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High resolution polymer gel dosimetry for small beam irradiation using a 7T micro-MRI scanner

The use of small field radiation beams has greatly increased with advanced radiation therapy techniques such as IMRT, rotational IMRT, and stereotactic body radiotherapy. In this work small field 3D dose distributions have been measured with high spatial resolution using polymer gels and 7T micro-MR imaging. A MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) polymer gel [1] phantom was used to capture the 3D dose distributions for two small field (5 × 5 mm2 and 10 × 10 mm2) for a 6MV x-ray beam. High resolution 3D T2 maps were obtained with 7T micro-MRI (0.156mm × 0.156mm × 1mm, MSME pulse sequence). For comparison T2 maps, the gel phantom was scanned in a 3T MRI clinical scanner (0.254mm × 0.254mm × 2mm, FSE pulse sequence). Normalized 3D dose maps were calculated in Matlab. Results show that 7T micro-MRI 3D gel dosimetry measurements are much more stable, less noisy, and have higher spatial resolution than those obtained using a 3T clinical scanner for the same amount of scan time. In general, 3D gel dosimetry results also agree with simultaneously-obtained radiochromic film dosimetry. This study indicates that the MAGIC polymer gel with 7T micro-MRI for 3D dose readout could potentially be used for small radiation beams, including measurements for micro-beams (field size ~ 100um).

Demonstration of dose and scatter reductions for interior computed tomography.

With continuing developments in computed tomography (CT) technology and its increasing use of CT imaging, the ionizing radiation dose from CT is becoming a major public concern particularly for high-dose applications such as cardiac imaging. We recently proposed a novel interior tomography approach for x-ray dose reduction that is very different from all the previously proposed methods. Our method only uses the projection data for the rays passing through the desired region of interest. This method not only reduces x-ray dose but scatter as well. In this paper, we quantify the reduction in the amount of x-ray dose and scattered radiation that could be achieved using this method. Results indicate that interior tomography may reduce the x-ray dose by 18% to 58% and scatter to the detectors by 19% to 59% as the FOV is reduced from 50 to 8.6 cm.

SU‐E‐T‐468: Gamma Knife Perfexion Dosimetry: A Monte Carlo Model of One Sector

PURPOSE We have implemented a Monte Carlo (MC) based dose computation model of one sector of the Gamma Knife Perfexion (GK PFX) using the Penelope MC dosimetry codes. The single sector simulation was rotated about the z-axis to model all eight GK sectors. GK dosimetric aspects examined include: 1) output factors (OF) for each of the three GK collimator sizes (4, 8, 16 mm), 2) OFs for each source row and collimator size, and 3) dose distribution profiles along the x- and z-axes, compared to film measurements and dose calculations from the Leksell GammaPlan (LGP) workstation. METHODS We defined the internal GK PFX geometry in Penelope with the aid of vendor-supplied proprietary information. A single source per row was modeled for five rows for each of the 3 collimators (15 beams modeled). MC simulations were carried out on a Linux cluster. Phase space files (PSFs) were collected for the 15 modeled collimators then rotated about the z-axis to model the sector of 24 sources per collimator. 3D dose distributions from the MC model, film, and LGP DICOM-RT dose exports were analyzed using Matlab. For OF calculations, a 16 cm diameter dosimetry sphere was modeled with a virtual detector volume at its center. RESULTS Good agreement is found for row- and total-output factors (greatest deviation of any type < 4%) compared to reference values. Off-axis factors closely follow LGP predicted dose distributions along the x-axis and differ on the inferior side of the z-axis. CONCLUSIONS Detailed geometric representations (radiation source, device components) of the GK PFX are required for high fidelity MC simulations. Calculated GK PFX OF values depend on the simulated detector volume size (4 mm OF most dependent). Our model shows strong agreement for the GK PFX OFs and dose profile curves compared to reference values. Non-disclosure agreement for proprietary information with Elekta AB. No financial contribution.

