Background Magnetic resonance (MR) imaging devices have been integrated with medical linear accelerators (linac) in radiation therapy. Both perpendicular linac-MR (LMR-B perpendicular to) and parallel (LMR-B parallel to) systems exist, where due to the MR's magnetic field dose can be perturbed in the patient. Dose perturbations from the electron return effect (ERE) and electron streaming effects (ESEs) are present in LMR-B perpendicular to systems, where a dose collimating effect has been observed in LMR-B parallel to systems . Purpose To report on an asymmetric dose perturbation which is present at the interface between two different materials during treatment in parallel linac-MR (LMR-B parallel to) systems. To the best of our knowledge, these asymmetric dose effects, "Lateral Scattered Electron Return Effect" (LS-ERE) have not been previously reported. Methods BEAMnrc and EGSnrc Monte Carlo (MC) radiation transport codes were used with the EEMF macro to emulate a 6 FFF beam from the 0.5-T Alberta linac-MR (LMR). Simulations were performed at 0.5 and 1.5 T in several different phantom material-interface combinations and field sizes including from modulated MLC-like fields. MC simulations quantified LS-ERE in patient CT datasets for the head, breast, and lung. LS-ERE cancellation techniques were investigated. LS-ERE asymmetries were quantified by subtracting an antiparallel dose from the parallel dose, dividing by two and normalizing to the global 0-T maximum dose. GafChromic film measurements were made in the 0.5-T Alberta LMR-B parallel to system using solid water at the water-air interface to validate MC simulations. ERE was simulated for an emulated LMR-B perpendicular to system and compared to LMR-B parallel to dose perturbations. Results LS-ERE is mostly independent of field size for fields >1 x 1 cm(2). For 5 x 5-cm(2) fields at 0.5T/1.5T, LS-ERE asymmetries are <=+/- 6.9%/6.9% at bone-air and <=+/- 9.0%/7.0% at tissue-air for nonair doses, and <=+/- 4.1%/5.5% at tissue-lung interfaces. LS-ERE increases as the density gradient increases, where the magnitude and extent of LS-ERE are reduced as field strength increases. For a single 5 x 5-cm(2) field at 0.5T/1.5T, the LS-ERE asymmetry is <=+/- 10.2%/8.5% at the tissue-air sinus interface for head, <=+/- 4.2%/5.3% at the spine-lung interface for the lung, and <=+/- 5.7%/4.9% at the skin-air interface for a breast tangent plan at 0.5T/1.5T. POP fields mostly remove LS-ERE asymmetries, with magnetic field reversal during treatment being the most effective method. Skin dose was investigated and compared to 0-T treatments for 0.5T/1.5T LMR-B parallel to single field breast and head treatments. Including all dosimetric magnetic field perturbations, a 21%/24% and 22%/22% increase in skin dose to head and breast, respectively, was observed, of which LS-ERE is responsible for approximately 30% of the total. Measured LS-ERE asymmetries and dose enhancements at the water-air interface using GafChromic film were in excellent agreement with MC simulations. ERE in 1.5-T LMR-B perpendicular to systems are on average 5.5 times larger than total dose perturbations at 0.5 T in LMR-B parallel to systems. Conclusion LS-ERE is present at the interface between materials and awareness of LS-ERE is crucial for proper TPS evaluation for LMR-B parallel to treatments, especially in areas where large tissue density gradients exist.

The entire data set contained 592 structures at the on-set of analysis. After applying the rejection filters, the remaining data had 294 structures; a large portion of filtering was due to unmatched names. Out of these, OARs with contours from at least 10 patients (N>9) were further analyzed to include the 25 and 75 percentile for DSC and HD95%. Results from this analysis (structures with N>9) are presented in the table below, where results of left-right paired structures are combined into one row. The submandibular glands, larynx, and optical nerves (5 < N < 10) all had median DSC < 0.77 while lungs had median DSC > 0.98 (N = 6). Parotids and lenses had poor DSC and HD95% scores and may require significant contour editing to achieve agreement with our clinical conventions. This study highlights the difficulty with retrospective analysis of contours with an external trained model due to variations in the superior/inferior extent of tubular structures such as rectum, cord, esophagus, etc. CONCLUSION: Results are encouraging, given that the pre-trained commercial model has not seen our institutional data. The pre-trained AI contouring model matched very well to manual contours for large volume, higher contrast structures but did not match well for parotids and lenses. Variability in conventions regarding the superior/inferior extent of some structures hinders retrospective comparison with a pre-trained AI model.

