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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.

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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

A dosimetric evaluation of the Eclipse AAA algorithm and Millennium 120 MLC for cranial intensity-modulated radiosurgery

The aim of this study is to assess the accuracy of a convolution-based algorithm (anisotropic analytical algorithm [AAA]) implemented in the Eclipse planning system for intensity-modulated radiosurgery (IMRS) planning of small cranial targets by using a 5-mm leaf-width multileaf collimator (MLC). Overall, 24 patient-based IMRS plans for cranial lesions of variable size (0.3 to 15.1 cc) were planned (Eclipse, AAA, version 10.0.28) using fixed field-based IMRS produced by a Varian linear accelerator equipped with a 120 MLC (5-mm width on central leaves). Plan accuracy was evaluated according to phantom-based measurements performed with radiochromic film (EBT2, ISP, Wayne, NJ). Film 2D dose distributions were performed with the FilmQA Pro software (version 2011, Ashland, OH) by using the triple-channel dosimetry method. Comparison between computed and measured 2D dose distributions was performed using the gamma method (3%/1 mm). Performance of the MLC was checked by inspection of the DynaLog files created by the linear accelerator during the delivery of each dynamic field. The absolute difference between the calculated and measured isocenter doses for all the IMRS plans was 2.5% ± 2.1%. The gamma evaluation method resulted in high average passing rates of 98.9% ± 1.4% (red channel) and 98.9% ± 1.5% (blue and green channels). DynaLog file analysis revealed a maximum root mean square error of 0.46 mm. According to our results, we conclude that the Eclipse/AAA algorithm provides accurate cranial IMRS dose distributions that may be accurately delivered by a Varian linac equipped with a Millennium 120 MLC.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2014

SU-E-J-110: TG 51 Dosimetry : With Or Without Lead

TG-51 Dosimetry: With or Without Lead. Purpose: In this project, an analytical method has been introduced for adjustment of the TG-51 recommended KQ in order to produce accurate dosimetric data for high energy photons without the lead foil. Methods: These investigations were performed using a 30 cm × 30 cm × 30 cm CIVCO water tank, A12 EXRADIN Water proof Farmer Chamber, a Standard Imaging MAX 4000 electrometer, and 1 mm thick lead foil from Standard Imaging. Complete TG-51 was performed every month with and without lead. The results were analyzed and an analytical model has been developed for comparing the values of KQ. TG-51 Table I was used to obtain KQ values. Results: The dosimetric evaluations were obtained for Varian Linear accelerators Model 21ix and 21ex. These results indicates that the measured data with lead foil in place as recommended by TG-51 is in excellent agreement (within 0.1%) with the calculated data obtained by the new model, from our dosimetry data without-lead. If equation 15 of the TG-51 report is used without any adjustments, it will lead to differences of about 1.6 % (on the average) in relative data which will Resultin differences of about 0.3 % (on the average) in the KQ Values. The KQ value for 18 MV obtained consistently with the equation of TG-51 “with lead” and “without lead” were 0.971 and 0.974, respectively. The 0.3 % higher results for KQ without lead eventually will lead to 0.3% larger output. However, by considering this model the KQ value was found to be 0.971 for dosimetry without lead. Conclusion: The analytical model that was introduced in this project was able to reproduce the dosimetric data of the high energy linear accelerators to within 0.1% without the use of the lead foil.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

SU-E-J-44: Design a Platform and Phantom Model for Photoacoustic Imaging in Combination with CT