Improved volumetric imaging in tomosynthesis using combined multiaxial sweeps

This study explores the volumetric reconstruction fidelity attainable using tomosynthesis with a kV imaging system which has a unique ability to rotate isocentrically and with multiple degrees of mechanical freedom. More specifically, we seek to investigate volumetric reconstructions by combining multiple limited‐angle rotational image acquisition sweeps. By comparing these reconstructed images with those of a CBCT reconstruction, we can gauge the volumetric fidelity of the reconstructions. In surgical situations, the described tomosynthesis‐based system could provide high‐quality volumetric imaging without requiring patient motion, even with rotational limitations present. Projections were acquired using the Digital Integrated Brachytherapy Unit, or IBU‐D. A phantom was used which contained several spherical objects of varying contrast. Using image projections acquired during isocentric sweeps around the phantom, reconstructions were performed by filtered backprojection. For each image acquisition sweep configuration, a contrasting sphere is analyzed using two metrics and compared to a gold standard CBCT reconstruction. Since the intersection of a reconstructed sphere and an imaging plane is ideally a circle with an eccentricity of zero, the first metric presented compares the effective eccentricity of intersections of reconstructed volumes and imaging planes. As another metric of volumetric reconstruction fidelity, the volume of one of the contrasting spheres was determined using manual contouring. By comparing these manually delineated volumes with a CBCT reconstruction, we can gauge the volumetric fidelity of reconstructions. The configuration which yielded the highest overall volumetric reconstruction fidelity, as determined by effective eccentricities and volumetric contouring, consisted of two orthogonally‐offset 60° L‐arm sweeps and a single C‐arm sweep which shared a pivot point with one the L‐arm sweeps. When compared to a similar configuration that lacked the C‐arm component, it is shown that the C‐arm improves the delineation of volumes along the transverse axis. The results described herein suggest that volumetric reconstruction using multiple, unconstrained orthogonal sweeps can provide an improvement compared with traditional cone beam CT using standard axial rotations. PACS number: 87.57.nf

Mechanisms and prevention of thermal injury from gamma radiosurgery headframes during 3T MR imaging

Magnetic resonance imaging (MRI) is regularly used for stereotactic imaging of Gamma Knife (GK) radiosurgery patients for GK treatment planning. MRI‐induced thermal injuries have occurred and been reported for GK patients with attached metallic headframes. Depending on the specific MR imaging and headframe conditions, a skin injury from MRI‐induced heating can potentially occur where the four headframe screws contact the skin surface of the patients head. Higher MR field strength has a greater heating potential. Two primary heating mechanisms, electromagnetic induction and the antenna effect, are possible. In this study, MRI‐induced heating from a 3T clinical MRI scanner was investigated for stereotactic headframes used in gamma radiosurgery and neurosurgery. Using melons as head phantoms, optical thermometers were used to characterize the temperature profile at various points of the melon headframe composite as a function of two 3T MR pulse sequence protocols. Different combinations of GK radiosurgery headframe post and screw designs were tested to determine best and worst combinations for MRI‐induced heating. Temperature increases were measured for all pulse sequences tested, indicating that the potential exists for MRI‐induced skin heating and burns at the headframe attachment site. This heating originates with electromagnetic induction caused by the RF fields inducing current in a loop formed by the headframe, mounting screws, and the region of the patients head located between any of the two screws. This induced current is then resistively dissipated, with the regions of highest resistance, located at the headframe screw–patient head interface, experiencing the most heating. Significant heating can be prevented by replacing the metallic threads holding the screw with electrically insulated nuts, which is the heating prevention and patient safety recommendation of the GK manufacturer. Our results confirm that the manufacturers recommendation to use insulating nuts reduces the induced currents in the headframe nearly to zero, effectively preventing heating and minimizing the likelihood of thermal injury. PACS numbers: 87.57.‐s, 87.61.‐c, 87.61.Tg, 87.57.c‐