Purpose. To construct a fast-calculating fluence modelfor the Siemens HD120 multi leaf collimator (MLC) using convolution-superposition techniques, and to develop a 3D VMAT dose engine using this fluence model. This work offers analternative to time-consuming open-source Monte Carlo simulations for thosedeveloping in-house dose-calculating software for research or clinical needs.Methods. EPID-acquired images of sweeping-window and sweeping-checker field profiles were used to commission transmission, 2 Dinterleaf leakage, and tongue-and-groove maps specific to the HD120 MLC. These maps, along with a 2D head-scattermodel were incorporated into a convolution-superposition algorithm to provide a fluence model for the HD120 MLC. This fluence model was used to develop a 3D VMAT dose engine, where 3D pre-computed 6MV dose kernels (EGSnrc) and a 3D fluence curvature-correction map were incorporatedto calculate 3D VMAT doses in a 22 cm diameter cylindrical phantom. Four VMAT patient plans witha large range of PTV sizes (36 cc to 604 cc) were chosen to test the fluence model and dose engine. Results. Excellent agreement was observed between the simulated commissioning fields and measured EPID-responses. 2D 2%/2 mm gamma analysis yielded a 98.9% pass rate for 1 cm, 2 cm, and 4 cm sweeping-window fields. 2D 2%/2mm gamma analysis for outer/inner MLC leaves yielded 89.1%/77.0% and 95.2%/91.1% pass rates from 1 cm and 2 cm sweeping-checker fields. Mean 3%/3 mm gamma analysis showed excellent agreement between our dose engine and Eclipse (Acuros) regardless of PTV size: 98.7% pass rate, with 95.1% pass rate in the high-dose volume. Fluence calculation times were13.6 seconds per dynamic MLC field and 1.4 minutes/arc for 3D VMAT dose on a standard PC. Conclusions. A fast-calculating convolution-superposition fluence model has been commissioned for the Siemens HD120 MLC and incorporatedinto a 3D VMAT dose engine. This work can be used to facilitate the development of fast in-house dose-calculating software.

PURPOSE Electronic portal imaging devices (EPIDs) are potentially useful for dosimetric verification in integrated MRI-linac systems. This work presents the reproducibility, linearity, image lag, and radiation field profiles in a conventional EPID, with and without a 0.5 T parallel magnetic field present in a 6 MV photon beam. METHODS An aS500 EPID was modified to function in strong magnetic fields. All measurements were made using the linac-MR installed at the Cross Cancer Institute. The EPID remained stationary on the couch between the measurements made with and without magnetic field. We measured short-term reproducibility of dark and flood fields, signal linearity from 1 to 500 MU irradiations, and image lag post 100 MU irradiation. An ion chamber was used to measure any linac output variations to correct the EPID signal due to these variations for the duration of experiment. X-axis and Y-axis radiation field profiles were obtained from the EPID image resulting from a 10 × 10 cm2 radiation field delivery. RESULTS The average pixel value (±standard deviation) of flood field with and without magnetic fields were 57,876 ± 379 and 57,703 ± 366, respectively, and the corresponding average dark field pixel values were -32.05 ± 0.85 and -32.19 ± 0.97. The maximum difference in image linearity data with and without magnetic field is 0.2% which is well within the measurement uncertainty of 0.65%. Similarly, the image lag curves, with and without the magnetic field, were nearly identical. The first measured point, with mean lag signal of 1.44% without and 1.41% with magnetic field, shows that the largest difference is well below the uncertainty in the EPID signal measurement. The radiation field profiles obtained with and without magnetic fields were nearly identical; 91.3% of the X-axis and 95.2% of the Y-axis profile points pass a gamma criterion of 1% and 1 mm. CONCLUSIONS A conventional EPID imager with a 0.1 cm copper plate responds to 6 MV photons similarly irrespective of the strong magnetic field being off or on in the parallel orientation to the radiation beam. Thus, the EPID is a potentially useful tool for pretreatment dosimetric verification in linac-MR systems using parallel magnetic field.

