Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “dose”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Dose rate effects on array CCDs exposed by Co-60 γ rays induce saturation output degradation and annealing tests

The experimental tests of dose rate and annealing effects on array charge-coupled devices (CCDs) are presented. The saturation output voltage (V{sub S}) versus the total dose at the dose rates of 0.01, 0.1, 1.0, 10.0 and 50 rad(Si)/s are compared. Annealing tests are performed to eliminate the time-dependent effects. The V{sub S} degradation levels depend on the dose rates. The V{sub S} degradation mechanism induced by dose rate and annealing effects is analyzed. The V{sub S} at 20 krad(Si) with the dose rate of 0.03 rad(Si)/s are supplemented to assure the degradation curves between the dose rates of 0.1 and 0.01 rad(Si)/s. The CCDs are divided into two groups, with one group biased and the other unbiased during {sup 60}Co γ radiation. The V{sub S} degradation levels of the biased CCDs during radiation are more severe than that of the unbiased CCDs.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Automated treatment planning for a dedicated multi-source intra-cranial radiosurgery treatment unit accounting for overlapping structures and dose homogeneity

Purpose: The purpose of this work is to advance the two-step approach for Gamma Knife{sup ®} Perfexion™ (PFX) optimization to account for dose homogeneity and overlap between the planning target volume (PTV) and organs-at-risk (OARs).Methods: In the first step, a geometry-based algorithm is used to quickly select isocentre locations while explicitly accounting for PTV-OARs overlaps. In this approach, the PTV is divided into subvolumes based on the PTV-OARs overlaps and the distance of voxels to the overlaps. Only a few isocentres are selected in the overlap volume, and a higher number of isocentres are carefully selected among voxels that are immediately close to the overlap volume. In the second step, a convex optimization is solved to find the optimal combination of collimator sizes and their radiation duration for each isocentre location.Results: This two-step approach is tested on seven clinical cases (comprising 11 targets) for which the authors assess coverage, OARs dose, and homogeneity index and relate these parameters to the overlap fraction for each case. In terms of coverage, the mean V{sub 99} for the gross target volume (GTV) was 99.8% while the V{sub 95} for the PTV averaged at 94.6%, thus satisfying the clinical objectives of 99% for GTV and 95% for PTV, respectively. The mean relative dose to the brainstem was 87.7% of the prescription dose (with maximum 108%), while on average, 11.3% of the PTV overlapped with the brainstem. The mean beam-on time per fraction per dose was 8.6 min with calibration dose rate of 3.5 Gy/min, and the computational time averaged at 205 min. Compared with previous work involving single-fraction radiosurgery, the resulting plans were more homogeneous with average homogeneity index of 1.18 compared to 1.47.Conclusions: PFX treatment plans with homogeneous dose distribution can be achieved by inverse planning using geometric isocentre selection and mathematical modeling and optimization techniques. The quality of the obtained treatment plans are clinically satisfactory while the homogeneity index is improved compared to conventional PFX plans.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2013

Dose equations for tube current modulation in CT scanning and the interpretation of the associated CTDI{sub vol}

Purpose: The scanner-reported CTDI{sub vol} for automatic tube current modulation (TCM) has a different physical meaning from the traditional CTDI{sub vol} at constant mA, resulting in the dichotomy “CTDI{sub vol} of the first and second kinds” for which a physical interpretation is sought in hopes of establishing some commonality between the two.Methods: Rigorous equations are derived to describe the accumulated dose distributions for TCM. A comparison with formulae for scanner-reported CTDI{sub vol} clearly identifies the source of their differences. Graphical dose simulations are also provided for a variety of TCM tube current distributions (including constant mA), all having the same scanner-reported CTDI{sub vol}.Results: These convolution equations and simulations show that the local dose at z depends only weakly on the local tube current i(z) due to the strong influence of scatter from all other locations along z, and that the “local CTDI{sub vol}(z)” does not represent a local dose but rather only a relative i(z) ≡ mA(z). TCM is a shift-variant technique to which the CTDI-paradigm does not apply and its application to TCM leads to a CTDI{sub vol} of the second kind which lacks relevance.Conclusions: While the traditional CTDI{sub vol} at constant mA conveys useful information (the peak dose at the center of the scan length), CTDI{sub vol} of the second kind conveys no useful information about the associated TCM dose distribution it purportedly represents and its physical interpretation remains elusive. On the other hand, the total energy absorbed E (“integral dose”) as well as its surrogate DLP remain robust between variable i(z) TCM and constant current i{sub 0} techniques, both depending only on the total mAs = t{sub 0}=i{sub 0} t{sub 0} during the beam-on time t{sub 0}.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2014

