked matching all electrical and dosimetric measurements to high accuracy. The results showed non-Gaussian spatial intensity and energy distributions for the linac modeled.

First MR images obtained during megavoltage photon irradiation from a prototype integrated linac-MR system.

The authors report the first magnetic resonance (MR) images produced by their prototype MR system integrated with a radiation therapy source. The prototype consists of a 6 MV linac mounted onto the open end of a biplanar 0.2 T permanent MR system which has 27.9 cm pole-to-pole opening with flat gradients (40 mT/m) running under a TMX NRC console. The distance from the magnet isocenter to the linac target is 80 cm. The authors' design has resolved the mutual interferences between the two devices such that the MR magnetic field does not interfere with the trajectory of the electron in the linac waveguide, and the radiofrequency (RF) signals from each system do not interfere with the operation of the other system. Magnetic and RF shielding calculations were performed and confirmed with appropriate measurements. The prototype is currently on a fixed gantry; however, in the very near future, the linac and MR magnet will rotate in unison such that the linac is always aimed through the opening in the biplanar magnet. MR imaging was found to be fully operational during linac irradiation and proven by imaging a phantom with conventional gradient echo sequences. Except for small changes in SNR, MR images produced during irradiation were visually and quantitatively very similar to those taken with the linac turned off. This prototype system provides proof of concept that the design has decreased the mutual interferences sufficiently to allow the development of real-time MR-guided radiotherapy. Low field-strength systems (0.2-0.5 T) have been used clinically as diagnostic tools. The task of the linac-MR system is, however, to provide MR guidance to the radiotherapy beam. Therefore, the 0.2 T field strength would provide adequate image quality for this purpose and, with the addition of fast imaging techniques, has the potential to provide 4D soft-tissue visualization not presently available in image-guided radiotherapy systems. The authors' initial design incorporates a permanent magnet; however, other types of magnets and field strengths could also be incorporated. Usable MR images were obtained during linac irradiation from the linac-MR prototype. The authors' prototype design can be used as the functional starting point in developing real-time MR guidance offering soft-tissue contrast that can be coupled with tumor tracking for real-time adaptive radiotherapy.

Sci-Thurs PM: Delivery-06: 2-D lag and response nonlinearity corrections for dynamic IMRT verification using an EPID.

In recent years, EPIDs have been used for pre-treatment IMRT verification. Although EPID lag and signal nonlinearities have been investigated, they have not been implemented in the verification process. In dynamic sliding-window IMRT delivery, the dose delivered, and the time between the end of dose delivery and the end of image acquisition differ between pixels. The resulting differences in lag and signal-response across the image can cause artificial asymmetries and amplitude changes in measured EPID dose images. These artifacts alter the agreement between measured and predicted images, potentially complicating the assessment of clinical IMRT verifications. A method of 2-D (pixel-by-pixel) correction was developed based on data from sets of experiments performed to independently quantify the lag and nonlinearity characteristics of Varian's aS500 EPID. To test the correction, it was applied to two sweeping window 10×10 cm2 fields that differ only in sweeping direction. The correction resolved discrepancies in the symmetry between these two cases, and the differences between measured and predicted amplitudes evident when small numbers of MUs were delivered. To illustrate its potential use, the correction technique was applied to a measured image of a clinical IMRT field that produced a relatively poor verification result. The correction partially accounted for discrepancies between measured and Eclipse-predicted images of this field, reducing the percentage of pixels failing a Gamma analysis (3 %, 3 mm) from 8.5 to 5.6 %. This correction technique can be used to help resolve the source of discrepancies in troublesome clinical IMRT verifications.

Sci-Fri AM: YIS-09: The effect of magnetic interference on a coupled MR-linac system: Optimization of 3D FEM linac model.

PURPOSE: The coupling of a 0.2T bi-planar Magnetic Resonance Imager and medical linear accelerator (linac) is proposed to provide real-time Image Guided Radiotherapy. This coupling necessitates the linac to be within the fringe fields of the bi-planar magnets causing magnetic interference. The design and optimization of the minimum required shielding is necessary to reduce the fringe field magnitudes to a point where a clinically useful radiation beam is produced. METHOD: A first step to designing shielding is the full 3D radio-frequency modeling of the linac waveguide using the Finite Element Method. Various optimizations were performed on the linac model in order to achieve a desired resonant frequency, π/2 phase shift per cavity and other desired properties. An accelerating cavity (AC) and coupling cavity (CC) was first optimized in 3D to have identical resonant frequencies before the full 3D model was generated. RESULTS: In order to increase the capture efficiency of the injected electrons, the electric field in the first AC was reduced by shifting the first CC towards the gun end of the linac. The input waveguide AC dimensions were adjusted to account of the additional coupling iris and the last full AC had its gap length decreased. CONCLUSION: This work is the first step to determining the minimum magnetic shielding required to produce a clinically useful radiation beam from a coupled MR-Linac system. The fully optimized 3D model more accurately calculates the electric and magnetic field values since it includes the effects of coupling.