Purpose To modify a previously designed, short, 10 MV linac waveguide, so that it can produce any energy from 4 to 10 MV. The modified waveguide is designed to be a drop-in replacement for the 6 MV waveguide used in the author's current linear accelerator-magnetic resonance imager (Linac-MR). Methods Using our group's previously designed short 10 MV linac as a starting point, the port was moved to the fourth cavity, the shift to the first coupling cavity was removed and a tuning cylinder added to the first coupling cavity. Each cavity was retuned using finite element method (FEM) simulations to resonate at the desired frequency. FEM simulations were used to determine the RF field distributions for various tuning cylinder depths, and electron trajectories were computed using a particle-in-cell model to determine the required RF power level and tuning cylinder depth to produce electron energy distributions for 4, 6, 8, and 10 MV photon beams. Monte Carlo simulations were then used to compare the depth dose profiles with those produced by published electron beam characteristics for Varian linacs. Results For each desired photon energy, the electron beam energy was within 0.5% of the target mean energy, the depth of maximum dose was within 1.5 mm of that produced by the Varian linac, and the ratio of dose at 10 cm depth to 20 cm depth was within 1%. Conclusions A new 27.5 cm linear accelerator waveguide design capable of producing any photon energy between 4 and 10 MV has been simulated, however coupling port design and the implications of increased electron beam current at 10 MV remain to be investigated. For the specific cases of 4, 6, and 10 MV, this linac produces depth dose profiles similar to those produced by published spectra for Varian linacs.

Purpose: Integrating a linac with a magnetic resonance imager (MRI) will revolutionize the accuracy of external beam radiation treatments. Irradiating in the presence of a strong magnetic field, however, will modify the dose distribution. These dose modifications have been investigated previously, mainly using Monte Carlo simulations. The purpose of this work is to experimentally verify the use of the EGSnrc Monte Carlo (MC) package for calculating percent depth closes (PDDs) in a homogeneous phantom, in the presence of a realistic parallel magnetic field.Methods: Two cylindrical electromagnets were used to produce a 0.207 T magnetic field parallel to the central axis of a 6 MV photon beam from a clinical linac. The magnetic field was measured at discrete points along orthogonal axes, and these measurements were used to validate a full 3D magnetic field map generated using COMSOL Multiphysics. Using a small parallel plate ion chamber, the depth dose was measured in a polystyrene phantom placed inside the electromagnet bore at two separate locations: phantom top surface coinciding with top of bore, and phantom top surface coinciding with center of bore. BEAMnrc MC was used to model the linac head which was benchmarked against the linac's commissioning measurements. The depth close in polystyrene was simulated using DOS-XYZnrc MC. For the magnetic field case, the DOSXYZnrc code was slightly modified to implement the previously calculated 3D magnetic field map to be used in the standard electromagnetic macros.Results: The calculated magnetic field matched the measurements within 2% of the maximum central field (0.207 T) with most points within the experimental uncertainty (1.5%). For the MC linac head model, over 93% of all simulated points passed the 2%, 2 mm gamma acceptance criterion, when comparing measured and simulated lateral beam and depth dose profiles. The parallel magnetic field caused a surface dose increase, compared to the no magnetic field case, due to the Lorentz force confining contaminant electrons within the beam. The surface dose increase was measured to he approximately 10% (relative to no field D-max) when the phantom surface coincided with the top of the electromagnet's bore. This effect was enhanced by moving the phantom surface to the center of the magnet's bore in relatively high magnetic field (> 0.13 T). The surface dose for this setup increased by 30% and the entire buildup region was affected. When the dimensions and composition of the ion chamber air cavity and entrance window were included, EGSnrc was able to accurately simulate these close increases, both at the surface and in the buildup region. All the simulated points were within 1% of the measurements for both setups. The ferromagnetic linac head was determined to have a negligible effect on the final PDD comparison.Conclusions: Irradiating in the presence of a parallel magnetic field causes measurable surface and buildup depth dose increases. We have experimentally verified that the EGSnrc Monte Carlo package is able to accurately calculate the PDDs with these surface and buildup dose modifications in a homogeneous phantom. (C) 2017 American Association of Physicists in Medicine

