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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 1,675 records · Page 93
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DOE OSTI2014

SU-E-T-184: Feasibility of Superabsorbent Polymers as a Buildup Material

Purpose: To investigate the feasibility of superabsorbent polymers as a buildup material for radiation therapy Methods: A standard bolus, a layered damp towel, and a superabsorbent polymer (SAP) phantom were created and scanned to compare the Hounsfield units of each buildup material. A single field plan was developed on Eclipse TPS with AAA dose calculation algorithm to examine dose buildup. Relative film dosimetery (EBT3) was performed to evaluate the surface dose with each buildup material. Each buildup material had an approximate thickness of 0.5 cm and 100 monitor units with 6MV were delivered with solid water placed underneath film to simulate backscatter and more realistic surface dose. Results: The average HU units of the bolus, wet towel, and SAP phantom were 75 (SD=3), -378 (SD=113), -198 (SD=45) respectively. AAA dose calculation demonstrated sufficient dose buildup in all three materials. The relative surfaces doses to film were 23.7% without buildup, 87.5% with 0.5 cm bolus, 92.4% for the SAP phantom, and 87.1% for the damp towel. Conclusion: We demonstrate that superabsorbent polymers can provide sufficient dose buildup. Furthermore, due to the form in which SAPs are traditionally manufactured, this material is less expensive conforms more easily to irregular surfaces than standard sheets of bolus. Also, as a substance which is designed to absorb and retain water efficiently, SAPs are much more comfortable and more consistent than damp towels.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-192: FMEA Severity Scores - Do We Really Know?

Purpose: Failure modes and effects analysis (FMEA) is a subjective risk mitigation technique that has not been applied to physics-specific quality management practices. There is a need for quantitative FMEA data as called for in the literature. This work focuses specifically on quantifying FMEA severity scores for physics components of IMRT delivery and comparing to subjective scores. Methods: Eleven physical failure modes (FMs) for head and neck IMRT dose calculation and delivery are examined near commonly accepted tolerance criteria levels. Phantom treatment planning studies and dosimetry measurements (requiring decommissioning in several cases) are performed to determine the magnitude of dose delivery errors for the FMs (i.e., severity of the FM). Resultant quantitative severity scores are compared to FMEA scores obtained through an international survey and focus group studies. Results: Physical measurements for six FMs have resulted in significant PTV dose errors up to 4.3% as well as close to 1 mm significant distance-to-agreement error between PTV and OAR. Of the 129 survey responses, the vast majority of the responders used Varian machines with Pinnacle and Eclipse planning systems. The average years of experience was 17, yet familiarity with FMEA less than expected. Survey reports perception of dose delivery error magnitude varies widely, in some cases 50% difference in dose delivery error expected amongst respondents. Substantial variance is also seen for all FMs in occurrence, detectability, and severity scores assigned with average variance values of 5.5, 4.6, and 2.2, respectively. Survey shows for MLC positional FM(2mm) average of 7.6% dose error expected (range 0–50%) compared to 2% error seen in measurement. Analysis of ranking in survey, treatment planning studies, and quantitative value comparison will be presented. Conclusion: Resultant quantitative severity scores will expand the utility of FMEA for radiotherapy and verify accuracy of FMEA results compared to highly variable subjective scores.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-219: Investigation of IMRT Out-Of-Field Dose Calculation Accuracy for a Commercial Treatment Planning System