Purpose: Our (long-term) objective is to develop a US manipulator that will provide in situ radiation response and image-guided therapy for bladder cancer based on photoacoustic molecular imaging. Methods: A platform was devised to provide a reproducible positional frame of reference for targeting anatomic structure between MDCT and US scans, in lieu of CBCT, and to fuse photoacoustic molecular imaging. US and photoacoustic scans are taken of a patient while in the CT scanner and IRMT. Through co-registration, based on anatomical positions, we identified a common coordinate system to be used in Eclipse. A bladder phantom was constructed to validate anatomical tracking via US and photoacoustic imaging. We tested the platform using phantom model to demonstrate validity once moved from the CT couch to the linear accelerator couch. Results: This platform interlocks with Varian exact couch index points for reproducibility of positioning. Construction from low Z material and sized appropriately to fit in CT/IMRT gantry. Error in conversion from cylindrical coordinates of the manipulator to X, Y, Z coordinates of the treatment couch was less than 1mm. We measured the bladder size in 3 different directions in both Eclipse from the CT and Acuson from US. The error was less than 2mm in all directions. CT and US images were co-registered in MATLAB. Co-registration of photoacoustic images is still being developed. Conclusion: For Linear Accelerators without on board imaging, MV portal images are not a viable option for the localization of soft tissue anatomy. We believe our manipulator provides an alternative using US imaging, which will be examined in an upcoming clinical trial. We plan to examine the value of hypoxia guided treatment through photoacoustic imaging during this trial.

60 APPLIED LIFE SCIENCES↗
NASA NTRS2001

The human vestibulo-ocular reflex during linear locomotion

During locomotion, there is a translation and compensatory rotation of the head in both the vertical and horizontal planes. During moderate to fast walking (100 m/min), vertical head translation occurs at the frequency of stepping (2 Hz) and generates peak linear acceleration of 0.37 g. Lateral head translation occurs at the stride frequency (1 Hz) and generates peak linear acceleration of 0.1 g. Peak head pitch and yaw angular velocities are approximately 17 degrees/s. The frequency and magnitude of these head movements are within the operational range of both the linear and angular vestibulo-ocular reflex (IVOR and aVOR). Vertical eye movements undergo a phase reversal from near to far targets. When viewing a far (>1 m) target, vertical eye velocity is typical of an aVOR response; that is, it is compensatory for head pitch. At close viewing distances (<1 m), vertical eye velocity is in phase with head pitch and is compensatory for vertical head translation, suggesting that the IVOR predominantly generates the eye movement response. Horizontal head movements during locomotion occur at the stride frequency of 1 Hz, where the IVOR gain is low. Horizontal eye movements are compensatory for head yaw at all viewing distances and are likely generated by the aVOR.

NASA Discipline Neuroscience↗
DOE OSTI2016

SU-F-T-271: Comparing IMRT QA Pass Rates Before and After MLC Calibration

Purpose: To compare IMRT QA pass rates before and after an in-house MLC leaf calibration procedure. Methods: The MLC leaves and backup jaws on four Elekta linear accelerators with MLCi2 heads were calibrated using the EPID-based RIT Hancock Test as the means for evaluation. The MLCs were considered to be successfully calibrated when they could pass the Hancock Test with criteria of 1 mm jaw position tolerance, and 1 mm leaf position tolerance. IMRT QA results were collected pre- and postcalibration and analyzed using gamma analysis with 3%/3mm DTA criteria. AAPM TG-119 test plans were also compared pre- and post-calibration, at both 2%/2mm DTA and 3%/3mm DTA. Results: A weighted average was performed on the results for all four linear accelerators. The pre-calibration IMRT QA pass rate was 98.3 ± 0.1%, compared with the post-calibration pass rate of 98.5 ± 0.1%. The TG-119 test plan results showed more of an improvement, particularly at the 2%/2mm criteria. The averaged results were 89.1% pre and 96.1% post for the C-shape plan, 94.8% pre and 97.1% post for the multi-target plan, 98.6% pre and 99.7% post for the prostate plan, 94.7% pre and 94.8% post for the head/neck plan. Conclusion: The patient QA results did not show statistically significant improvement at the 3%/3mm DTA criteria after the MLC calibration procedure. However, the TG-119 test cases did show significant improvement at the 2%/2mm level.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-505: Inter-Comparison of Clinical Plans Created On Truebeam with Trilogy and Clinac

