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At least 19 records

Nuclear Data Sheets for A = 61

The evaluated spectroscopic data are presented for known nuclides of mass 61 (Sc,Ti,V,Cr,Mn,Fe,Co,Ni, Cu,Zn,Ga,Ge). Excited-state data are nonexistent for {sup 61}Sc, {sup 61}Ti, {sup 61}V, and {sup 61}Ge. Significant amounts of new data have been added since the previous NDS evaluation of A=61 nuclides (1999Bh04). {sup 61}Sc nuclide is now experimentally known, but without any knowledge of its half-life. The ground state half-lives of {sup 61}Ti and {sup 61}V are now determined. Excited-state data have become available for {sup 61}Cr, {sup 61}Mn and {sup 61}Ga, while for {sup 61}Ge, five excited states were erroneously assigned in the previous evaluation, these have been omitted here. Improved and extensive high-spin excitations are available for {sup 61}Fe, {sup 61}Cu and {sup 61}Zn, including several new superdeformed structures in {sup 61}Cu and {sup 61}Zn. Thermal neutron capture γ-ray data are available in detail from 2004Ra23. The radioactive decay schemes of {sup 61}Sc and {sup 61}Ti are not known, while those for {sup 61}V, {sup 61}Fe and {sup 61}Ge are poorly known. This work benefited from the earlier full evaluations of A=61 published by 1999Bh04, 1983Ek01 and 1975Au05 and the one published in an ‘update’ mode by 1992Zh31. The data and conclusions presented in the current work supersede those in all the previous evaluations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Thermal-structural panel buckling tests

A titanium-matrix-composite (TMC) hat-stiffened panel of 61 cm sq area and 3.175 cm thick was nondestructively tested to 649 C to examine its buckling characteristics. Compressive loads were applied to the panel in a 978.6 kN uniaxial load frame system. High-temperature testing was performed using quartz lamp heating. A single-strain-age force/stiffness buckling prediction technique was developed to predict panel buckling loads. For the monolithic panel, these test predictions correlated within 10 percent with a finite-element buckling analyses performed elsewhere. Comparisons between force/stiffness predictions and analyses for the TMC panel are in progress.

Thompson, Randolph C.↗

SU-F-E-02: A Feasibility Study for Application of Metal Artifact Reduction Techniques in MR-Guided Brachytherapy Gynecological Cancer with Titanium Applicators

Purpose: Utilization of Titanium Tandem and Ring (T&R) applicators in MR-guided brachytherapy has become widespread for gynecological cancer treatment. However, Titanium causes magnetic field disturbance and susceptibility artifact, which complicate image interpretation. In this study, metal artifact reduction techniques were employed to improve the image quality and reduce the metal related artifacts. Methods: Several techniques were employed to reduce the metal artifact caused by titanium T&R applicator. These techniques include Metal Artifact Reduction Sequence (MARS), View Angle Tilting (VAT) to correct in-plane distortion, and Slice Encoding for Metal Artifact Correction (SEMAC) for through-plane artifact correction. Moreover, MARS can be combined with VAT to further reduce the in-plane artifact by reapplying the selection gradients during the readout (MARS+VAT). SEMAC uses a slice selective excitation but acquires additional z-encodings in order to resolve off-resonant signal and to reduce through-plane distortions. Results: Comparison between the clinical sequences revealed that increasing the bandwidth reduces the error in measured diameter of T&R. However, the error is larger than 4mm for the best case with highest bandwidth and spatial resolution. MARS+VAT with isotropic resolution of 1mm reduced the error to 1.9mm which is the least among the examined 2D sequences. The measured diameter of tandem from SEMAC+VAT has the closest value to the actual diameter of tandem (3.2mm) and the error was reduced to less than 1mm. In addition, SEMAC+VAT significantly reduces the blooming artifact in the ring compared to clinical sequences. Conclusion: A higher bandwidth and spatial resolution sequence reduces the artifact and diameter of applicator with a slight compromise in SNR. Metal artifact reduction sequences decrease the distortion associated with titanium applicator. SEMAC+VAT sequence in combination with VAT revealed promising results for titanium imaging and can be utilized for MR-guided brachytherapy in gynecological cancer. The author is employee with Philips Healthcare.