Fast dose kernel interpolation using Fourier transform with application to permanent prostate brachytherapy dosimetry

Purpose: Boyer and Mok proposed a fast calculation method employing the Fourier transform (FT), for which calculation time is independent of the number of seeds but seed placement is restricted to calculation grid points. Here an interpolation method is described enabling unrestricted seed placement while preserving the computational efficiency of the original method. Methods: The Iodine-125 seed dose kernel was sampled and selected values were modified to optimize interpolation accuracy for clinically relevant doses. For each seed, the kernel was shifted to the nearest grid point via convolution with a unit impulse, implemented in the Fourier domain. The remaining fractional shift was performed using a piecewise third-order Lagrange filter. Results: Implementation of the interpolation method greatly improved FT-based dose calculation accuracy. The dose distribution was accurate to within 2% beyond 3 mm from each seed. Isodose contours were indistinguishable from explicit TG-43 calculation. Dose-volume metric errors were negligible. Computation time for the FT interpolation method was essentially the same as Boyer's method. Conclusions: A FT interpolation method for permanent prostate brachytherapy TG-43 dose calculation was developed which expands upon Boyer's original method and enables unrestricted seed placement. The proposed method substantially improves the clinically relevant dose accuracy with negligible additional computation cost, preserving the efficiency of the original method.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2014

SU-E-I-59: Image Quality and Dose Measurement for Partial Cone-Beam CT

Purpose: To characterize performance of cone beam CT (CBCT) used in dentistry investigating quantitatively the image quality and radiation dose during dental CBCT over different settings for partial rotation of the x-ray tube. Methods: Image quality and dose measurements were done on a variable field of view (FOV) dental CBCT (Carestream 9300). X-ray parameters for clinical settings were adjustable for 2–10 mA, 60–90 kVp, and two optional voxel size values, but time was fixed for each FOV. Image quality was assessed by scanning cylindrical poly-methyl methacrylate (PMMA) image quality phantom (SEDENTEXCT IQ), and then the images were analyzed using ImageJ to calculate image quality parameters such as noise, uniformity, and contrast to noise ratio (CNR). A protocol proposed by SEDENTEXCT, dose index 1 (DI1), was applied to dose measurements obtained using a thimble ionization chamber and cylindrical PMMA dose index phantom (SEDENTEXCT DI). Dose distributions were obtained using Gafchromic film. The phantoms were positioned in the FOV to imitate a clinical positioning. Results: The image noise was 6–12.5% which, when normalized to the difference of mean voxel value of PMMA and air, was comparable between different FOVs. Uniformity was 93.5ß 99.7% across the images. CNR was 1.7–4.2 and 6.3–14.3 for LDPE and Aluminum, respectively. Dose distributions were symmetric about the rotation angle's bisector. For large and medium FOVs at 4 mA and 80–90 kVp, DI1 values were in the range of 1.26–3.23 mGy. DI1 values were between 1.01–1.93 mGy for small FOV (5×5 cm{sup 2}) at 4–5 mA and 75–84 kVp. Conclusion: Noise decreased by increasing kVp, and the CNR increased for each FOV. When FOV size increased, image noise increased and CNR decreased. DI1 values were increased by increasing tube current (mA), tube voltage (kVp), and/or FOV. Funding for this project from NSERC Discovery grant, UBC Faculty of Dentistry Research Equipment Grant and UBC Faculty of Dentistry S. Wah Leung Endowment Fund.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