Purpose The purpose of this study is to investigate the dosimetric differences in clinical treatment plans when machine parameters are switched from Varian Truebeam to older Varian models and vice-versa. This is used to determine the clinically-safe limits to directly transfer patients from Truebeam to Trilogy or Clinac. Methods Thirty-two patients with cancer of different treatment sites such as lung, head-and-neck, prostate and breast were studied. The clinically approved IMRT, VMAT or 3D conformal treatment plans were delivered either on Truebeam or Clinac/Trilogy. Keeping the monitor units, fraction size and other machine parameters the same, the plans were recomputed on a different Varian linear accelerator (e.g., Truebeam plans were recomputed on Trilogy and vice-versa). The plans comparison was done using the target D98 and normal organ D2, D10, D20, D30, D50 and D70 dose metrics. Results The perfraction dose-difference in the PTV-D98 for all 32 patients varied from 0.01 to −0.06 Gy with median being −0.04 Gy. For a 2Gy/fraction treatment course, this would be maximum PTV-D98 dose-difference of 6 cGy/fraction or 30 cGy/5fractions. For organs-at-risks the maximum per-fraction dosedifference in D2, D5, D10, D20, D30, D50 and D70 between Truebeam and Trilogy/Clinac plans varied from 0.06 to −0.07 Gy with median being with — 0.02 Gy. The 3D-CRT plans had relatively lower dose-difference in comparison to IMRT and VMAT plans. The dose computed on Truebeam was systematically lower than the dose computed on Trilogy/ Clinac. Conclusions for conventional fractionation schedules, assuming the maximum uncertainty of <2%, it is clinically safe to switch the treatment machine for 5 fractions. For hypo-fractionated treatments with higher dose per-fraction, plans may need to be revisited before switching the linear accelerator from Truebeam to Clinac/Trilogy or vice-versa.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2012

The effect of small field output factor measurements on IMRT dosimetry

Purpose: To evaluate how changes in the measured small field output factors affect the doses in intensity-modulated treatment planning. Methods: IMRT plans were created using Philips Pinnacle treatment planning system. The plans were optimized to treat a cylindrical target 2 cm in diameter and 2 cm in length. Output factors for 2 Multiplication-Sign 2 and 3 Multiplication-Sign 3 cm{sup 2} field sizes were changed by {+-}5%, {+-}10%, and {+-}20% increments from the baseline measurements and entered into the planning system. The treatment units were recommissioned in the treatment planning system after each modification of the output factors and treatment plans were reoptimized. All plans were delivered to a solid water phantom and dose measurements were made using an ionization chamber. The percentage differences between measured and computed doses were calculated. An Elekta Synergy and a Varian 2300CD linear accelerator were separately evaluated. Results: For the Elekta unit, decreasing the output factors resulted in higher measured than computed doses by 0.8% for -5%, 3.6% for -10%, and 8.7% for -20% steps. Increasing the output factors resulted in lower doses by 2.9% for +5%, 5.4% for +10%, and 8.3% for +20% steps. For the Varian unit no changes were observed for either increased or decreased output factors. Conclusions: The measurement accuracy of small field output factors are of importance especially when the treatment plan consists of small segments as in IMRT. The method proposed here could be used to verify the accuracy of the measured small field output factors for certain linear accelerators as well as to test the beam model. The Pinnacle treatment planning system model uses output factors as a function of jaw setting. Consequently, plans using the Elekta unit, which conforms the jaws to the segments, are sensitive to small field measurement accuracy. On the other hand, for the Varian unit, jaws are fixed and segments are modeled as blocked fields hence, the impact of small field output factors on IMRT monitor unit calculation is not evaluable by this method.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2014

Characterization of radiation beams used to determinate the correction factor for a CyberKnife® unit reference field using ionization chambers