60 APPLIED LIFE SCIENCES↗

Approaches to reducing photon dose calculation errors near metal implants

Purpose: Dose calculation errors near metal implants are caused by limitations of the dose calculation algorithm in modeling tissue/metal interface effects as well as density assignment errors caused by imaging artifacts. The purpose of this study was to investigate two strategies for reducing dose calculation errors near metal implants: implementation of metal-based energy deposition kernels in the convolution/superposition (C/S) dose calculation method and use of metal artifact reduction methods for computed tomography (CT) imaging. Methods: Both error reduction strategies were investigated using a simple geometric slab phantom with a rectangular metal insert (composed of titanium or Cerrobend), as well as two anthropomorphic phantoms (one with spinal hardware and one with dental fillings), designed to mimic relevant clinical scenarios. To assess the dosimetric impact of metal kernels, the authors implemented titanium and silver kernels in a commercial collapsed cone C/S algorithm. To assess the impact of CT metal artifact reduction methods, the authors performed dose calculations using baseline imaging techniques (uncorrected 120 kVp imaging) and three commercial metal artifact reduction methods: Philips Healthcare’s O-MAR, GE Healthcare’s monochromatic gemstone spectral imaging (GSI) using dual-energy CT, and GSI with metal artifact reduction software (MARS) applied. For the simple geometric phantom, radiochromic film was used to measure dose upstream and downstream of metal inserts. For the anthropomorphic phantoms, ion chambers and radiochromic film were used to quantify the benefit of the error reduction strategies. Results: Metal kernels did not universally improve accuracy but rather resulted in better accuracy upstream of metal implants and decreased accuracy directly downstream. For the clinical cases (spinal hardware and dental fillings), metal kernels had very little impact on the dose calculation accuracy (<1.0%). Of the commercial CT artifact reduction methods investigated, the authors found that O-MAR was the most consistent method, resulting in either improved dose calculation accuracy (dental case) or little impact on calculation accuracy (spine case). GSI was unsuccessful at reducing the severe artifacts caused by dental fillings and had very little impact on calculation accuracy. GSI with MARS on the other hand gave mixed results, sometimes introducing metal distortion and increasing calculation errors (titanium rectangular implant and titanium spinal hardware) but other times very successfully reducing artifacts (Cerrobend rectangular implant and dental fillings). Conclusions: Though successful at improving dose calculation accuracy upstream of metal implants, metal kernels were not found to substantially improve accuracy for clinical cases. Of the commercial artifact reduction methods investigated, O-MAR was found to be the most consistent candidate for all-purpose CT simulation imaging. The MARS algorithm for GSI should be used with caution for titanium implants, larger implants, and implants located near heterogeneities as it can distort the size and shape of implants and increase calculation errors.

60 APPLIED LIFE SCIENCES↗

SU-F-T-54: Determination of the AAPM TG-43 Brachytherapy Dosimetry Parameters for A New Titanium-Encapsulated Yb-169 Source by Monte Carlo Calculations

Purpose: To determine the AAPM TG-43 brachytherapy dosimetry parameters of a new titanium-encapsulated Yb-169 source designed to maximize the dose enhancement during gold nanoparticle-aided radiation therapy (GNRT). Methods: An existing Monte Carlo (MC) model of the titanium-encapsulated Yb-169 source, which was described in the current investigators’ published MC optimization study, was modified based on the source manufacturer’s detailed specifications, resulting in an accurate model of the titanium-encapsulated Yb-169 source that was actually manufactured. MC calculations were then performed using the MCNP5 code system and the modified source model, in order to obtain a complete set of the AAPM TG-43 parameters for the new Yb-169 source. Results: The MC-calculated dose rate constant for the new titanium-encapsulated Yb-169 source was 1.05 ± 0.03 cGy per hr U, indicating about 10% decrease from the values reported for the conventional stainless steel-encapsulated Yb-169 sources. The source anisotropy and radial dose function for the new source were found similar to those reported for the conventional Yb-169 sources. Conclusion: In this study, the AAPM TG-43 brachytherapy dosimetry parameters of a new titanium-encapsulated Yb-169 source were determined by MC calculations. The current results suggested that the use of titanium, instead of stainless steel, to encapsulate the Yb-169 core would not lead to any major change in the dosimetric characteristics of the Yb-169 source, while it would allow more low energy photons being transmitted through the source filter thereby leading to an increased dose enhancement during GNRT. Supported by DOD/PCRP grant W81XWH-12-1-0198 This investigation was supported by DOD/PCRP grant W81XWH-12-1- 0198.