SU‐D‐BRB‐02: Monte Carlo Modeling of the Gamma Knife Perfexion

Purpose: For dosimetric and research irradiation studies, we have implemented a Monte Carlo (MC)dose computation model based on the physical and radiological characteristics of the Gamma Knife Perfexion (GK PFX) using the Penelope MCdosimetry codes. GK dosimetric aspects examined include: 1) output factors (OF) for each of the three GK collimator sizes (4, 8, 16 mm), 2) OFs for each source row and collimator size, and 3) dose distribution profiles. Methods: Vendor proprietary information facilitated our modeling of the GK PFX irradiation geometry, which was mathematically defined within Penelope. MC simulations were carried out on a Linux cluster. 3D dose distributions were analyzed using Matlab. A 16 cm diameter dosimetry sphere was modeled with a virtual detector volume at its center. Detector volume varied from 33 to 590 mm3 to study detector volume effects. A single source per row was modeled for five rows for each collimator (15 beams modeled). Single‐source dose distributions were rotated about the z‐axis of the axially symmetric geometry and summed to simulate all 192 sources. Results: Good agreement is found for row‐ and total‐output factors (greatest deviation <2% for the 4 mm collimator) compared to reference values. Simulated and measured full‐width at half‐ max values of 3D dose distribution profiles show sub‐millimeter differences (0.4 mm, 4 and 8 mm collimators; 0.9 mm, 16 mm collimator). There is excellent agreement for integrated profile shapes. Conclusions: Detailed geometric representations (radiation source, device components) of the GK PFX are required for high fidelity MC simulations. Calculated GK PFX OF values are dependent on the simulated detector volume size (4 mm OF most dependent). Our model shows strong agreement for the GK PFX OFs and dose profile shapes compared to reference values. Acknowledgement: Non‐disclosure agreement for proprietary information with Elekta AB. No financial contribution.

SU-E-T-99: Small Field Output Factor Measurement Using MAGIC Gel Dosimeter in 3T MRI

PURPOSE Small field dosimetry is very important because of radiation therapy techniques that use small fields such as IMRT, gamma and body radiosurgery, cyberknife and tomotherapy. We investigated use of a MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) gel dosimeter to quantitatively measure small field output factors (OFs) for 6MV x rays. METHODS In this work, MAGIC (Gelatin 9%; Methacrylic acid 4%; CuSO4 0.1mM; Ascorbic ascid 2mM; Glucose 10%) gel phantoms were developed to measure the 6MV x-ray output factors for 1×1 up to 10×10 cm2 square fields (Varian 2100C linear accelerator). For comparison, 3 ion chambers (PTW:TN30013, Exradin A12, Capintec PR-05P), Gafchromic film (EBT2), and TLD (LiF-100) were used to measure the small field OFs under identical experimental conditions: 6cm depth (solid water), SAD=100cm, SSD = 94cm, 6MV, 512 MUs per irradiation. Relative OFs were normalized to a reference field (10×10 cm2 @ SAD =100cm). MAGIC gel dosimeters were scanned in a 3T GE signa® EXCITETM clinical scanner using a Spin Echo pulse sequence for dose distribution readout (pixel size = 0.4mm, slice thickness 3mm, TR = 4000ms, TE = 10ms and 110ms, respectively). Gel dose distributions were then calculated using custom Matlab code. RESULTS 6MV x-ray OFs versus field size for all detectors were graphically compared. The MAGIC polymer gel dosimeter OF for a 1×1 cm2 field is 0.612 (+/- 5%), approximately 2% different from the OFs measured using small volume dosimeters (TLD, EBT2 film and the Capintec PR-05P ion chamber). Larger ion chamber (PTW:TN30013 and Exradin A12, both ∼ 0.6cc) OFs were low (OF = 0.26 for 1×1cm2 ) due to nonequilibrium and partial volume conditions. CONCLUSIONS MAGIC gel dosimeter with 3T MRI scanner as a read-out makes it an ideal tool for small field dosimetry.

SU‐E‐T‐116: Measuring Dose Distribution Accuracy in Stereotactic Radiosurgery and Gamma Knife Treatment Using MR Or CT Imaging

Purpose: This study aimed to establish a standard dosimetry protocol for HDR Ir‐192 sources using an ion chamber calibrated with a Co‐60 beam. We developed a dedicated device for ion chambermeasurements with a sandwich method and examined its measurement accuracy. Methods: A microSelectron‐v2 HDR Ir‐192 source was modeled with the EGSnrc/egs_chamber code. The accuracy of modeling was confirmed by comparing calculated results for gL (r) and F(r, angle) with those of TG‐43. First, an optimal source‐to‐chamber (SCD) separation for Ir‐192 dosimetry was determined from measurements with a PTW 31010 chamber at distances of 1.5–5 cm from the source center in water. The measuredionization chamber reading was corrected with the Monte Carlo‐calculated energy response for Co‐60 and Ir‐192, and was converted to the absorbed dose to water. The measured doses were compared with TPS values based on TG‐43. We developed a dedicated device for ion chambermeasurements with a sandwich method at the optimal SCD separation. The average dose measured with two EXRADIN A1SL chambers was compared with the TPS value. Results: Calculated gL (r) and F(r, angle) values agreed well with those of TG‐43. The absorbed dose to water measured with the PTW31010 chamber was 3% lower than that of TPS at a distance of 5 cm and was 3%‐7% lower at distances less than 5 cm. This was addressed to the uncertainty of the chamber positioning. We made a sandwich measurement device with the separation of 5 cm, considering the uncertainty of positioning and measurement time. The dose to water with the sandwich method was in agreement with that of TG‐43 within −1.2%. Conclusions: The optimal distance for ion chambermeasurements was at 5 cm from the Ir‐192 source. The dose to water measurement with the sandwich method is useful for daily dose management for Ir‐192 sources.