This paper aimed to characterize a 6 MV x-ray beam from a Varian® iX linear accelerator in order to obtain the correction factors needed by the IAEA/AAPM new formalism{sup 1}. The experiments were performed in a liquid water phantom under different irradiation conditions: a) Calibration of the reference field of 10 cm × 10 cm at 90 cm SSD and 10 cm depth was carried out according to the TRS-398 protocol using three ionization chambers (IC) calibrated in different reference laboratory and b) Measurement of the absorbed dose rate at 70 cm SSD and 10 cm depth in a 10 cm × 10 cm and 5.4 cm × 5.4 cm fields was obtained in order to simulate the CyberKnife® conditions where maximum distance between the source and the detector is equal to 80 cm and the maximum field size is 6 cm diameter. Depending where the IC was calibrated, differences between 0.16% and 2.24% in the absorbed dose rate measured in the 10 cm × 10 cm field at 90 cm SSD were observed, while for the measurements at 70 cm SSD, differences between 1.27% and 3.88% were obtained. For the 5.4 cm × 5.4 cm field, the absorbed dose measured with the three ICs varies between 1.37% and 3.52%. The increase in the difference on the absorbed dose when decreasing the SSD could possibly be associated to scattering radiation generated from the collimators and/or the energy dependence of the ionization chambers to low-energy radiation. The results presented in this work suggest the importance of simulating the CyberKnife® conditions using other linear accelerator for obtaining the correction factors as proposed by the IAEA/AAPM new formalism in order to measure the absorbed dose with acceptable accuracy.

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

SU-E-T-90: Accuracy of Calibration of Lithium-6 and -7 Enriched LiF TLDs for Neutron Measurements in High Energy Radiotherapy

Purpose: To determine the potential error involved in the interpretation of neutron measurements from medical linear accelerators (linacs) using TLD-600H and TLD-700H if standard AmBe and {sup 252}Cf neutron sources are used for calibration without proper inclusion of neutron energy spectrum information. Methods: The Kerma due to neutrons can be calculated from the energy released by various nuclear interactions (elastic and inelastic scatter, (n,α), (n,p), (n,d), (n,t), (n,2n), etc.). The response of each TLD can be considered the sum of the neutron and gamma components; each proportional to the Kerma. Using the difference between the measured TLD responses and the ratio of the calculated Kerma for each material, the neutron component of the response can be calculated. The Monte Carlo code MCNP6 has been used to calculate the neutron energy spectra resulting from photonuclear interactions in a Varian 21EX linac. TLDs have been exposed to the mixed (γ-n) field produced by a linac and AmBe and {sup 252}Cf standard neutron sources. Results: For dosimetry of neutrons from AmBe or {sup 252}Cf sources, assuming TLD-700H insensitivity to neutrons will Result in 10% or 20% overestimation of neutron doses respectively.For dosimetry of neutrons produced in a Varian 21EX, applying a calibration factor derived from a standard AmBe or {sup 252}Cf source will Result in an overestimation of neutron fluence, by as much as a factor of 47.The assumption of TLD-700H insensitivity to neutrons produced by linacs leads to a negligible error due to the extremely high Kerma ratio (600H/700H) of 3000 for the assumed neutron spectrum. Conclusion: Lithium-enriched TLDs calibrated with AmBe and/or {sup 252}Cf neutron sources are not accurate for use under the neutron energy spectrum produced by a medical linear accelerator.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

An online x-ray based position validation system for prostate hypofractionated radiotherapy

Purpose: Accurate positioning of the target volume during treatment is paramount for stereotactic body radiation therapy (SBRT). In this work, the authors present the development of an in-house software tool to verify target position with an Elekta-Synergy linear accelerator using kV planar images acquired during treatment delivery. Methods: In-house software, SeedTracker, was developed in MATLAB to perform the following three functions: 1. predict intended seed positions in a planar view perpendicular to any gantry angle, simulating a portal imaging device, from the 3D seed co-ordinates derived from the treatment planning system; 2. autosegment seed positions in kV planar images; and 3. report the position shift based on the seed positions in the projection images. The performance of SeedTracker was verified using a CIRS humanoid phantom (CIRS, VA, USA) implanted with three Civco gold seed markers (Civco, IA, USA) in the prostate. The true positive rate of autosegmentation (TPR{sub seg}) and the accuracy of the software in alerting the user when the isocenter position was outside the tolerance (TPR{sub trig}) were studied. Two-dimensional and 3D static position offsets introduced to the humanoid phantom and 3D dynamic offsets introduced to a gel phantom containing gold seeds were used for evaluation of the system. Results: SeedTracker showed a TPR{sub seg} of 100% in the humanoid phantom for projection images acquired at all angles except in the ranges of 80°–100° and 260°–280° where seeds are obscured by anatomy. This resulted in a TPR{sub trig} of 88% over the entire treatment range for considered 3D static offsets introduced to the phantom. For 2D static offsets where the position offsets were only introduced in the anterior–posterior and lateral directions, the TPR{sub trig} of SeedTracker was limited by both seed detectability and positional offset. SeedTracker showed a false positive trigger in the projection angle range between 130°–170° and 310°–350° (a maximum of 24% of treatment time) due to limited information that can be derived from monoscopic images. The system accurately determined the dynamic trajectory of the isocenter position in the superior and inferior direction for the studied dynamic offset scenarios based on the seed position in monoscopic images. Conclusions: The developed software has been shown to accurately autosegment the seed positions in kV planar images except for two 20° arcs where seeds are obscured by anatomical structures. The isocenter trajectories determined by the system, based on the monoscopic images, provide useful information for monitoring the prostate position. The developed system has potential application for monitoring prostate position during treatment delivery in linear accelerator based SBRT.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2014