60 APPLIED LIFE SCIENCES↗

SU-F-J-212: Enabling Conventional Cone Beam CT with the Capability of Dual Energy Imaging Using a Simple Add-On Beam Modifier

Purpose: In searching for a robust, efficient and cost-effective dual energy cone beam CT (DECBCT) solution for various radiation oncology applications, in particularly for improved proton dose planning/replanning accuracy and DE-CBCT guided radiation therapy, we investigate a novel energy modulation scheme using a beam modifier placed between the source and patient and optimize its geometric configuration for routine clinical use. Methods: The study was performed using a Hitachi CBCT scanner and the tube voltage was set at 125 kVp. The higher energy beam was obtained by filtering the incident utilizing a beam modulation layer (material: copper, thickness: 1.8 mm). To avoid the need for double scans (one with and one without the energy modulator), the modulation layer was configured to cover only the half of the X-ray beam so that two sets of sinograms corresponding low and high energies were collected after a single gantry rotation of 360 deg. The average high energy and low energy HU numbers (HUhigh and HUlow) were derived for pixels in a defined region-of-interest, respectively. Results: The beam modifier increased the threshold of the energy spectrum from ∼20 keV up to ∼50 keV. Two complete sets of images were obtained with good alignment between the high energy and low-energy cases without any artifact observed (Fig. 2). The HUlow/HUhigh is ∼0/0 (water), ∼394/238 (brain), ∼1283/1085 (cortical bone) and ∼3000/1800 (titanium). Conclusion: The feasibility of the proposed DECT implementation using a beam modifier has been demonstrated. Compared to the existing DECT solutions, the proposed scheme is much more cost-effective and requires minimum hardware modification. The work lays foundation for us to study the quantification of HU values to derive material density images and atomic number (and electron density) of substances.

60 APPLIED LIFE SCIENCES↗

SU-E-T-328: Dosimetric Impact of Cobalt-Chrome Stabilization Hardware in Paraspinal Radiation Therapy

Purpose: Due to saturation, high density materials Result in an apparent density of 3.2 g/cm{sup 3} in CT images. The true density of traditional titanium stabilization rods (∼4.4 g/cm{sup 3}) is typically ignored in treatment planning. This may not be acceptable for new cobalt-chrome rods with a density of 8.5 g/cm{sup 3}. This study reports the dosimetric impact of cobalt-chrome rods in paraspinal radiotherapy. Methods: For titanium and cobalt-chrome rods, two planning studies were done for both IMRT and VMAT in Varian Eclipse using AAA. 1) The effect of planning without assigning the true rod density was assessed by comparing plans generated with the apparent density and recalculated with the true density for titanium and cobalt-chrome. 2) To test if TPS can compensate for high density rods during optimization. Furthermore, TPS calculation accuracy was verified using MapCheck for a single 20 x 10 cm{sup 2} field. The MapCheck was incrementally shifted to achieve measurement resolution of 1 mm. Results: PTV coverage was ∼0.3% and ∼4.7% lower in plans that were recalculated with the true rod density of titanium and cobalt-chrome, respectively. PTV coverage can be maintained if the correct density is used in optimization. Measurements showed that TPS overestimated the dose locally by up to 11% for cobalt-chrome rods and up to 4% for titanium rods if the density is incorrect. With density corrected, maximum local differences of 6% and 3% were seen for cobalt-chrome and titanium rods, respectively. At 2 cm beneath a rod, electrons scattered from the side of the rod increased the lateral dose and diminished as depth increases. TPS was not able to account for this effect properly even with the true rod density assigned. Conclusion: Neglecting the true density of cobalt-chrome rods can cause under coverage to the PTV. Assigning the correct density during treatment planning can minimize unexpected decrease in PTV dose.