[https://doi.org/10.1002/mp.12413] In the cited article,1 some of the references on page 4814, column 2 and on page 4815 column 1 are missing the last page. Thus, references 1, 4, 5, 10, and 15 should read: 1. Fallone BG, Murray B, Rathee S, Stanescu T, Steciw S, Vidakovic S, Blosser E, and Tymofichuk T. First MR images obtained during megavoltage photon irradiation from a prototype integrated linac-MR system. Med Phys. 2009;36:2084-2088. 4. Kirkby C, Murray B, Rathee S, Fallone BG. Lung dosimetry in a linac-MRI radiotherapy unit with a longitudinal magnetic field. Med Phys. 2010;37:4722-4732. 5. Keyvanloo A, Burke B, St Aubin J, Baillie D, Wachowicz K, Warkentin B, Steciw S and Fallone BG. Minimal skin dose increase in longitudinal rotating biplanar linac-MR systems : examination of radiation energy and flattening filter design. Phys Med Biol. 2016;61:3527-3539. 10. Kirkby C, Stanescu T, Rathee S, Carlone M, Murray B, Fallone BG. Patient dosimetry for hybrid MRI-radiotherapy systems. Med Phys. 2008;35:1019-1027. 15. Keyvanloo A, Burke B, Warkentin B, Tadic T, Rathee S, Kirkby C, Santos DM, Fallone BG. Skin dose in longitudinal and transverse linac-MRIs using Monte Carlo and realistic 3D MRI field models. Med Phys. 2012;39:6509-6521.

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 cm2 to 20  ×  20 cm2. For 10 MV, skin dose increases by ~6% for a 5  ×  5 cm2 field, and decreases by ~1.5% for a 20  ×  20 cm2 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 Dmax). 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.

Purpose:Integrated linac‐MR systems are uniquely suited for real time tumor tracking during radiation treatment. Understanding the magnetic field dose effects and incorporating them in treatment planning is paramount for linac‐MR clinical implementation. We experimentally validated the EGSnrc dose calculations in the presence of a magnetic field parallel to the radiation beam travel.Methods:Two cylindrical bore electromagnets produced a 0.21 T magnetic field parallel to the central axis of a 6 MV photon beam. A parallel plate ion chamber was used to measure the PDD in a polystyrene phantom, placed inside the bore in two setups: phantom top surface coinciding with the magnet bore center (183 cm SSD), and with the magnet bore's top surface (170 cm SSD). We measured the field of the magnet at several points and included the exact dimensions of the coils to generate a 3D magnetic field map in a finite element model. BEAMnrc and DOSXYZnrc simulated the PDD experiments in parallel magnetic field (i.e. 3D magnetic field included) and with no magnetic field.Results:With the phantom surface at the top of the electromagnet, the surface dose increased by 10% (compared to no‐magnetic field), due to electrons being focused by the smaller fringe fields of the electromagnet. With the phantom surface at the bore center, the surface dose increased by 30% since extra 13 cm of air column was in relatively higher magnetic field (>0.13T) in the magnet bore. EGSnrc Monte Carlo code correctly calculated the radiation dose with and without the magnetic field, and all points passed the 2%, 2 mm Gamma criterion when the ion chamber's entrance window and air cavity were included in the simulated phantom.Conclusion:A parallel magnetic field increases the surface and buildup dose during irradiation. The EGSnrc package can model these magnetic field dose effects accurately.Dr. Fallone is a co‐founder and CEO of MagnetTx Oncology Solutions (under discussions to license Alberta bi‐planar linac MR for commercialization).

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 mu m 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 similar to 6%-19% and similar to 1%-9% for the 6 and 10 MV beams, respectively. For 6 MV, skin dose increases from similar to 10.5% to similar to 1.5% for field-size increases of 5 x 5 cm(2) to 20 x 20 cm(2). For 10 MV, skin dose increases by similar to 6% for a 5 x 5 cm(2) field, and decreases by similar to 1.5% for a 20 x 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 similar to 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.

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.