A compact proton spectrometer for measurement of the absolute DD proton spectrum from which yield and pR are determined in thin-shell inertial-confinement-fusion implosions

A compact, step range filter proton spectrometer has been developed for the measurement of the absolute DD proton spectrum, from which yield and areal density (ρR) are inferred for deuterium-filled thin-shell inertial confinement fusion implosions. This spectrometer, which is based on tantalum step-range filters, is sensitive to protons in the energy range 1-9 MeV and can be used to measure proton spectra at mean energies of ~1-3 MeV. It has been developed and implemented using a linear accelerator and applied to experiments at the OMEGA laser facility and the National Ignition Facility (NIF). Modeling of the proton slowing in the filters is necessary to construct the spectrum, and the yield and energy uncertainties are ±<10% in yield and ±120 keV, respectively. This spectrometer can be used for in situ calibration of DD-neutron yield diagnostics at the NIF

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

SU-E-T-191: Commissioning and Dosimetric Characteristics of Elekta Agility for Total Skin Electron Beam (TSEB) Therapy

Purpose The purpose of this work is to publish beam data from Elekta Synergy(R) linear accelerators with Agility(TM) MLC for total skin electron beam (TSEB) therapy using the HDRE1 (High Dose Rate Electron 6MeV) energy. Method & Materials The optimal gantry angles for TSEB were determined using ion chamber measurements along a vertical profile at 450cm SSD. After gantry angles were chosen, field uniformity was measured over the entire treatment area. Uniformity was measured with and without the patient support device, allowing the dosimetric effect of the support device to be determined. Beam output and PDD were measured at the calibration point (450cm SSD) for a dual beam using a parallel plate chamber in solid water. These measurements were repeated with the chamber and phantom rotated about the patient isocenter at various angles, in order to measure the contribution from oblique beams. This technique provides a precise measurement of the treatment skin dose (TSD). Lastly, ion chamber measurements were verified by film and diodes. Results The optimal gantry angle for 450 cm SSD was determined to be 90±16°. This achieved uniformity better than 96% on the vertical axis, and 92% along the horizontal axis. HDRE1 was calibrated to deliver 10 cGy/MU at standard geometry (100 cm SSD, 1.2 cm depth). Thus at TSEB geometry (450 cm SSD, 0.1 cm depth) the output of the AP dual field was measured to be 0.35 cGy/MU. The TSD of a 20 cm radius cylinder for six (equally, 60° spaced) dual fields was measured to be 1.19 cGy/MU. Percent Depth Dose data for the AP dual field and TSD are shown in Figure 2. Conclusion This paper provides a modern procedure for commissioning TSEB therapy on a linear accelerator, and clinical beam data for the Elekta Synergy(R) with Agility(TM) MLC.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Dosimetry of cone-defined stereotactic radiosurgery fields with a commercial synthetic diamond detector