Purpose: Inaccuracies in out-of-field calculations could lead to underestimation of dose to organs-at-risk. This study evaluates the dose calculation accuracy of a model-based calculation algorithm at points outside the primary treatment field for an intensity modulated radiation therapy (IMRT) plan using experimental measurements. Methods: The treatment planning system investigated is Varian Eclipse V.10 with Analytical Anisotropic Algorithm (AAA). The IMRT fields investigated are from real patient treatment plans. The doses from a dynamic (DMLC) IMRT brain plan were calculated and compared with measured doses at locations outside the primary treatment fields. Measurements were performed with a MatriXX system (2-D chamber array) placed in solid water. All fields were set vertically incident on the phantom and were 9 cm × 6 cm or smaller. The dose was normalized to the central axis for points up to 15 cm off isocenter. The comparisons were performed at depths of 2, 10, 15, and 20 cm Results: The measurements have shown that AAA calculations underestimate doses at points outside the primary treatment field. The underestimation occurs at 2 cm depth and decreases down to a factor of 2 as depth increases to 20 cm. In low dose (<2% of target dose) regions outside the primary fields the local dose underestimations can be >200% compared to measured doses. Relative to the plan target dose, the measured doses to points outside the field were less than 1% at shallow depths and less than 2% at greater depths. Conclusion: Compared to measurements, the AAA algorithm underestimated the dose at points outside the treatment field with the greatest differences observed at shallow depths. Despite large local dose uncertainties predicted by the treatment planning system, the impact of these uncertainties is expected to be insignificant as doses at these points were less than 1-2% of the prescribed treatment dose.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-248: Near Real-Time Analysis of Radiation Delivery and Imaging, Accuracy to Ensure Patient Safety

Purpose: To develop and optimize an effective software method for comparing planned to delivered control point machine parameters for all VARIAN TrueBeam treatments so as to permit (1) assessment of a large patient pool throughout their treatment course to quantify treatment technique specific systematic and random uncertainty of observables, (2) quantify the site specific daily imaging shifts required for target alignment, and (3) define tolerance levels for mechanical parameters and imaging parameters based on statistical analysis data gathered, and the dosimetric impact of variations. Methods: Treatment and imaging log files were directly compared to plan parameters for Eclipse and Pinnacle planned treatments via 3D, IMRT, control point, RapidArc, and electrons. Each control point from all beams/arcs (7984) for all fractions (1940) of all patients treated over six months were analyzed. At each control point gantry angle, collimator angle, couch angle, jaw positions, MLC positions, MU were compared. Additionally per-treatment isocenter shifts were calculated. Results were analyzed as a whole in treatment type subsets: IMRT, 3D, RapidArc; and in treatment site subsets: brain, chest/mediastinum, esophagus, H and N, lung, pelvis, prostate. Results: Daily imaging isocenter shifts from initial external tattoo alignment were dependent on the treatment site with < 0.5 cm translational shifts for H and N, Brain, and lung SBRT, while pelvis, esophagus shifts were ∼1 cm. Mechanical delivery parameters were within tolerance levels for all sub-beams. The largest variations were for RapidArc plans: gantry angle 0.11±0.12,collimator angle 0.00±0.00, jaw positions 0.48±0.26, MLC leaf positions 0.66±0.08, MU 0.14±0.34. Conclusion: Per-control point validation reveals deviations between planned and delivered parameters. If used in a near real-time error checking system, patient safety can be improved by equipping the treatment delivery system with additional forcing functions which by-pass human error avenues.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-272: Radiation Damage Comparison Between Intensity Modulated Radiotherapy and Field-In-Field Technique in Breast Cancer Treatments

Purpose: To compare normal tissue complications between IMRT and FIF treatment in breast cancer. Methods: 16 patients treated with IMRT plan and 20 patients treated with FIF plan were evaluated in this study. Both kinds of plans were generated using Eclipse treatment planning system by dosimetrist following clinical radiotherapy treatment guidelines. The plans were reviewed and approved by radiation oncologist. The average survival fraction (SF) for three different normal tissue cells of each concerned structure can be calculated from differential dose volume histogram (DVH) using linear quadratic model. The three types of normal tissues include radiosensitive, moderately radiosensitive and radio-resistant that represents 70%, 50% and 30% survival fractions, respectively, for a 2-Gy open field. Equivalent uniform doses (EUD) for corresponding normal tissues of each structure were calculated. Results: The EUDs of the lungs, heart, healthy breast and spinal cord with both IMRT and FIF treatments were calculated. Considering the average value of all IMRT plans, the lung of treated side absorbed 16.0% of dosage prescribed to the tumor if the radiosensitivity of the lung is similar to the radiosensitive cell line. For moderately radiosensitive and radio-resistant lung tissue, the average EUDs can be 18.9% and 22.4% of prescription. In contrast, patients treated with FIF plans were delivered 6.0%, 7.5% and 10.3% of prescribed dose for radiosensitive, moderately radiosensitive and radio-resistant lung tissue, respectively. Comparing heart EUDs between IMRT and FIF plans, average absorbed doses in IMRT treatment were 7.7%, 8.7% and 9.7% of prescription for three types of heart normal tissue cell lines while FIF treatments delivered only 1.3%, 1.5% and 1.6% of prescription dose. For the other organs, the results were similar. Conclusion: The results indicated that breast cancer treatment using IMRT technique had more normal tissue damage than FIF treatment. FIF demonstrated more effective normal tissue dose reduction in breast cancer treatment.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-274: Radiation Therapy with Very High-Energy Electron (VHEE) Beams in the Presence of Metal Implants