SU-E-T-58: Calculation of Dose Distribution of Accuboost Brachytherapy in Deformable Polyvinil Alcohol Breast Phantom Using Biomechanical Modeling and Monte Carlo Simulation

Purpose: the accuboost is the most modern method of breast brachytherapy that is a boost method in compressed tissue by a mammography unit. the dose distribution in uncompressed tissue, as compressed tissue is important that should be characterized. Methods: In this study, the mechanical behavior of breast in mammography loading, the displacement of breast tissue and the dose distribution in compressed and uncompressed tissue, are investigated. Dosimetry was performed by two dosimeter methods of Monte Carlo simulations using MCNP5 code and thermoluminescence dosimeters. For Monte Carlo simulations, the dose values in cubical lattice were calculated using tally F6. The displacement of the breast elements was simulated by Finite element model and calculated using ABAQUS software, from which the 3D dose distribution in uncompressed tissue was determined. The geometry of the model is constructed from MR images of 6 volunteers. Experimental dosimetery was performed by placing the thermoluminescence dosimeters into the polyvinyl alcohol breast equivalent phantom and on the proximal edge of compression plates to the chest. Results: The results indicate that using the cone applicators would deliver more than 95% of dose to the depth of 5 to 17mm, while round applicator will increase the skin dose. Nodal displacement, in presence of gravity and 60N forces, i.e. in mammography compression, was determined with 43% contraction in the loading direction and 37% expansion in orthogonal orientation. Finally, in comparison of the acquired from thermoluminescence dosimeters with MCNP5, they are consistent with each other in breast phantom and in chest's skin with average different percentage of 13.7±5.7 and 7.7±2.3, respectively. Conclusion: The major advantage of this kind of dosimetry is the ability of 3D dose calculation by FE Modeling. Finally, polyvinyl alcohol is a reliable material as a breast tissue equivalent dosimetric phantom that provides the ability of TLD dosimetry for validation.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-112: Dose Distribution of Praseodymium-142 Microspheres in Microcapillary Using Radiochromic Film Dosimetry and Applications in Hepatocellular Carcinoma Microsphere Brachytherapy

Purpose: This work verified simulations of beta-minus emitter Praseodymium-142 (Pr-142) for microsphere brachytherapy by performing absolute dose measurements for Pr 142 microspheres in a microcapillary as a simplified model for a single blood vessel for the treatment of Hepatocellular Carcinoma (HCC). Methods: Pr-142 microspheres (mass: 0.169g, average diameter: 29.7±3.9μm) were activated by thermal neutron activation at the University of Missouri Research Reactor. Experimental setup consisted of a microsphere solution (initial activity 36.6mCi in 0.1ml of sterile water) within a glass microcapillary (internal and external diameter: 305μm and 453μm, respectively) placed for 51h in a custom made Gammex Solid Water™ phantom. GAFCHROMIC™ EBT2 film calibrated with a 6MeV electron beam was used to access the dose fall-off of microspheres. The microcapillary was modeled in MCNPX2.6 in order to compare with experiments. Results: The radial dose fall-off on the transverse plane due to scatter and attenuation in the solid water phantom was analyzed using ImageJ for both film and MCNPX2.6 simulations. Isodose analysis showed close agreement among the methods used, i.e. measurements and simulations agree within 3.9% for doses below 1600cGy. Experimental and simulated doses obtained at 0.5 cm radially from the source were 1547cGy and 1610cGy respectively. Discrepancies for points close to the microcapillary surface were observed between MCNPX2.6 and measurements due to film saturation for high doses. Dose due to Pr-142 3.7% gamma emission was below the threshold of detection for the film. Conclusion: A detailed dosimetric study was performed for Pr-142 glass microspheres within a single microcapillary. MCNPX2.6 simulations were verified by means of direct measurement. Based on these results, Pr-142 appears to be a viable choice of radionuclide for treating HCC.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