60 APPLIED LIFE SCIENCES↗

Defining the Shape Density of Cerium Oxide and Titanium Dioxide Aerosol

The “shape density” of CeO 2 (cerium oxide) powder (American Elements, Los Angeles CA) is estimated to be 0.82 ± 0.02 g cm -3 and the shape density for TiO 2 (titanium dioxide) powder (American Elements, Los Angeles CA) is estimated as 0.54 ± 0.01 g cm -3 . The “shape density” is defined as the measured loose density (ASTM 2018 and ASTM 2021) of a powder divided by the ICRP (International Commission on Radiological Protection) default aerosol shape factor (Valetin 2002). This information is used for experimental measurements of the RRF (respirable release factor) and other respirable source term parameter (US DOE 1994) experiments at the Los Alamos National Laboratory (LANL). The shape density is the input parameter to the TSI Inc (Shoreview MN) AIM (Aerosol Instrument Manager) software for the TSI model 3321 APS (Aerodynamic Particle Sizer) (Figure 1).

61 RADIATION PROTECTION AND DOSIMETRY↗

SU-F-J-74: High Z Geometric Integrity and Beam Hardening Artifact Assessment Using a Retrospective Metal Artifact Reduction (MAR) Reconstruction Algorithm

Purpose: To test the efficacy of a retrospective metal artifact reduction (MAR) reconstruction algorithm for a commercial computed tomography (CT) scanner for radiation therapy purposes. Methods: High Z geometric integrity and artifact reduction analysis was performed with three phantoms using General Electric’s (GE) Discovery CT. The three phantoms included: a Computerized Imaging Reference Systems (CIRS) electron density phantom (Model 062) with a 6.5 mm diameter titanium rod insert, a custom spine phantom using Synthes Spine hardware submerged in water, and a dental phantom with various high Z fillings submerged in water. Each phantom was reconstructed using MAR and compared against the original scan. Furthermore, each scenario was tested using standard and extended Hounsfield Unit (HU) ranges. High Z geometric integrity was performed using the CIRS phantom, while the artifact reduction was performed using all three phantoms. Results: Geometric integrity of the 6.5 mm diameter rod was slightly overestimated for non-MAR scans for both standard and extended HU. With MAR reconstruction, the rod was underestimated for both standard and extended HU. For artifact reduction, the mean and standard deviation was compared in a volume of interest (VOI) in the surrounding material (water and water equivalent material, ∼0HU). Overall, the mean value of the VOI was closer to 0 HU for the MAR reconstruction compared to the non-MAR scan for most phantoms. Additionally, the standard deviations for all phantoms were greatly reduced using MAR reconstruction. Conclusion: GE’s MAR reconstruction algorithm improves image quality with the presence of high Z material with minimal degradation of its geometric integrity. High Z delineation can be carried out with proper contouring techniques. The effects of beam hardening artifacts are greatly reduced with MAR reconstruction. Tissue corrections due to these artifacts can be eliminated for simple high Z geometries and greatly reduced for more complex geometries.

60 APPLIED LIFE SCIENCES↗

SU-F-P-40: Analysis of Pelvic Lymph Node Margin Using Prostate Fiducial Markers, for SBRT Treatments