Purpose:To design a new compact S‐band linac waveguide capable of producing a 10 MV x‐ray beam, while maintaining the length (27.5 cm) of current 6 MV waveguides. This will allow higher x‐ray energies to be used in our linac‐MRI systems with the same footprint.Methods:Finite element software COMSOL Multiphysics was used to design an accelerator cavity matching one published in an experiment breakdown study, to ensure that our modeled cavities do not exceed the threshold electric fields published. This cavity was used as the basis for designing an accelerator waveguide, where each cavity of the full waveguide was tuned to resonate at 2.997 GHz by adjusting the cavity diameter. The RF field solution within the waveguide was calculated, and together with an electron‐gun phase space generated using Opera3D/SCALA, were input into electron tracking software PARMELA to compute the electron phase space striking the x‐ray target. This target phase space was then used in BEAM Monte Carlo simulations to generate percent depth doses curves for this new linac, which were then used to re‐optimize the waveguide geometry.Results:The shunt impedance, Q‐factor, and peak‐to‐mean electric field ratio were matched to those published for the breakdown study to within 0.1% error. After tuning the full waveguide, the peak surface fields are calculated to be 207 MV/m, 13% below the breakdown threshold, and a d‐max depth of 2.42 cm, a D10/20 value of 1.59, compared to 2.45 cm and 1.59, respectively, for the simulated Varian 10 MV linac and brehmsstrahlung production efficiency 20% lower than a simulated Varian 10 MV linac.Conclusion:This work demonstrates the design of a functional 27.5 cm waveguide producing 10 MV photons with characteristics similar to a Varian 10 MV linac.

Purpose: To design a short (27.5 cm) S‐band linear accelerator capable of producing a 10 MV x‐ray beam to enable higher photon energies for linear accelerator‐MRI hybrid systems, where the waveguide length and linac‐MR orientation is a concern. Methods: Finite element software COMSOL Multiphysics was used to design an accelerator cavity matching one published in an experiment on breakdown, to ensure that the threshold electric field published are applicable to the final waveguide geometry. This cavity was used as the basis for designing an accelerator waveguide, and the full RF field solution within the waveguide was calculated. This RF solution and an electron‐gun phase space generated using Opera3D/SCALA are input into electron tracking software PARMELA to compute the electron phase space striking the x‐ray target. The resulting electron beam is used for Monte Carlo simulations to calculate PDD curves and the beam penumbra, and compared against those produced by a Varian 10 MV. Results: The shunt impedance, Q factor, and ratio of peak‐to‐mean electric fields of the cavity were matched to those in the published study to within 0.1% error. After tuning the full waveguide, the peak surface fields are 207 MV/m, well below the breakdown threshold of 239 MV/m. The peak of the electron spectrum is centered at 10.4 MeV, with a width of 0.2 MeV, compared with 10.5 MeV and 0.3 MeV, respectively, for a Varian 10 MV linac. Monte Carlo results show dmax is 2.15 cm for a 10×10 cm2 field and the penumbra is 5.0 mm wide at 10 cm depth, compared with dmax at 2.3 cm and 5.6 mm width for a Varian 10 MV linac. Conclusion: This work demonstrates the design and modeling of a waveguide capable of producing 10 MV energy x‐rays yet short enough to be compatible with a linac‐MR system.

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.

This work outlines the development of a robust method of calculating modulation factors used for the independent verification of MUs for IMRT and VMAT treatments, to replace onerous ion chamber measurements. Two-dimensional fluence maps were calculated for dynamic MLC fields that include MLC interleaf leakage, transmission, and tongue-and-groove effects, as characterized from EPID-acquired images. Monte Carlo-generated dose kernels were then used to calculate doses for a modulated field and that field with the modulation removed at a depth specific to the calculation point in the patient using in-house written software, Mod_Calc. The ratio of these two doses was taken to calculate modulation factors. Comparison between Mod_Calc calculation and ion chamber measurement of modulation factors for 121 IMRT fields yielded excellent agreement, where the mean difference between the two was -0.3% ± 1.2%. This validated use of Mod_Calc clinically. Analysis of 5,271 dynamic fields from clinical use of Mod_Calc gave a mean difference of 0.3% ± 1.0% between Mod_Calc and Eclipse-generated factors. In addition, 99.3% and 96.5% fields pass 5% and 2% criteria, respectively, for agreement between these two predictions. The development and use of Mod_Calc at our clinic has considerably streamlined our QA process for IMRT and RapidArc fields, compared to our previous method based on ion chamber measurements. As a result, it has made it feasible to maintain our established and trusted current in-house method of MU verification, without resorting to commercial software alternatives.