Purpose: To evaluate the effect of metal implants on treatment plans for radiation therapy with very high-energy electron (VHEE) beams. Methods: The DOSXYZnrc/BEAMnrc Monte Carlo (MC) codes were used to simulate 50–150MeV VHEE beam dose deposition and its effects on steel and titanium (Ti) heterogeneities in a water phantom. Heterogeneities of thicknesses ranging from 0.5cm to 2cm were placed at 10cm depth. MC was also used to calculate electron and photon spectra generated by the VHEE beams' interaction with metal heterogeneities. The original VMAT patient dose calculation was planned in Eclipse. Patient dose calculations with MC-generated beamlets were planned using a Matlab GUI and research version of RayStation. VHEE MC treatment planning was performed on water-only geometry and water with segmented prostheses (steel and Ti) geometries with 100MeV and 150MeV beams. Results: 100MeV PDD 5cm behind steel/Ti heterogeneity was 51% less than in the water-only phantom. For some cases, dose enhancement lateral to the borders of the phantom increased the dose by up to 22% in steel and 18% in Ti heterogeneities. The dose immediately behind steel heterogeneity decreased by an average of 6%, although for 150MeV, the steel heterogeneity created a 23% increase in dose directly behind it. The average dose immediately behind Ti heterogeneities increased 10%. The prostate VHEE plans resulted in mean dose decrease to the bowel (20%), bladder (7%), and the urethra (5%) compared to the 15MV VMAT plan. The average dose to the body with prosthetic implants was 5% higher than to the body without implants. Conclusion: Based on MC simulations, metallic implants introduce dose perturbations to VHEE beams from lateral scatter and backscatter. However, when performing clinical planning on a prostate case, the use of multiple beams and inverse planning still produces VHEE plans that are dosimetrically superior to photon VMAT plans. BW Loo and P Maxim received research support from RaySearch laboratories; B Hardemark and E Hynning are employees of RaySearch.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-287: Robustness Study of Passive-Scattering Proton Therapy in Lung: Is Range and Setup Uncertainty Calculation On the Initial CT Enough to Predict the Plan Robustness?

Purpose: Plan robustness of the passive-scattering proton therapy treatment of lung tumors has been studied previously using combined uncertainties of 3.5% in CT number and 3 mm geometric shifts. In this study, we investigate whether this method is sufficient to predict proton plan robustness by comparing to plans performed on weekly verification CT scans. Methods: Ten lung cancer patients treated with passive-scattering proton therapy were randomly selected. All plans were prescribed 6660cGy in 37 fractions. Each initial plan was calculated using +/− 3.5% range and +/− 0.3cm setup uncertainty in x, y and z directions in Eclipse TPS(Method-A). Throughout the treatment course, patients received weekly verification CT scans to assess the daily treatment variation(Method-B). After contours and imaging registrations are verified by the physician, the initial plan with the same beamline and compensator was mapped into the verification CT. Dose volume histograms (DVH) were evaluated for robustness study. Results: Differences are observed between method A and B in terms of iCTV coverage and lung dose. Method-A shows all the iCTV D95 are within +/− 1% difference, while 20% of cases fall outside +/−1% range in Method-B. In the worst case scenario(WCS), the iCTV D95 is reduced by 2.5%. All lung V5 and V20 are within +/−5% in Method-A while 15% of V5 and 10% of V20 fall outside of +/−5% in Method-B. In the WCS, Lung V5 increased by 15% and V20 increased by 9%. Method A and B show good agreement with regard to cord maximum and Esophagus mean dose. Conclusion: This study suggests that using range and setup uncertainty calculation (+/−3.5% and +/−3mm) may not be sufficient to predict the WCS. In the absence of regular verification scans, expanding the conventional uncertainty parameters(e.g., to +/−3.5% and +/−4mm) may be needed to better reflect plan actual robustness.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-313: The Accuracy of the Acuros XB Advanced Dose Calculation Algorithm for IMRT Dose Distributions in Head and Neck