SU-E-T-209: Independent Dose Calculation in FFF Modulated Fields with Pencil Beam Kernels Obtained by Deconvolution

Purpose: To obtain the pencil beam kernels that characterize a megavoltage photon beam generated in a FFF linac by experimental measurements, and to apply them for dose calculation in modulated fields. Methods: Several Kodak EDR2 radiographic films were irradiated with a 10 MV FFF photon beam from a Varian True Beam (Varian Medical Systems, Palo Alto, CA) linac, at the depths of 5, 10, 15, and 20cm in polystyrene (RW3 water equivalent phantom, PTW Freiburg, Germany). The irradiation field was a 50 mm diameter circular field, collimated with a lead block. Measured dose leads to the kernel characterization, assuming that the energy fluence exiting the linac head and further collimated is originated on a point source. The three-dimensional kernel was obtained by deconvolution at each depth using the Hankel transform. A correction on the low dose part of the kernel was performed to reproduce accurately the experimental output factors. The kernels were used to calculate modulated dose distributions in six modulated fields and compared through the gamma index to their absolute dose measured by film in the RW3 phantom. Results: The resulting kernels properly characterize the global beam penumbra. The output factor-based correction was carried out adding the amount of signal necessary to reproduce the experimental output factor in steps of 2mm, starting at a radius of 4mm. There the kernel signal was in all cases below 10% of its maximum value. With this correction, the number of points that pass the gamma index criteria (3%, 3mm) in the modulated fields for all cases are at least 99.6% of the total number of points. Conclusion: A system for independent dose calculations in modulated fields from FFF beams has been developed. Pencil beam kernels were obtained and their ability to accurately calculate dose in homogeneous media was demonstrated.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

SU-E-T-227: Least Restrictive Assignment of Dose-Distance Differences (LRAD) in IMRT On a Conventional Linac

Purpose: To develop a method to compare dose distributions based on the least restrictive of dose and distance differences, and to evaluate the method for IMRT QA on a conventional linac. Methods: Lrad is defined as the least restrictive of the dose difference at each point in the distributions being compared (%D), with dose normalized by the same factor, and the distance to agreement (DTA in millimeters), limited to directions parallel and perpendicular to MLC motion. If %D<%Do, Lrad=%D. Otherwise, if L=%D/DTA>Lo, Lrad=DTA, else Lrad=%D. Lrad fails at any point when Lrad>DTAo or Lrad>%Do. We compared LRAD (Lo=1%/mm, DTAo=2mm, %Do=2%) with Gamma index tool (2%/2mm pass/fail criteria) for IMRT QA with 2D detector arrays. Dose distribution measured for 6 MV and 18 MV x-rays for various field sizes and shapes, including 6 IMRT plans, were compared to measurements made with output, beam flatness, beam symmetry, MLC leaf positions, and detector array translation and rotation adjusted at up to twice published acceptable tolerance. Results: LRAD was more effective than Gamma in detecting a 2% drift in output (dose per monitor unit), 1–2 mm drifts in MLC leaf position for 1–8 leaves in the field, and beam flatness and symmetry drifts that resulted in a 3% change in the off-axis ratio. LRAD and Gamma were equally effective for larger output changes and for detector translations and rotations. For all ∼400 measurements performed, LRAD detected 78% of the error and Gamma detected 52% of the errors. By using a color scale to display dose differences and grey scale for spatial errors, the reason for failure was self evident, whether it be beam or mechanical changes. Conclusion: LRAD proved to be advantageous in its ability to detect and distinguish between realistic changes in beam and mechanical settings in comparison to the Gamma Index.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-233: Modeling Linac Couch Effects On Attenuation and Skin Dose

