B Oborn

Impact of the MLC on the MRI field distortion of a prototype MRI-linac

PURPOSE To cope with intrafraction tumor motion, integrated MRI-linac systems for real-time image guidance are currently under development. The multileaf collimator (MLC) is a key component in every state-of-the-art radiotherapy treatment system, allowing for accurate field shaping and tumor tracking. This work quantifies the magnetic impact of a widely used MLC on the MRI field homogeneity for such a modality. METHODS The finite element method was employed to model a MRI-linac assembly comprised of a 1.0 T split-bore MRI magnet and the key ferromagnetic components of a Varian Millennium 120 MLC, namely, the leaves and motors. Full 3D magnetic field maps of the system were generated. From these field maps, the peak-to-peak distortion within the MRI imaging volume was evaluated over a 30 cm diameter sphere volume (DSV) around the isocenter and compared to a maximum preshim inhomogeneity of 300 μT. Five parametric studies were performed: (1) The source-to-isocenter distance (SID) was varied from 100 to 200 cm, to span the range of a compact system to that with lower magnetic coupling. (2) The MLC model was changed from leaves only to leaves with motors, to determine the contribution to the total distortion caused by MLC leaves and motors separately. (3) The system was configured in the inline or perpendicular orientation, i.e., the linac treatment beam was oriented parallel or perpendicular to the magnetic field direction. (4) The treatment field size was varied from 0 × 0 to 20×20 cm(2), to span the range of clinical treatment fields. (5) The coil currents were scaled linearly to produce magnetic field strengths B0 of 0.5, 1.0, and 1.5 T, to estimate how the MLC impact changes with B0. RESULTS (1) The MLC-induced MRI field distortion fell continuously with increasing SID. (2) MLC leaves and motors were found to contribute to the distortion in approximately equal measure. (3) Due to faster falloff of the fringe field, the field distortion was generally smaller in the perpendicular beam orientation. The peak-to-peak DSV distortion was below 300 μT at SID≥130 cm (perpendicular) and SID≥140 cm (inline) for the 1.0 T design. (4) The simulation of different treatment fields was identified to cause dynamic changes in the field distribution. However, the estimated residual distortion was below 1.2 mm geometric distortion at SID≥120 cm (perpendicular) and SID≥130 cm (inline) for a 10 mT/m frequency-encoding gradient. (5) Due to magnetic saturation of the MLC materials, the field distortion remained constant at B0>1.0 T. CONCLUSIONS This work shows that the MRI field distortions caused by the MLC cannot be ignored and must be thoroughly investigated for any MRI-linac system. The numeric distortion values obtained for our 1.0 T magnet may vary for other magnet designs with substantially different fringe fields, however the concept of modest increases in the SID to reduce the distortion to a shimmable level is generally applicable.

SU-E-J-03: A Comprehensive Comparison Between Alpha and Beta Emitters for Cancer Radioimmunotherapy

PURPOSE The purpose of this study is to perform a comprehensive comparison of the therapeutic efficacy and cytotoxicity of alpha and beta emitters for Radioimmunotherapy (RIT). For each stage of cancer development, specific models were built for the separate objectives of RIT to be addressed:a) kill isolated cancer cells in transit in the lymphatic and vascular circulation,b) regress avascular cell clusters,c) regress tumor vasculature and tumors. METHODS Because of the nature of short range, high LET alpha and long energy beta radiation and heterogeneous antigen expression among cancer cells, the microdosimetric approach is essential for the RIT assessment. Geant4 based microdosimetric models are developed for the three different stages of cancer progression: cancer cells, cell clusters and tumors. The energy deposition, specific energy resulted from different source distribution in the three models was calculated separately for 4 alpha emitting radioisotopes (211 At, 213 Bi, 223 Ra and 225 Ac) and 6 beta emitters (32 P, 33 P, 67 Cu, 90 Y, 131 I and 177 Lu). The cell survival, therapeutic efficacy and cytotoxicity are determined and compared between alpha and beta emitters. RESULTS We show that internal targeted alpha radiation has advantages over beta radiation for killing isolated cancer cells, regressing small cell clusters and also solid tumors. Alpha particles have much higher dose specificity and potency than beta particles. They can deposit 3 logs more dose than beta emitters to single cells and solid tumor. Tumor control probability relies on deep penetration of radioisotopes to cancer cell clusters and solid tumors. CONCLUSION The results of this study provide a quantitative understanding of the efficacy and cytotoxicity of RIT for each stage of cancer development.