Purpose: To investigate the accuracy of the Acuros XB version 11 (AXB11) advanced dose calculation algorithm by comparing with Monte Caro (MC) calculations. The comparisons were performed with dose distributions for a virtual inhomogeneity phantom and intensity-modulated radiotherapy (IMRT) in head and neck. Methods: Recently, AXB based on Linear Boltzmann Transport Equation has been installed in the Eclipse treatment planning system (Varian Medical Oncology System, USA). The dose calculation accuracy of AXB11 was tested by the EGSnrc-MC calculations. In additions, AXB version 10 (AXB10) and Analytical Anisotropic Algorithm (AAA) were also used. First the accuracy of an inhomogeneity correction for AXB and AAA algorithms was evaluated by comparing with MC-calculated dose distributions for a virtual inhomogeneity phantom that includes water, bone, air, adipose, muscle, and aluminum. Next the IMRT dose distributions for head and neck were compared with the AXB and AAA algorithms and MC by means of dose volume histograms and three dimensional gamma analysis for each structure (CTV, OAR, etc.). Results: For dose distributions with the virtual inhomogeneity phantom, AXB was in good agreement with those of MC, except the dose in air region. The dose in air region decreased in order of MC<AXB11<AXB10. This may be caused by the difference of the electron cut-off energy for algorithms, ie: 0.700 MeV for MC, 0.711 MeV for AXB11, and 1.011 MeV for AXB 10. Since the AAA algorithm is based on the dose kernel of water, the doses in regions for air, bone, and aluminum considerably became higher than those of AXB and MC. The pass rates of the gamma analysis for IMRT dose distributions in head and neck were similar to those of MC in order of AXB11<AXB10<AAA. Conclusion: The dose calculation accuracy of AXB11 was almost equivalent to the MC dose calculation.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-317: The Development of a DIBH Technique for Left Sided Breast Patients Undergoing Radiation Therapy Utilizing Varians RPM System in a Community Hospital

Purpose: To develop and implement a Deep Inhalation Breath Hold program (DIBH) for treatment of patients with Left-sided breast cancer in a community hospital. Methods: All patients with left sided breast cancer underwent a screening free breathing CT. Evaluation of the conventional tangent treatment fields and the heart was conducted. If the heart would not be excluded using tangents, the patient then received DIBH breathe coaching. The patients returned for a 4D CT simulation. The patients breathing cycle was monitored using the Varian Real-Time position ManagementTM (RPM) system to assess duration of DIBH, amplitude, phase and recovery time to normal breathing. Then a DIBH CT was obtained at the desired amplitude. Duplicate plans were developed for both free breathing and DIBH on the Eclipse planning system and comparison DVH's were created. The plan that provided the prescribed treatment coverage and the least doses to the OAR (heart, Lt. Lung) was determined. Those patients selected to receive treatment with DIBH were set up for treatment, and breathing was monitored using the RPM system. Practice trials were used to confirm that the amplitude, phase and recovery were consistent with findings from simulation. Results: 10 patients have been treated using the DIBH procedure in our clinic. The DIBH patients had an average increase of 80% lung volume on DIBH, decreased lung volume receiving 50% of the dose, and decreases in the V20 dose. Significant reduction in the maximum and mean dose to the heart, as well as the dose to 1CC of the volume for the DIBH plans. Conclusion: Using the RPM system already available in the clinic, staff training, and patient coaching a simple DIBH program was setup. The use of DIBH has shown promise in reducing doses to the critical organs while maintaining PTV coverage for left sided breast treatments.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-319: The Effect of Slice Thickness On IMRT Planning