Purpose: Treatment couch tops in medical LINAC rooms lead to attenuation to beams penetrating them, plus higher skin dose which can become a significant concern with the high fraction doses associated with Stereotactic Body Radiation Therapy. This work measures the attenuation and shallow depth dose due to a BrainLab couch, and studies the modeling of the couch top in our treatment planning system (TPS) as a uniform solid material with homogeneous density. Methods: LINAC photon beams of size 10×10 cm and nominal energy 6 MV were irradiated from different gantry angles on a stack of solid water. Depth dose were measured with two types of parallel plate chambers, MPPK and Markus. In the Philips Pinnacle TPS, the couch was modeled as a slab with varying thickness and density. A digital phantom of size 30×30×10 cm with density 1 g/cc was created to simulate the measurement setup. Both the attenuation and skin dose effects due to the couch were studied. Results: An orthogonal attenuation rate of 3.2% was observed with both chamber measurements. The attenuation can be modeled by couch models of varying thicknesses. Once the orthogonal attenuation was modeled well, the oblique beam attenuation in TPS agreed with measurement within 1.5%. The depth dose at shallow depth (0.5 cm) was also shown to be modeled correctly within 1.5% of the measurement using a 12 mm thick couch model with density of 0.9 g/cc. Agreement between calculation and measurement diverges at very shallow depths (≤1 mm) but remains acceptable (<5%) with the aforementioned couch model parameters. Conclusion: Modeling the couch top as a uniform solid in a treatment planning system can predict both the attenuation and surface dose simultaneously well within clinical tolerance in the same model.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-358: Monte Carlo Dose Calculation of Small Field Electron Beams

Purpose: Dynamic radiotherapy involving electron beams such as Dynamic Electron Arc Radiotherapy (DEAR) requires accurate dose modelling of small field sizes, similar to the requirement of IMRT field on the small photon field. The current commercial electron Monte Carlo algorithms such as eMC v11 in Eclipse were developed for standard field sizes and do not support the planning of dynamic therapy yet. The purpose of this study is to develop a method to accurately model small field electron beam dosimetry using Monte Carlo simulations. Methods: Comparison between eMC, phantom measurements (diode), and Monte Carlo (MC) simulations (BEAMnrc/DOSYZnrc) were performed for a Varian TrueBeam linac. MC simulations utilized Varian TrueBeam phase space files which had been validated in another study. Static single small field was assessed by comparing dose distributions in water for a 16 MeV beam for circular (2 cm diameter) and rectangular (1×10 cm{sup 2}) cut-out. MC was performed with a resolution of 2.5×2.5×2 mm{sup 2} and statistical uncertainty < 4%. The dose distribution was averaged over adjacent bins to improve precision. Depth dose and orthogonal profiles were evaluated. Results: Small field PDDs differ from those with standard cones. For both circular and rectangular cutouts, the difference in range R8 0-R1 0 is less than 2 mm and in dose within 2%. For the orthogonal profiles, field size and penumbra differences were within 1 mm at depth of maximum dose. The eMC displayed a distinctive “step” in the out-field dose profile in disagreement with both measurement and MC results and needs further investigation. Conclusion: MC was able to characterize the small field dosimetry with good agreement with the measurement data, and thus offers the opportunity for treatment planning of dynamic radiotherapy. Analyses for all other electron energies and cut-out sizes are under way and results will be included in the presentation.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

SU-E-T-425: Spherical Dose Distributions for Radiosurgery Using a Standardized MLC Plan