Purpose: The use of fiducials markers in prostate treatment allows a precise localization of this volume. Typical prostate SBRT margins with fiducials markers are 5mm in all directions, except toward the rectum, where 3mm is used. For some patients nearby pelvic lymph nodes with 5mm margin need to be irradiate assuming that its localization is linked to the prostate fiducial markers instead of bony anatomy. The purpose of this work was to analyze the geometric impact of locate the lymph node regions through the patient positioning by prostate fiducial markers. Methods: 10 patients with prostate SBRT with lymph nodes irradiation were selected. Each patient had 5 implanted titanium fiducial markers. A Novalis TX (BrainLAB-Varian) with ExacTrac and aSi1000 portal image was used. Treatment plan uses 11 beams with a dose prescription (D95%) of 40Gy to the prostate and 25Gy to the lymph node in 5 fractions. Daily positioning was carried out by ExacTrac system based on the implanted fiducials as the reference treatment position; further position verification was performed using the ExacTrac and two portal images (gantry angle 0 and 90) based on bony structures. Comparison between reference position with bony based ExacTrac and portal image localization, was done for each treatment fraction Results: A total of 50 positioning analysis were done. The average discrepancy between reference treatment position and ExacTrac based on bony anatomy (pubic area) was 4.2mm [0.3; 11.2]. The discrepancy was <5mm in 61% of the cases and <9mm in 92%. Using portal images the average discrepancy was 3.7mm [0.0; 11.1]. The discrepancy was <5mm in 69% of the cases and <9mm in 96%. Conclusion: Localizing lymph node by prostate fiducial markers may produce large discrepancy as large as 11mm compared to bony based localization. Dosimetric impact of this discrepancy should be studied.

60 APPLIED LIFE SCIENCES↗

SU-F-T-441: Dose Calculation Accuracy in CT Images Reconstructed with Artifact Reduction Algorithm

Purpose: Accuracy of radiotherapy dose calculation in patients with surgical implants is complicated by two factors. First is the accuracy of CT number, second is the dose calculation accuracy. We compared measured dose with dose calculated on CT images reconstructed with FBP and an artifact reduction algorithm (OMAR, Philips) for a phantom with high density inserts. Dose calculation were done with Varian AAA and AcurosXB. Methods: A phantom was constructed with solid water in which 2 titanium or stainless steel rods could be inserted. The phantom was scanned with the Philips Brillance Big Bore CT. Image reconstruction was done with FBP and OMAR. Two 6 MV single field photon plans were constructed for each phantom. Radiochromic films were placed at different locations to measure the dose deposited. One plan has normal incidence on the titanium/steel rods. In the second plan, the beam is at almost glancing incidence on the metal rods. Measurements were then compared with dose calculated with AAA and AcurosXB. Results: The use of OMAR images slightly improved the dose calculation accuracy. The agreement between measured and calculated dose was best with AXB and image reconstructed with OMAR. Dose calculated on titanium phantom has better agreement with measurement. Large discrepancies were seen at points directly above and below the high density inserts. Both AAA and AXB underestimated the dose directly above the metal surface, while overestimated the dose below the metal surface. Doses measured downstream of metal were all within 3% of calculated values. Conclusion: When doing treatment planning for patients with metal implants, care must be taken to acquire correct CT images to improve dose calculation accuracy. Moreover, great discrepancies in measured and calculated dose were observed at metal/tissue interface. Care must be taken in estimating the dose in critical structures that come into contact with metals.

60 APPLIED LIFE SCIENCES↗

SU-F-T-47: MRI T2 Exclusive Based Planning Using the Endocavitary/interstitial Gynecological Benidorm Applicator: A Proposed TPS Library and Preplan Efficient Methodology

Purpose: ABS and GEC-ESTRO have recommended MRI T2 for image guided brachytherapy. Recently, a new applicator (Benidorm Template, TB) has been developed in our Department (Rodriguez et al 2015). TB is fully MRI compatible because the Titanium needles and it allows the use of intrauterine tandem. Currently, TPS applicators library are not currently available for non-rigid applicators in case of interstitial component as the TB.The purpose of this work is to present the development of a library for the TB, together with its use on a pre-planning technique. Both new goals allow a very efficient and exclusive T2 MRI based planning clinical TB implementation. Methods: The developed library has been implemented in Oncentra Brachytherapy TPS, version 4.3.0 (Elekta) and now is being implemented on Sagiplan v 2.0 TPS (Eckert&Ziegler BEBIG). To model the TB, free and open software named FreeCAD and MeshLab have been used. The reconstruction process is based on three inserted A-vitamin pellets together with the data provided by the free length. The implemented preplanning procedure is as follow: 1) A MRI T2 acquisition is performed with the template in place just with the vaginal cylinder (no uterine tube nor needles). 2) The CTV is drawn and the required needles are selected using a developed Java based application and 3) A post-implant MRI T2 is performed. Results: This library procedure has been successfully applied by now in 25 patients. In this work the use of the developed library will be illustrated with clinical examples. The preplanning procedure has been applied by now in 6 patients, having significant advantages: needle depth estimation, needle positions and number are optimized a priori, time saving, etc Conclusion: TB library and pre-plan techniques are feasible and very efficient and their use will be illustrated in this work.