SU‐E‐J‐80: Small Beam Dosimetry Using MAGIC Gel with a 7T Micro‐MRI Scanner

Purpose: Dose distribution characteristics for small radiation fields can be very difficult to determine. In this work a MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) 3D polymergel is combined with 7T micro‐MR imaging for high resolution measurements of the small field (<1cm) 3D dose distributions. Methods: MAGIC(Gelatin 9%; Methacrylic acid 4%; CuSO4 0.1 mM; Ascorbic ascid 2mM; Glucose 22%) 3D gel phantoms were irradiated with very small 6MV x‐ray beams (5x5 mm2 and 10 × 10 mm2 square fields; 2mm diameter round field). Gel dose measurements were performed in Bruker 7T mirco‐MRI and GE Signa 3T scanners and with simultaneously‐obtained radiochromic films. T2 maps were acquired using a 10‐echo‐Multi‐Spin Multi‐Echo (MSME) pulse sequence on both MR scanners. Normalized 3D dose maps were calculated in Matlab. Results: Dose distributions determined from 30 minute scans for the 5×5 mm2 and 10×10 mm2 square fields on the 7T MR unit were superior to 3T MR unit in spatial resolution (7T: 0.156mm × 0.156mm × 1mm voxel ; 3T: 0.254mm × 0.254mm × 2mm voxel). For the very small field size (2mm diameter), the MAGIC gel with 7T MRI provided even better quality dose distribution images (1.5hour scan; spatial resolution 79um ×79um × 1mm and ; 12 hour scan: spatial resolution 38um × 38um × 1mm), 3T MRI was not able to read accurate dose profile due to a low SNR in the same 1.5hour scan time. Conclusions: This study indicates that the MAGIC polymergel with 7T micro‐MRI for 3D dose readout can potentially be used for dosimetric characterization of very small radiation beams, including measurements for micro‐beams (field size ∼ 100um). Techniques and limitations for MAGIC geldosimetry via high field MR imaging for dosimetric assessment are detailed in this work.

SU‐GG‐T‐512: Causes and Prevention of MR‐Induced Skin Heating for Patients with Attached Headframes for Gamma Radiosurgery

Purpose: We have investigated the potential for magnetic resonance imaging(MRI) induced skin heating for gamma radiosurgery patients with attached rigid headframes. MRI‐induced heating through three mechanisms may be possible where the four headframe screws contact the skin surface of the patients head. Method and Materials: Using melons as head phantoms, optical thermometers were inserted sub‐surface at selected points to measure the temperature profile of the melon‐headframe composite as a function of the applied 3T MR pulse sequence. Multiple headframe post and screw combinations, representing possible clinical scenarios, were evaluated for MRI‐induced heating.Results: The potential exists for a range of MRI‐induced skin heating from 2–10 C or more at the attachment sites of the radiosurgical headframe. This localized heating originates with the RF fields inducing current in a loop formed by the headframe, mounting screws and the region of the patients head located between any of the two screws, with the loop in a position perpendicular to the RF field. This current is then resistively dissipated, with the regions of highest resistance, the screw‐patient interface, experiencing the most heating. Thus skin heating, including burns, is a potential hazard for gamma radiosurgery patients during MRI scans. However, this hazard is easily prevented by replacing the metallic threads holding the screws with electrically insulated nuts that prevent the formation of current loops. This method has been confirmed and is a recommendation of the gamma units manufacturer. Conclusion: MRI‐induced heating of the skin has been investigated for patients with rigidly attached headframes. Using a melon‐phantom system the cause of heating and potential burns has been determined for selected 3T MR imaging sequences and headframe‐screw combinations. The recommended method for prevention of MRI‐induced skin heating with an attached gamma radiosurgery headframe has been verified. Disclosure: Supported in part by NIH T32‐CA113267.