Purpose: The accuracy of volume estimated of a treatment planning system is investigated in this study. In addition, the effect of slice thickness on IMRT planning is also studied. Methods: The accuracy in volume determination was investigated using a water phantom containing various objects with known volumes ranging from 1–100cm{sup 3}. The phantom was scanned with different slice thickness (1–10 mm). The CT data sets were sent to Eclipse TPS for contour delineation and volume calculation. The effect of slice thickness on IMRT planning was studied using a commercial phantom containing four different shaped objects. The phantom was scanned with different slice thickness (1–5 mm). IMRT plans were generated for the different CT datasets to calculate TCP, homogeneity (HI) and conformity indices (CI). Results: The variability of volumes with CT slice thickness was significant especially for small volume structures. The minimum and maximum error in the volume estimation is in the range of −2.3% to 92%. On the other hand, with increasing slice thickness, the PTV mean dose and TCP values decreases. Maximum variation of ∼5% was observed in mean dose and ∼2% in TCP with slice thickness change from 1–5 mm. The relative decrease in target volume receiving 95% of prescribed dose is ∼5% slice thickness change from 1–5 mm. HI increases up to 163% and CI decreases by 4% between 1–5 mm slice thickness change, producing highly inhomogeneous and least conformal plan. Conclusion: Accuracy of volume estimation is dependent on CT slice thickness and the contouring algorithm in a TPS. During TPS commissioning and for all clinical protocols, evaluation of volume should be included to provide the limit of accuracy in DVH calculation. A smaller slice thickness provides superior dosimetry with improved TCP values. Thus, the smallest possible slice thickness should be used for IMRT planning.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

SU-E-T-351: Verification of Monitor Unit Calculation for Lung Stereotactic Body Radiation Therapy Using a Secondary Independent Planning System

Purpose: To compare isocenter (IC) dose between X-ray Voxel Monte Carlo (XVMC) and Acuros XB (AXB) as part of an independent verification of monitor unit (MU) calculation for lung stereotactic body radiation therapy (SBRT) using a secondary independent treatment planning system (TPS). Methods: Treatment plans of 110 lesions from 101 patients who underwent lung SBRT with Vero4DRT (Mitsubishi Heavy Industries, Ltd., Japan, and BrainLAB, Feldkirchen, Germany) were evaluated retrospectively. Dose distribution was calculated with X-ray Voxel Monte Carlo (XVMC) in iPlan 4.5.1 (BrainLAB, Feldkirchen, Germany) on averaged intensity projection images. A spatial resolution and mean variance were 2 mm and 2%, respectively. The clinical treatment plans were transferred from iPlan to Eclipse (Varian Medical Systems, Palo Alto, CA, USA), and doses were recalculated with well commissioned AXB ver. 11.0.31 while maintaining the XVMC-calculated MUs and beam arrangement. Dose calculations were made in the dose-to-medium dose reporting mode with the calculation grid size of 2.5 mm. The mean and standard deviation (SD) of the IC dose difference between XVMC and AXB were calculated. The tolerance level was defined as |mean|+2SD. Additionally, the relationship between IC dose difference and the size of planning target volume (PTV) or computed tomography (CT) value of internal target volume (ITV) was evaluated. Results: The mean±SD of the IC dose difference between XVMC and AXB was −0.32±0.73%. The tolerance level was 1.8%. Absolute IC dose differences exceeding the tolerance level were observed in 3 patients (2.8%). There were no strong correlations between IC dose difference and PTV size (R=−0.14) or CT value of ITV (R=−0.33). Conclusion: The present study suggested that independent verification of MU calculation for lung SBRT using a secondary TPS is useful.

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-378: Limits and Possibilities of a Simplistic Approach to Whole Breast Radiation Therapy Planning