Purpose: To investigate a standardized MLC treatment plan to generate small spherical dose distributions. Methods: The static virtual cone plan comprised six table positions with clockwise and counterclockwise arcs having collimator angles 45 and 135 degrees, respectively, at each position. The central two leaves of a 2.5 mm leaf width MLC were set to a constant gap. Control points were weighted proportional to the sine of the gantry angle. Plans were created for the 10 MV flattening-filter-free beam of a TrueBeam STx (Varian Medical Systems) with gaps of 1, 1.5, 2, and 3 mm and were delivered to a phantom containing radiochromic film. Dose was calculated using the Eclipse AAA (Varian Medical Systems). A dynamic plan in which the table and gantry moved simultaneously with 1.5 mm gap was also created and delivered using the TrueBeam developer mode. Results: The full-width-half-max (FWHM) varied with leaf gap, ranging from 5.2 to 6.2 mm. Calculated FWHM was smaller than measured by 0.7 mm for the 1 mm gap and ≤ 0.4 mm for the larger gaps. The measured-to-calculated dose ratio was 0.93, 0.96, 1.01, and 0.99 for 1 mm, 1.5 mm, 2 mm, and 3 mm gaps, respectively. The dynamic results were the same as the static. The position deviations between the phantom target position and the center of the dose distribution were < 0.4 mm. Conclusion: The virtual cone can deliver spherical dose distributions suitable for radio surgery of small targets such as the trigeminal nerve. The Eclipse AAA accurately calculates the expected dose, particularly for leaf gap ≥ 1.5 mm. The measured dose distribution is slightly larger than the calculation, which is likely due to systematic leaf position error, isocenter variation due to gantry sag and table eccentricity, and inaccuracy in MLC leaf end modeling.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-426: Dose Delivery Accuracy in Breast Field Junction for Free Breath and Deep Inspiration Breath Hold Techniques

Purpose: The purpose of this work was to verify the accuracy of the dose distribution along the field junction in a half beam irradiation technique for breast cancer patients receiving radiation to the breast or chest wall (CW) and the supraclavicular LN region for both free breathing and deep inspiration breath hold (DIBH) technique. Methods: We performed in vivo measurements for nine breast cancer patients receiving radiation to the breast/CW and to the supraclavicular LN region. Six patients were treated to the left breast/CW using DIBH technique and three patients were treated to the right breast/CW in free breath. We used five microMOSFET dosimeters: three located along the field junction, one located 1 cm above the junction and the fifth microMOSFET located 1 cm below the junction. We performed consecutive measurements over several days for each patient and compared the measurements to the TPS calculation (Eclipse, Varian™). Results: The calculated and measured doses along the junction were 0.97±0.08 Gy and 1.02±0.14 Gy, respectively. Above the junction calculated and measured doses were 0.91±0.08 Gy and 0.98±0.09 Gy respectively, and below the junction calculated and measured doses were 1.70±0.15 Gy and 1.61±0.09 Gy, respectively. All differences were not statistically significant. When comparing calculated and measured doses for DIBH patients only, there was still no statistically significant difference between values for all dosimeter locations. Analysis was done using the Mann-Whitney Rank-Sum Test. Conclusion: We found excellent correlation between calculated doses from the TPS and measured skin doses at the junction of several half beam fields. Even for the DIBH technique, where there is more potential for variance due to depth of breath, there is no over or underdose along the field junction. This correlation validates the TPS, as well an accurate, reproducible patient setup.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-626: Accuracy of Dose Calculation Algorithms in MultiPlan Treatment Planning System in Presence of Heterogeneities