Three-dimensional IMRT verification with a flat-panel EPID.

A three-dimensional (3D) intensity-modulated radiotherapy (IMRT) pretreatment verification procedure has been developed based on the measurement of two-dimensional (2D) primary fluence profiles using an amorphous silicon flat-panel electronic portal imaging device (EPID). As described in our previous work, fluence profiles are extracted from EPID images by deconvolution with kernels that represent signal spread in the EPID due to radiation and optical scattering. The deconvolution kernels are derived using Monte Carlo simulations of dose deposition in the EPID and empirical fitting methods, for both 6 and 15 MV photon energies. In our new 3D verification technique, 2D fluence modulation profiles for each IMRT field in a treatment are used as input to a treatment planning system (TPS), which then generates 3D doses. Verification is accomplished by comparing this new EPID-based 3D dose distribution to the planned dose distribution calculated by the TPS. Thermoluminescent dosimeter (TLD) point dose measurements for an IMRT treatment of an anthropomorphic phantom were in good agreement with the EPID-based 3D doses; in contrast, the planned dose under-predicts the TLD measurement in a high-gradient region by approximately 16%. Similarly, large discrepancies between EPID-based and TPS doses were also evident in dose profiles of small fields incident on a water phantom. These results suggest that our 3D EPID-based method is effective in quantifying relevant uncertainties in the dose calculations of our TPS for IMRT treatments. For three clinical head and neck cancer IMRT treatment plans, our TPS was found to underestimate the mean EPID-based doses in the critical structures of the spinal cord and the parotids by approximately 4 Gy (11%-14%). According to radiobiological modeling calculations that were performed, such underestimates can potentially lead to clinically significant underpredictions of normal tissue complication rates.

Modeling scintillator-photodiodes as detectors for megavoltage CT.

The use of cadmium tungstate (CdWO4) and cesium iodide [CsI(Tl)] scintillation detectors is studied in megavoltage computed tomography (MVCT). A model describing the signal acquired from a scintillation detector has been developed which contains two steps: (1) the calculation of the energy deposited in the crystal due to MeV photons using the EGSnrc Monte Carlo code; and (2) the transport of the optical photons generated in the crystal voxels to photodiodes using the optical Monte Carlo code DETECT2000. The measured detector signals in single CdWO4 and CsI(Tl) scintillation crystals of base 0.275 x 0.8 cm2 and heights 0.4, 1, 1.2, 1.6 and 2 cm were, generally, in good agreement with the signals calculated with the model. A prototype detector array which contains 8 CdWO4 crystals, each 0.275 x 0.8 x 1 cm3, in contact with a 16-element array of photodiodes was built. The measured attenuation of a Cobalt-60 beam as a function of solid water thickness behaves linearly. The frequency dependent modulation transfer function [MTF(f)], noise power spectrum [NPS(f)], and detective quantum efficiency [DQE(f)] were measured for 1.25 MeV photons (in a Cobalt-60 beam). For 6 MV photons, only the MTF(f) was measured from a linear accelerator, where large pulse-to-pulse fluctuations in the output of the linear accelerator did not allow the measurement of the NPS(f). A two-step Monte Carlo simulation was used to model the detector's MTF(f), NPS(f) and DQE(f). The DQE(0) of the detector array was found to be 26% and 19% for 1.25 MeV and 6 MV photons, respectively. For 1.25 MeV photons, the maximum discrepancies between the measured and modeled MTF(f), relative NPS(f) and the DQE(f) were found to be 1.5%, 1.2%, and 1.9%, respectively. For the 6 MV beam, the maximum discrepancy between the modeled and the measured MTF(f) was found to be 2.5%. The modeling is sufficiently accurate for designing appropriate detectors for MVCT.

Dosimetric IMRT verification with a flat-panel EPID.

A convolution-based calibration procedure has been developed to use an amorphous silicon flat-panel electronic portal imaging device (EPID) for accurate dosimetric verification of intensity-modulated radiotherapy (IMRT) treatments. Raw EPID images were deconvolved to accurate, high-resolution 2-D distributions of primary fluence using a scatter kernel composed of two elements: a Monte Carlo generated kernel describing dose deposition in the EPID phosphor, and an empirically derived kernel describing optical photon spreading. Relative fluence profiles measured with the EPID are in very good agreement with those measured with a diamond detector, and exhibit excellent spatial resolution required for IMRT verification. For dosimetric verification, the EPID-measured primary fluences are convolved with a Monte Carlo kernel describing dose deposition in a solid water phantom, and cross-calibrated with ion chamber measurements. Dose distributions measured using the EPID agree to within 2.1% with those measured with film for open fields of 2 x 2 cm2 and 10 x 10 cm2. Predictions of the EPID phantom scattering factors (SPE) based on our scatter kernels are within 1% of the SPE measured for open field sizes of up to 16 x 16 cm2. Pretreatment verifications of step-and-shoot IMRT treatments using the EPID are in good agreement with those performed with film, with a mean percent difference of 0.2 +/- 1.0% for three IMRT treatments (24 fields).