60 APPLIED LIFE SCIENCES↗

Polyoxometalate derivatized metal oxide electrodes

A porous transparent electrode is formed where a film comprising of semiconducting nanoparticles is decorated with polyoxometalates (POMs) bonded to their surfaces. The semiconducting nanoparticles are transparent metal oxide. The semiconducting nanoparticles include tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), or titanium dioxide (TiO 2 ). In an embodiment, the POM is [SiW 12 O 40 ]L 4− ; [α-P 2 W 18 O 62 ] 6− ; or [α 2 -P 2 W 17 O 61 ] 10− . The semiconducting nanoparticles bond to the POM through a combination of electrostatic interactions and hydrogen bonds. The porous transparent electrode can be placed in a protonated form or ion-paired with alkali metal cations or tetraalkylammonium cations.

Dares, Christopher J.↗

SU-F-J-163: In Vivo Quantification of Sequence Parameter Effect On Geometric Distortion Caused by Implanted Titanium Brachytherapy Applicator

Purpose: The use of MR to plan and evaluate brachytherapy treatment for cervical cancer is increasing given the availability of MR conditional or safe applicators and MRI’s proven superiority to CT for characterizing soft tissue lesions. The titanium applicators, however, cause geometric distortions or imaging artifacts, which reduce the utility of MRI for dosimetry. We sought to quantify the observed volume of the same applicator on a previously optimized T2 sequence in comparison to the conventional T2 sequence and CT obtained for brachytherapy planning. Methods: Prior work with testing in phantoms showed that increases in readout bandwidth yielded reductions in artifact area and distortion measurements even with voxel increases. Following IRB approval, nine patients with titanium tandem & ovoid applicator (Varian Medical Systems) in place were scanned with a standard periprocedural protocol which included sagittal T2 fast spin echo (FSE) acquisition (res 0.98×0.78×4.0 mm{sup 3}; BW 200Hz). An additional T2-weighted FSE sequence (res 0.98×0.98×3–4 mm{sup 3}; BW500Hz) with increased readout bandwidth, readout voxel size, and echo train length was added to the protocol. Volume measurements of the applicator (from tip to cervical stop) were hand-segmented in Velocity AI 3.1 (Velocity Medical Solutions) for the two T2 FSE sequences and a planning CT obtained shortly after MRI. Differences were analyzed using a paired t-test. Results: Average apparent volumes of the applicator on standard T2 sequence, decreased bandwidth T2 sequence and CT were 5.922±1.283 cm{sup 3}, 4.544±1.524 cm3, and 2.304±0.509 cm{sup 3} respectively. Conclusion: Apparent volumes of a brachytherapy applicator can be compared in vivo. The modified sequence results in decreased apparent size of the cervical applicator. Both MR sequence volumes were larger than the planning CT, which was expected. Future work will focus on the diagnostic quality of the new sequence and quantifying any geometric shifts after CT to MRI registration based on anatomical landmarks.

60 APPLIED LIFE SCIENCES↗

Analysis of hafnium configuration of FLUOLE-2 program - 25409

The purpose of this paper is to present an analysis of the hafnium configuration of FLUOLE-2 program. FLUOLE-2 is a set of benchmark-type experiments dedicated to neutron attenuation analysis with the aim of improving the TRIPOLI-4{sup R} Monte Carlo code validation and industrial neutron physics scheme. The CEA developed this program, with the support of EDF, in order to be representative of 900 and 1450 MWe Pressurized Water Reactors operated in France. For that purpose, different stainless steel structures have been designed and appropriately positioned inside the EOLE facility located at Cadarache CEA center. EOLE is a pool type zero power reactor, composed of a cylindrical aluminum vessel with an over-structure of stainless steel, that is able to contain various types of core and related structures. The FLUOLE-2 core has been designed as a 29 x 29 pins square lattice of fuel rods. Different kind of dosimeters (cobalt, gold, tin, rhodium, indium, iron, nickel, titanium, aluminum, and vanadium) were irradiated inside and outside the core. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗

SU-E-T-404: Evaluation of the Effect of Spine Hardware for CyberKnife Spinal Stereotactic Radiosurgery

Purpose: Spine hardware made of high-Z materials such as titanium has the potential to affect the dose distribution around the metal rods in CyberKnife spinal stereotactic radiosurgery (SRS) treatments. The purpose of this work was to evaluate the magnitude of such effect retrospectively for clinical CyberKnife plans. Methods: The dose calculation was performed within the MultiPlan treatment planning system using the ray tracing (RT) and Monte Carlo (MC) method. A custom density model was created by extending the CT-to-Density table to titanium density of 4.5 g/cm3 with the CT number of 4095. To understand the dose perturbation caused by the titanium rod, a simple beam setup (7.5 mm IRIS collimator) was used to irradiate a mimic rod (5 mm) with overridden high density. Five patient spinal SRS cases were found chronologically from 2010 to 2015 in our institution. For each case, the hardware was contoured manually. The original plan was re-calculated using both RT and MC methods with and without rod density override without changing clinical beam parameters. Results: The simple beam irradiation shows that there is 10% dose increase at the interface because of electron backscattering and 7% decrease behind the rod because of photon attenuation. For actual clinical plans, the iso-dose lines and DVHs are almost identical (<2%) for calculations with and without density override for both RT and MC methods. However, there is a difference of more than 10% for D90 between RT and MC method. Conclusion: Although the dose perturbation around the metal rods can be as large as 10% for a single beam irradiation, for clinical treatments with complex beam composition the effect of spinal hardware to the PTV and spinal dose is minimal. As such, the MC dose algorithm without rod density override for CyberKnife spinal SRS is acceptable.

60 APPLIED LIFE SCIENCES↗

SU-E-T-329: Dosimetric Impact of Implementing Metal Artifact Reduction Methods and Metal Energy Deposition Kernels for Photon Dose Calculations

Purpose: To investigate two strategies for reducing dose calculation errors near metal implants: use of CT metal artifact reduction methods and implementation of metal-based energy deposition kernels in the convolution/superposition (C/S) method. Methods: Radiochromic film was used to measure the dose upstream and downstream of titanium and Cerrobend implants. To assess the dosimetric impact of metal artifact reduction methods, dose calculations were performed using baseline, uncorrected images and metal artifact reduction Methods: Philips O-MAR, GE’s monochromatic gemstone spectral imaging (GSI) using dual-energy CT, and GSI imaging with metal artifact reduction software applied (MARs).To assess the impact of metal kernels, titanium and silver kernels were implemented into a commercial collapsed cone C/S algorithm. Results: The CT artifact reduction methods were more successful for titanium than Cerrobend. Interestingly, for beams traversing the metal implant, we found that errors in the dimensions of the metal in the CT images were more important for dose calculation accuracy than reduction of imaging artifacts. The MARs algorithm caused a distortion in the shape of the titanium implant that substantially worsened the calculation accuracy. In comparison to water kernel dose calculations, metal kernels resulted in better modeling of the increased backscatter dose at the upstream interface but decreased accuracy directly downstream of the metal. We also found that the success of metal kernels was dependent on dose grid size, with smaller calculation voxels giving better accuracy. Conclusion: Our study yielded mixed results, with neither the metal artifact reduction methods nor the metal kernels being globally effective at improving dose calculation accuracy. However, some successes were observed. The MARs algorithm decreased errors downstream of Cerrobend by a factor of two, and metal kernels resulted in more accurate backscatter dose upstream of metals. Thus, these two strategies do have the potential to improve accuracy for patients with metal implants in certain scenarios. This work was supported by Public Health Service grants CA 180803 and CA 10953 awarded by the National Cancer Institute, United States of Health and Human Services, and in part by Mobius Medical Systems.

60 APPLIED LIFE SCIENCES↗