Purpose: Advanced stereotactic radiotherapy (SRT) treatments require accurate dose calculation for treatment planning especially for treatment sites involving heterogeneous patient anatomy. The purpose of this study was to evaluate the accuracy of dose calculation algorithms, Raytracing and Monte Carlo (MC), implemented in the MultiPlan treatment planning system (TPS) in presence of heterogeneities. Methods: First, the LINAC of a CyberKnife radiotherapy facility was modeled with the PENELOPE MC code. A protocol for the measurement of dose distributions with EBT3 films was established and validated thanks to comparison between experimental dose distributions and calculated dose distributions obtained with MultiPlan Raytracing and MC algorithms as well as with the PENELOPE MC model for treatments planned with the homogenous Easycube phantom. Finally, bones and lungs inserts were used to set up a heterogeneous Easycube phantom. Treatment plans with the 10, 7.5 or the 5 mm field sizes were generated in Multiplan TPS with different tumor localizations (in the lung and at the lung/bone/soft tissue interface). Experimental dose distributions were compared to the PENELOPE MC and Multiplan calculations using the gamma index method. Results: Regarding the experiment in the homogenous phantom, 100% of the points passed for the 3%/3mm tolerance criteria. These criteria include the global error of the method (CT-scan resolution, EBT3 dosimetry, LINAC positionning …), and were used afterwards to estimate the accuracy of the MultiPlan algorithms in heterogeneous media. Comparison of the dose distributions obtained in the heterogeneous phantom is in progress. Conclusion: This work has led to the development of numerical and experimental dosimetric tools for small beam dosimetry. Raytracing and MC algorithms implemented in MultiPlan TPS were evaluated in heterogeneous media.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

TU-F-18A-03: Improving Tissue Segmentation for Monte Carlo Dose Calculation Using DECT Data

Purpose: To develop a new segmentation technique using dual energy CT (DECT) to overcome limitations related to segmentation from a standard Hounsfield unit (HU) to electron density (ED) calibration curve. Both methods are compared with a Monte Carlo analysis of dose distribution. Methods: DECT allows a direct calculation of both ED and effective atomic number (EAN) within a given voxel. The EAN is here defined as a function of the total electron cross-section of a medium. These values can be effectively acquired using a calibrated method from scans at two different energies. A prior stoichiometric calibration on a Gammex RMI phantom allows us to find the parameters to calculate EAN and ED within a voxel. Scans from a Siemens SOMATOM Definition Flash dual source system provided the data for our study. A Monte Carlo analysis compares dose distribution simulated by dosxyz-nrc, considering a head phantom defined by both segmentation techniques. Results: Results from depth dose and dose profile calculations show that materials with different atomic compositions but similar EAN present differences of less than 1%. Therefore, it is possible to define a short list of basis materials from which density can be adapted to imitate interaction behavior of any tissue. Comparison of the dose distributions on both segmentations shows a difference of 50% in dose in areas surrounding bone at low energy. Conclusion: The presented segmentation technique allows a more accurate medium definition in each voxel, especially in areas of tissue transition. Since the behavior of human tissues is highly sensitive at low energies, this reduces the errors on calculated dose distribution. This method could be further developed to optimize the tissue characterization based on anatomic site.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

TH-A-18C-11: An Investigation of KV CBCT Image Quality and Dose Reduction for Volume-Of-Interest Imaging Using Dynamic Collimation

Purpose: The focus of this work was to investigate the improvements in image quality and dose reduction for volume-of-interest (VOI) kV-CBCT using dynamic collimation. Methods: A prototype iris aperture was used to track a VOI during a CBCT acquisition. The current aperture design is capable of one-dimensional translation as a function of gantry angle and dynamic adjustment of the iris radius. The aperture occupies the location of the bow-tie filter on a Varian OBI system. CBCT and planar image quality was investigated as a function of aperture radius, while maintaining the same dose to the VOI, for a 20 cm diameter cylindrical water phantom with a 9 mm diameter bone insert centered on isocenter. Corresponding scatter-to-primary ratios (SPR) were determined at the detector plane with Monte Carlo simulation using EGSnrc. Dose distributions for various anatomical sites were modeled using a dynamic BEAMnrc library and DOSXYZnrc. The resulting VOI dose distributions were compared to full-field distributions. Results: SPR was reduced by a factor of 8.4 when decreasing iris diameter from 21.2 cm to 2.4 cm (at isocenter). Similarly, this change in iris diameter corresponds to a factor increase of approximately 1.4 and 1.5 in image contrast for CBCT and planar images, respectively, and similarly a factor decrease in image noise of approximately 1.7 and 1.5. This results in a measured gain in contrast-to-noise ratio of a factor of approximately 2.3 for both CBCT and planar images. Depending upon the anatomical site, dose was reduced to 10%–70% of the full field value along the central axis plane and down to 2% along the axial planes, while maintaining the same dose to the VOI compared to full-field techniques. Conclusion: The presented VOI technique offers improved image quality for image-guided radiotherapy while sparing the surrounding volume of unnecessary dose compared to full-field techniques.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-107: Development of a GPU-Based Dose Delivery System for Adaptive Pencil Beam Scanning