Passive magnetic shielding in MRI-Linac systems

Passive magnetic shielding refers to the use of ferromagnetic materials to redirect magnetic field lines away from vulnerable regions. An application of particular interest to the medical physics community is shielding in MRI systems, especially integrated MRI-linear accelerator (MRI-Linac) systems. In these systems, the goal is not only to minimize the magnetic field in some volume, but also to minimize the impact of the shield on the magnetic fields within the imaging volume of the MRI scanner. In this work, finite element modelling was used to assess the shielding of a side coupled 6 MV linac and resultant heterogeneity induced within the 30 cm diameter of spherical volume (DSV) of a novel 1 Tesla split bore MRI magnet. A number of different shield parameters were investigated; distance between shield and magnet, shield shape, shield thickness, shield length, openings in the shield, number of concentric layers, spacing between each layer, and shield material. Both the in-line and perpendicular MRI-Linac configurations were studied. By modifying the shield shape around the linac from the starting design of an open ended cylinder, the shielding effect was boosted by approximately 70% whilst the impact on the magnet was simultaneously reduced by approximately 10%. Openings in the shield for the RF port and beam exit were substantial sources of field leakage; however it was demonstrated that shielding could be added around these openings to compensate for this leakage. Layering multiple concentric shield shells was highly effective in the perpendicular configuration, but less so for the in-line configuration. Cautious use of high permeability materials such as Mu-metal can greatly increase the shielding performance in some scenarios. In the perpendicular configuration, magnetic shielding was more effective and the impact on the magnet lower compared with the in-line configuration.

TU-H-BRA-03: Performance of a Clinical Gridded Electron Gun in Magnetic Fields: Implications for MRI-Linac Therapy

PURPOSE Recent advances towards MRI Linac radiotherapy have motivated a wide range of studies characterizing electromagnetic interactions between the two devices. One of the most sensitive components is the linac electron gun. To data, only non gridded (diode) guns have been investigated however, most linac vendors utilize gridded (triode) guns, which enable efficient and robust beam gating. The purpose of this study was to develop a realistic model of a gridded gun used clinically, and to characterize its performance in magnetic fields. METHODS The gridded electron gun used on Varian high energy machines was measured using 3D laser scanning quoted as accurate to 0.1mm. Based on the scane, a detailed CAD mode was developed. From this, key geometry was extracted and a FEM model was developed (Opera/SCALA). Next, the high voltage (HV), grid voltage, and emission current were read from six dose matched TrueBeam linacs for the 6X, 10X and 15X photon modes (0 B-field). The mean values were used to represent each mode, which was simulated I constant magnetic fields from 0-200G in-line, and 0-35G perpendicular. RESULTS Experimentally measured HV, grid voltage, and emission current from 6X, 10X and 15X modes were respectively: 15±.03kV, 10±.08kV, 11±.03kV; 93±7V, 41±3V, and 70±6V; 327±27mA, 129±10mA, and 214±19mA. The error in simulated emission current of each mode was 3%,6%, and 3%. For in-line fields, 50% beam loss occurred at 114, 96, and 97G; for perpendicular; at 12, 13 and 14G. Sensitivity for a given geometry is primarily determined by HV setting. CONCLUSION Future MRI-Linac systems will almost certainly use gridded guns. We present the first model of a clinical gridded gun, and match the experimental emission current to within 6% across three different operating modes. This clinical gun shows increased sensitivity to magnetic fields than previous work,and different modes show different sensitivity.

TH‐CD‐BRA‐05: Proton Beam Deflection in MRI Fields: Implications for MRI‐Guided Proton Therapy

Purpose: To simulate the delivery of proton beams to the treatment zone inside a split-bore MRI-guided proton therapy system. Methods: Field maps from a split-bore 1 T MRI system are used as input to Monte Carlo simulations which model the trajectory of proton beams towards isocentre. Both inline (along the MRI bore) and perpendicular (through the split-bore gap) orientations are simulated. Monoenergetic diverging beams of energy 90 MeV, 195 MeV and 300 MeV starting from 1.5 m above isocentre were modelled. A phase space file detailing a 2D calibration pattern is used to set the particle starting positions, and their spatial location as they cross isocentre recorded. Results: Inline Orientation: The radial symmetry of the solenoidal style fringe field acts to rotate the protons around the beam’s central axis. For protons starting at 1.5 m from isocentre this rotation is 17° (90 MeV), and 8° (300 MeV). Perpendicular Orientation: Isocentre shifts of 135 mm (90 MeV) and 65 mm (300 MeV) were observed in the direction perpendicular to the main imaging field. Off-axis protons are also slightly deflected towards or away from the central axis in the direction perpendicular to the main deflection direction. This leads to a distortion of the phase space pattern, not just a shift. The distortion increases from zero at the central axis to 10 mm (90 MeV) and 5 mm (300 MeV) for a proton 150 mm off-axis. Conclusion: The complexity and energy-dependent nature of the magnetic deflection and distortion indicates the pencil beam scanning method will be the only choice for delivering a therapeutic proton beam inside a potential MRI-guided proton therapy system. Significant correction strategies will be required to account for the MRI fringe fields. The authors acknowledge funding from NHMRC Program Grant No. 1036078 and ARC Discovery Grant No. DP120100821.