Purpose: Small field x-ray beam dosimetry is difficult due to lack of lateral electronic equilibrium, source occlusion, high dose gradients, and detector volume averaging. Currently, there is no single definitive detector recommended for small field dosimetry. The objective of this work was to evaluate the performance of a new commercial synthetic diamond detector, namely, the PTW 60019 microDiamond, for the dosimetry of small x-ray fields as used in stereotactic radiosurgery (SRS). Methods: Small field sizes were defined by BrainLAB circular cones (4–30 mm diameter) on a Novalis Trilogy linear accelerator and using the 6 MV SRS x-ray beam mode for all measurements. Percentage depth doses (PDDs) were measured and compared to an IBA SFD and a PTW 60012 E diode. Cross profiles were measured and compared to an IBA SFD diode. Field factors, Ω{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}}, were calculated by Monte Carlo methods using BEAMnrc and correction factors, k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}}, were derived for the PTW 60019 microDiamond detector. Results: For the small fields of 4–30 mm diameter, there were dose differences in the PDDs of up to 1.5% when compared to an IBA SFD and PTW 60012 E diode detector. For the cross profile measurements the penumbra values varied, depending upon the orientation of the detector. The field factors, Ω{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}}, were calculated for these field diameters at a depth of 1.4 cm in water and they were within 2.7% of published values for a similar linear accelerator. The corrections factors, k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}}, were derived for the PTW 60019 microDiamond detector. Conclusions: The authors conclude that the new PTW 60019 microDiamond detector is generally suitable for relative dosimetry in small 6 MV SRS beams for a Novalis Trilogy linear equipped with circular cones.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2016

SU-F-R-12: Prediction of TrueBeam Hardware Issues Using Trajectory Log Analysis

Purpose: To predict potential failures of hardware within the Varian TrueBeam linear accelerator in order to proactively replace parts and decrease machine downtime. Methods: Machine downtime is a problem for all radiation oncology departments and vendors. Most often it is the result of unexpected equipment failure, and increased due to lack of in-house clinical engineering support. Preventative maintenance attempts to assuage downtime, but often is ineffective at preemptively preventing many failure modes such as MLC motor failures, the need to tighten a gantry chain, or the replacement of a jaw motor, among other things. To attempt to alleviate downtime, software was developed in house that determines the maximum value of each axis enumerated in the Truebeam trajectory log files. After patient treatments, this data is stored in a SQL database. Microsoft Power BI is used to plot the average maximum error of each day of each machine as a function of time. The results are then correlated with actual faults that occurred at the machine with the help of Varian service engineers. Results: Over the course of six months, 76,312 trajectory logs have been written into the database and plotted in Power BI. Throughout the course of analysis MLC motors have been replaced on three machines due to the early warning of the trajectory log analysis. The service engineers have also been alerted to possible gantry issues on one occasion due to the aforementioned analysis. Conclusion: Analyzing the trajectory log data is a viable and effective early warning system for potential failures of the TrueBeam linear accelerator. With further analysis and tightening of the tolerance values used to determine a possible imminent failure, it should be possible to pinpoint future issues more thoroughly and for more axes of motion.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

SU-E-T-784: Using MLC Log Files for Daily IMRT Delivery Verification

Purpose: To verify daily intensity modulated radiation therapy (IMRT) treatments using multi-leaf collimator (MLC) log files. Methods: The MLC log files from a NovalisTX Varian linear accelerator were used in this study. The MLC files were recorded daily for all patients undergoing IMRT or volumetric modulated arc therapy (VMAT). The first record of each patient was used as reference and all records for subsequent days were compared against the reference. An in house MATLAB software code was used for the comparisons. Each MLC log file was converted to a fluence map (FM) and a gamma index (γ) analysis was used for the evaluation of each daily delivery for every patient. The tolerance for the gamma index was set to 2% dose difference and 2mm distance to agreement while points with signal of 10% or lower of the maximum value were excluded from the comparisons. Results: The γ between each of the reference FMs and the consecutive daily fraction FMs had an average value of 99.1% (ranged from 98.2 to 100.0%). The FM images were reconstructed at various resolutions in order to study the effect of the resolution on the γ and at the same time reduce the time for processing the images. We found that the comparison of images with the highest resolution (768×1024) yielded on average a lower γ (99.1%) than the ones with low resolution (192×256) (γ 99.5%). Conclusion: We developed an in-house software that allows us to monitor the quality of daily IMRT and VMAT treatment deliveries using information from the MLC log files of the linear accelerator. The information can be analyzed and evaluated as early as after the completion of each daily treatment. Such tool can be valuable to assess the effect of MLC positioning on plan quality, especially in the context of adaptive radiotherapy.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Effect of MLC leaf width on treatment adaptation and accuracy for concurrent irradiation of prostate and pelvic lymph nodes