Purpose: A description of a GPU-based dose delivery system (G-DDS) to integrate a fast forward planning implementing in real-time the prescribed sequence of pencil beams. The system, which is under development, is designed to evaluate the dose distribution deviations due to range variations and interplay effects affecting mobile tumors treatments. Methods: The Dose Delivery System (DDS) in use at the Italian Centro Nazionale di Adroterapia Oncologica (CNAO), is the starting point for the presented system. A fast and partial forward planning (FP) tool has been developed to evaluate in few seconds the delivered dose distributions using the DDS data (on-line measurements of spot properties, i.e. number of particles and positions). The computation is performed during the intervals between synchrotron spills and, made available at the end of each spill. In the interval between two spills, the G-DDS will evaluate the delivered dose distributions taking into account the real-time target positions measured by a tracking system. The sequence of prescribed pencil beams for the following spill will be adapted taking into account the variations with respect to the original plan due to the target motion. In order to speed up the computation required to modify pencil beams distribution (up to 400 times has been reached), the Graphics Processing Units (GPUs) and advanced Field Programmable Gate Arrays (FPGAs) are used. Results: An existing offline forward planning is going to be optimized for the CUDA architecture: the gain in time will be presented. The preliminary performances of the developed GPU-based FP algorithms will be shown. Conclusion: A prototype of a GPU-based dose delivery system is under development and will be presented. The system workflow will be illustrated together with the approach adopted to integrate the three main systems, i.e. CNAO dose delivery system, fast forward planning, and tumor tracking system.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Poster — Thur Eve — 36: Implementation of constant dose rate and gantry speed arc therapy(CDR-CAS-IMAT) for thoracic esophageal carcinoma on Varian 23EX

Objective: The purpose of this study is to propose an alternative planning approach for VMAT using constant dose rate and gantry speed arc therapy(CDR-CAS-IMAT) implementation on conventional Linac Varian 23EX and used IMRT as a benchmark to evaluate the performance. Methods and materials: Eighteen patients with thoracic esophageal carcinoma who were previously treated with IMRT on Varian 23EX were retrospectively planned for CDR-CAS-IMAT plans. Dose prescription was set to 60 Gy to PTVs in 30 fractions. The planning objectives for PTVs and OAR were corresponding with the IMRT plans. Dose to the PTVs and OAR were compared to IMRT with respect to plan quality, MU, treatment time and delivery accuracy. Results: CDR-CAS-IMAT plans led to equivalent or superior plan quality as compared to IMRT, PTV's CI relative increased 16.2%, while small deviations were observed on minimum dose for PTV. Volumes in the cord receiving 40Gy were increased from 3.6% with IMRT to 7.0%. Treatment times were reduced significantly with CDR-CAS-IMAT(mean 85.7s vs. 232.1s, p < .05), however, MU increased by a factor of 1.3 and lung V10/5/3.5/aver were relative increase 6.7%,12%,17.9%,4.2%, respectively. And increased the E-P low dose area volume decreased the hight dose area. There were no significant difference in Delta4 measurements results between both planning techniques. Conclusion: CDR-CAS-IMAT plans can be implemented smoothly and quickly into a busy cancer center, which improved PTV CI and reduces treatment time but increased the MU and low dose irradiated area. An evaluation of weight loss must be performed during treatment for CDR-CAS-IMAT patients.

60 APPLIED LIFE SCIENCES↗