WE-G-BRD-05: Inline Magnetic Fields Enhance Tumor Dose for Small Lung Cancers

Purpose: To report on significant dose enhancement effects caused by magnetic fields aligned parallel to 6MV photon beam radiotherapy of small lung tumors. Findings are applicable to future inline MRI-guided radiotherapy systems. Methods: 9 clinical lung plans were recalculated using Monte Carlo methods and external inline (parallel to the beam direction) magnetic fields of 0.5 T, 1.0 T and 3 T were included. Three plans were 6MV 3D-CRT and six were 6MV IMRT. The GTV’s ranged from 0.8 cc to 73 cc, while the PTV ranged from 1 cc to 180 cc. Results: The inline magnetic field has a moderate impact in lung dose distributions by reducing the lateral scatter of secondary electrons and causing a small local dose increase. Superposition of multiple small beams acts to superimpose the small dose increases and can lead to significant dose enhancements, especially when the GTV is low density. Two plans with very small, low mean density GTV’s (<1 cc, ρ(mean)<0.35g/cc) showed uniform increases of 16% and 23% at 1 T throughout the PTV. Three plans with moderate mean density PTV’s (3–13 cc, ρ(mean)=0.58–0.67 g/cc) showed 6% mean dose enhancement at 1 T in the PTV, however not uniform throughout the GTV/PTV. Replanning would benefit these cases. The remaining 5 plans had large dense GTV’s (∼ 1 g/cc) and so only a minimal (<2%) enhancement was seen. In general the mean dose enhancement at 0.5 T was 60% less than 1 T, while 5–50% higher at 3 T. Conclusions: A paradigm shift in the efficacy of small lung tumor radiotherapy is predicted with future inline MRI-linac systems. This will be achieved by carefully taking advantage of the reduction of lateral electronic disequilibrium withing lung tissue that is induced naturally inside strong inline magnetic fields.

WE‐AB‐BRB‐04: A Novel Monolithic Silicon 2D Detector Array for Use in Stereotactic Applications

Purpose: To assess the capability of a novel 2D monolithic silicon detector array in measuring stereotactic photon fields. Methods: The silicon array detector used in this work, named Magic Plate-512 (MP512), is a thin monolithic silicon wafer (52 × 52 × 0.47 mm3) with 512 ion-implanted diodes (0.5 × 0.5 mm2). Adjacent pixels are spaced evenly with 2 mm pitch, covering a maximum detection area 46 mm wide. Its fast, FPGA based read-out system is synchronised with the linac to allow readout of all pixels pulse-by-pulse. A clinical SABR lung plan (consisting of 9 single segment beams, 6MV) was measured with the array at 1.5 cm depth in a solid water phantom (100 cm SSD). The typical field size was in the range of 3 × 3 cm2 to 4 × 4 cm2. Each beam was delivered at perpendicular incidence to the detector plane so as to avoid the need for angular dependence corrections. The fields were measured under the same conditions using Gafchromic EBT3 film for comparison. The film was scanned at 72 dpi resolution, with the red channel data used for analysis. Results: Average gamma passing rates of (92.3 ± 1.8) % for 2%/2mm criteria, and (86.6 ± 2.3) % for 1%/2mm criteria were achieved for MP512, using EBT3 film as the reference distribution. The detector array was able to accurately measure the full-width-at-half-maximum (FWHM), to within (0.77 ± 0.01) mm accuracy when compared to film. The penumbral widths (80%-20%) were measured to within (0.30 ± 0.01) mm accuracy to film. Conclusion: The MP512 is a feasible option for measurement of stereotactic photon fields, with its high density of detection points making it useful for small field applications. The prototype array has demonstrated merit; in the future the development of a larger array detection area would be beneficial for clinical applications.

SU‐F‐500‐05: MRI‐Linac Systems: Can a Standard MLC Be Incorporated Into Such a Device?