Purpose: The aim of the study was to evaluate the impact of multileaf collimator (MLC) leaf width on treatment adaptation and delivery accuracy for concurrent treatment of the prostate and pelvic lymph nodes with intensity modulated radiation therapy (IMRT). Methods: Seventy-five kilovoltage cone beam CTs (KV-CBCT) from six patients were included for this retrospective study. For each patient, three different IMRT plans were created based on a planning CT using three different MLC leaf widths of 2.5, 5, and 10 mm, respectively. For each CBCT, the prostate displacement was determined by a dual image registration. Adaptive plans were created by shifting selected MLC leaf pairs to compensate for daily prostate movements. To evaluate the impact of MLC leaf width on the adaptive plan for each daily CBCT, three MLC shifted plans were created using three different leaf widths of MLCs (a total of 225 adaptive treatment plans). Selective dosimetric endpoints for the tumor volumes and organs at risk (OARs) were evaluated for these adaptive plans. Using the planning CT from a selected patient, MLC shifted plans for three hypothetical longitudinal shifts of 2, 4, and 8 mm were delivered on the three linear accelerators to test the deliverability of the shifted plans and to compare the dose accuracy of the shifted plans with the original IMRT plans. Results: Adaptive plans from 2.5 and 5 mm MLCs had inadequate dose coverage to the prostate (D99 < 97%, or D{sub mean} < 99% of the planned dose) in 6%-8% of the fractions, while adaptive plans from 10 mm MLC led to inadequate dose coverage to the prostate in 25.3% of the fractions. The average V{sub 56Gy} of the prostate over the six patients was improved by 6.4% (1.6%-32.7%) and 5.8% (1.5%-35.7%) with adaptive plans from 2.5 and 5 mm MLCs, respectively, when compared with adaptive plans from 10 mm MLC. Pelvic lymph nodes were well covered for all MLC adaptive plans, as small differences were observed for D99, D{sub mean}, and V{sub 50.4Gy}. Similar OAR sparing could be achieved for the bladder and rectum with all three MLCs for treatment adaptation. The MLC shifted plans can be accurately delivered on all three linear accelerators with accuracy similar to their original IMRT plans, where gamma (3%/3 mm) passing rates were 99.6%, 93.0%, and 92.1% for 2.5, 5, and 10 mm MLCs, respectively. The percentages of pixels with dose differences between the measurement and calculation being less than 3% of the maximum dose were 85.9%, 82.5%, and 70.5% for the original IMRT plans from the three MLCs, respectively. Conclusions: Dosimetric advantages associated with smaller MLC leaves were observed in terms of the coverage to the prostate, when the treatment was adapted to account for daily prostate movement for concurrent irradiation of the prostate and pelvic lymph nodes. The benefit of switching the MLC from 10 to 5 mm was significant (p Much-Less-Than 0.01); however, switching the MLC from 5 to 2.5 mm would not gain significant (p= 0.15) improvement. IMRT plans with smaller MLC leaf widths achieved more accurate dose delivery.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2015

SU-D-207-05: Real-Time Intrafractional Motion Tracking During VMAT Delivery Using a Conventional Elekta CBCT System