PURPOSE Integrated MRI-linac systems for real-time image guidance are currently under development to master intrafraction tumor motion. As the multileaf collimator (MLC) is a crucial component of such systems, this study is aimed to answer if a standard MLC can be incorporated based on (a) its impact on the MRI field homogeneity and (b) the electromagnetic force acting on the MLC in the MRI field. METHODS A 1.0 T split-bore magnet and the key ferromagnetic components of a Varian Millennium 120 MLC, namely the leaves and motors, were modelled using the finite element method (FEM). In this work both the parallel and perpendicular configuration for such a system were investigated, i.e. the linac treatment beam was orientated parallel and perpendicular to the MRI field direction. Simulations were performed for isocentre distances in the range from 100 to 200 cm. From the resultant magnetic field distributions, the introduced field distortion over the MRI imaging volume, a 30 cm diameter sphere volume (DSV) around the isocentre, and the net electromagnetic force acting on the MLC were evaluated. RESULTS For all distances of 140 cm or larger, the field distortion was below 300 ppm which is commonly referred to as maximum pre-shim inhomogeneity of MRI devices. The total force pulling the MLC towards the MRI magnet was found to be maximum at a 120 cm isocentre distance with 1706 N and 3499 N in parallel and perpendicular configuration, respectively, and to decrease for larger distances. CONCLUSION The theoretical results suggest that the distortion introduced by a standard MLC can be corrected for by shimming for isocentre distances of 140 cm or larger. Appropriate mounting of the MLC to withstand the electromagnetic forces is considered mechanically challenging and may require an isocentre distance larger than 140 cm. NHMRC Australia Fellowship, Australian Endeavour Research Fellowship.

TU‐A‐BRA‐06: EPID Operation in a Bi‐Directional MRI‐Linac System: A Monte Carlo Study

Purpose: To examine via Monte Carlo simulation the dosimetric operation of an EPID inside a bi‐directional MRI‐Linac system. The proposed system is a 1 T MRI which supports both perpendicular (transverse) and inline (longitudinal) 6 MV MRIgRT. Methods: BEAMnrc Monte Carlo simulations were used to model a Varian 6 MV linac. Phase space files were produced below the MLCs and were inputs to Geant4 Monte Carlo simulations. These consist of transporting the radiation beam through a phantom and then on to a detailed model of a Varian EPID. The Geant4 simulations take place inside magnetic field maps matched to Finite Element Modeling of the MRI‐Linac system (COMSOL). Hence the EPID operation is simulated in the same conditions as a real MRI‐linac system. Various EPID SSDs were simulated in both inline and perpendicular field MRI‐linac systems. Dose to the EPID layers were taken for each field and compared with zero magnetic field. Results: The inclusion of the magnetic fringe field had little impact in the inline direction, however in the perpendicular direction the electron‐return effect (ERE) caused secondary electrons to return to the phosphor scoring level. This induced a lateral blur and overdosing of the images that was related directly to the SSD or strength of the fringe field in the transverse direction (5% at 0.25 T, 10% at 1 T). Including 1 cm of water material below the phosphor layer significantly reduced the ERE and improved the dose image quality. Conclusion: Novel Monte Carlo simulations of the operation of an EPID working inside a bidirectional field MRI‐linac have been performed. The main effect which induced an image or dose artifact was the ERE in the perpendicular field arrangement. This can however be almost eliminated by the use of an exit side bolus on the EPID.

SU‐E‐T‐20: Removal of Electron Contamination in Longitudinal Field MRI‐Linac Systems: A Monte Carlo Study

PURPOSE The prototype inline MRI-linac system has some advantages over perpendicular models including avoiding the electron return effect. One of the disadvantages of the inline approach is the increased skin dose, estimated to be 400-1000% of the dmax dose. The purpose of this work was to design a feasible method to reduce this skin dose to acceptable levels. METHODS Magnetic modeling of proposed MRI-linac designs have been simulated with the inclusion of an optimized permanent magnet system to purge/deflect the electron contamination. The region of air above the phantom was also replaced with a helium bag (region of helium gas) and a beam scrapper below the deflector was added to collect deflected off-axis contamination. Monte Carlo simulations were then performed including the accurate 3D magnetic field maps. Surface dosimetry was recorded to verify the changes to the skin doses. RESULTS Magnetic modelling showed that an optimized NdFeB permanent magnet system located outside the MRI coils (below the MLCs) can provide a strong enough region to purge/deflect a significant portion of the electron contamination from the x-ray beam. The impact on the MRI uniformity is around 100 ppm and hence is correctable via active/passive shimming of the MRI. The helium region also significantly limits the production of contamination traveling towards the phantom surface. Entry doses near CAX are predicted to be similar to the 0 T case. CONCLUSIONS Magnetic and Monte Carlo modeling were performed to estimate the effect that a permanent magnet purging system, beam scrapper, and helium bag would have on lowering the skin doses in an inline MRI-Linac system. MRI non-uniformities introduced by the deflector could be corrected, contamination is mostly purged or blocked, and the helium bag minimizes air-generated contamination. As a result skin doses are comparable to having zero magnetic field.