Purpose: Real-time kV projection streaming capability has become recently available for Elekta XVI version 5.0. This study aims to investigate the feasibility and accuracy of real-time fiducial marker tracking during CBCT acquisition with or without simultaneous VMAT delivery using a conventional Elekta linear accelerator. Methods: A client computer was connected to an on-board kV imaging system computer, and receives and processes projection images immediately after image acquisition. In-house marker tracking software based on FFT normalized cross-correlation was developed and installed in the client computer. Three gold fiducial markers with 3 mm length were implanted in a pelvis-shaped phantom with 36 cm width. The phantom was placed on a programmable motion platform oscillating in anterior-posterior and superior-inferior directions simultaneously. The marker motion was tracked in real-time for (1) a kV-only CBCT scan with treatment beam off and (2) a kV CBCT scan during a 6-MV VMAT delivery. The exposure parameters per projection were 120 kVp and 1.6 mAs. Tracking accuracy was assessed by comparing superior-inferior positions between the programmed and tracked trajectories. Results: The projection images were successfully transferred to the client computer at a frequency of about 5 Hz. In the kV-only scan, highly accurate marker tracking was achieved over the entire range of cone-beam projection angles (detection rate / tracking error were 100.0% / 0.6±0.5 mm). In the kV-VMAT scan, MV-scatter degraded image quality, particularly for lateral projections passing through the thickest part of the phantom (kV source angle ranging 70°-110° and 250°-290°), resulting in a reduced detection rate (90.5%). If the lateral projections are excluded, tracking performance was comparable to the kV-only case (detection rate / tracking error were 100.0% / 0.8±0.5 mm). Conclusion: Our phantom study demonstrated a promising Result for real-time motion tracking using a conventional Elekta linear accelerator. MV-scatter suppression is needed to improve tracking accuracy during MV delivery. This research is funded by Motion Management Research Grant from Elekta.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Improved image quality of cone beam CT scans for radiotherapy image guidance using fiber-interspaced antiscatter grid

Purpose: Medical linear accelerator mounted cone beam CT (CBCT) scanner provides useful soft tissue contrast for purposes of image guidance in radiotherapy. The presence of extensive scattered radiation has a negative effect on soft tissue visibility and uniformity of CBCT scans. Antiscatter grids (ASG) are used in the field of diagnostic radiography to mitigate the scatter. They usually do increase the contrast of the scan, but simultaneously increase the noise. Therefore, and considering other scatter mitigation mechanisms present in a CBCT scanner, the applicability of ASGs with aluminum interspacing for a wide range of imaging conditions has been inconclusive in previous studies. In recent years, grids using fiber interspacers have appeared, providing grids with higher scatter rejection while maintaining reasonable transmission of primary radiation. The purpose of this study was to evaluate the impact of one such grid on CBCT image quality. Methods: The grid used (Philips Medical Systems) had ratio of 21:1, frequency 36 lp/cm, and nominal selectivity of 11.9. It was mounted on the kV flat panel detector of an Elekta Synergy linear accelerator and tested in a phantom and a clinical study. Due to the flex of the linac and presence of gridline artifacts an angle dependent gain correction algorithm was devised to mitigate resulting artifacts. Scan reconstruction was performed using XVI4.5 augmented with inhouse developed image lag correction and Hounsfield unit calibration. To determine the necessary parameters for Hounsfield unit calibration and software scatter correction parameters, the Catphan 600 (The Phantom Laboratory) phantom was used. Image quality parameters were evaluated using CIRS CBCT Image Quality and Electron Density Phantom (CIRS) in two different geometries: one modeling head and neck and other pelvic region. Phantoms were acquired with and without the grid and reconstructed with and without software correction which was adapted for the different acquisition scenarios. Parameters used in the phantom study weret{sub cup} for nonuniformity and contrast-to-noise ratio (CNR) for soft tissue visibility. Clinical scans were evaluated in an observer study in which four experienced radiotherapy technologists rated soft tissue visibility and uniformity of scans with and without the grid. Results: The proposed angle dependent gain correction algorithm suppressed the visible ring artifacts. Grid had a beneficial impact on nonuniformity, contrast to noise ratio, and Hounsfield unit accuracy for both scanning geometries. The nonuniformity reduced by 90% for head sized object and 91% for pelvic-sized object. CNR improved compared to no corrections on average by a factor 2.8 for the head sized object, and 2.2 for the pelvic sized phantom. Grid outperformed software correction alone, but adding additional software correction to the grid was overall the best strategy. In the observer study, a significant improvement was found in both soft tissue visibility and nonuniformity of scans when grid is used. Conclusions: The evaluated fiber-interspaced grid improved the image quality of the CBCT system for broad range of imaging conditions. Clinical scans show significant improvement in soft tissue visibility and uniformity without the need to increase the imaging dose.

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