Purpose: Challenges for radiation therapy in developing countries include unreliable infrastructure and high patient load. We propose a system to treat whole breast in the prone position without computed tomography and/or planning software. Methods: Six parameters are measured using calipers and levels with the patient prone in the treatment position. (1) The largest separation; (2) the angle that separation makes with the horizontal; (3) the separation 2 cm posterior to the nipple; (4) the vertical distance between these two separations; (5) the sup/inf length and (6) angle of the desired posterior field edge. The data in (5) (6) and (2) provide field length, collimator and gantry angles. Isocenter is set to the midpoint of (1), anterior jaw setting is 20cm (half-beam setup), and the dose is prescribed to a point 1.5 cm anterior to isocenter. MUs and wedge angles are calculated using an MU calculator and by requiring 100% dose at that point and 100-105% at the midpoint of (3). Measurements on 30 CT scans were taken to obtain the data 1-6. To test the resulting MU/wedge combinations, they were entered into Eclipse (Varian) and dose distributions were calculated. The MU/wedge combinations were recorded and tabulated. Results: Performing a dose volume histogram analysis, the contoured breast V95 was 90.5%, and the average V90 was 94.1%. The maximum dose never exceeded 114.5%, (average 108%). The lung V20 was <5% for 96.7%, and the heart V5 was <10% for 93.3% of our sample. Conclusion: A method to provide prone whole breast treatment without CT-planning was developed. The method provides reasonable coverage and normal tissue sparing. This approach is not recommended if imaging and planning capabilities are available; it was designed to specifically avoid the need for CT planning and should be reserved to clinics that need to avoid that step.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-383: Can Stereotactic Body Radiotherapy Mimic the Dose Distribution of High-Dose-Rate Tandem and Ovoids/ring Brachytherapy?

Purpose: To investigate whether stereotactic body radiotherapy (SBRT) using volumetric modulated arc therapy (VMAT) can mimic the dosimetry of tandem and ovoids/ring brachytherapy. Methods: We selected 5 patients treated with 3D-CT based high-dose rate (HDR) brachytherapy using 4 tandem and ovoid and 1 tandem and ring case. Manual optimization based on the Manchester system followed by graphical optimization (Nucletron Oncentra MasterPlan or Varian BrachyVision) was performed to deliver 6.0 Gy per fraction to a high-risk CTV while maintaining dose to organs at risk (OAR) below the ABS recommendations. For theoretical SBRT plans, CT images and OAR contours from the HDR plans were imported into Eclipse (Varian). The SBRT plan was created to mimic the heterogeneity of HDR plans by using a simultaneous integrated boost technique to match the V100, V150, and V200 isodose volumes from HDR. The OAR Dmax from HDR was used to define the OAR dose constraints for SBRT. Target coverage, dose spill-out, and OAR doses (D0.1cc, D1cc, and D2cc) between the HDR and SBRT plans were compared for significance using a two-tail paired ttest. Results: The mean isodose volumes for HDR vs. SBRT were 29.4 cc vs. 29.0 cc (V200, p = 0.674), 49.2 cc vs. 56.3 cc (V150, p = 0.017), 95.4 cc vs. 127.7 cc (V100, p = 0.001), and 271.9 cc vs. 581.6 cc (V50, p = 0.001). The D2cc to OAR for HDR vs. SBRT was 71.6% vs. 96.2% (bladder, p = 0.002), 69.2% vs. 101.7% (rectum, p = 0.0003), and 56.9% vs. 68.6% (sigmoid, p = 0.004). Conclusion: SBRT with VMAT can provide similar dose target coverage (V200), but dose spill-out and doses to OAR were statistically significantly higher than HDR. This study clearly demonstrated that brachytherapy can not be substituted with SBRT in gynecologic cervical cancer treatment.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-384: Experimental Verification of a Monte Carlo Linear Accelerator Model Using a Radiochromic Film Stack Dosimeter

Purpose: To experimentally verify a Monte Carlo (MC) linear accelerator model for the simulation of intensity-modulated radiation therapy (IMRT) treatments of moving targets. Methods: A Varian Clinac™ 21EX linear accelerator was modeled using the EGSnrc user code BEAMnrc. The mean energy, radial-intensity distribution, and divergence of the electron beam incident on the bremsstrahlung target were adjusted to achieve agreement between simulated and measured percentage-depth-dose and transverse field profiles for a 6 MV beam. A seven-field step-and-shoot IMRT lung procedure was prepared using Varian Eclipse™ treatment planning software. The plan was delivered using a Clinac™ 21EX linear accelerator and measured with a Gafchromic™ EBT2 film stack dosimeter (FSD) in two separate static geometries: within a cylindrical water-equivalent-plastic phantom and within an anthropomorphic chest phantom. Two measurements were completed in each setup. The dose distribution for each geometry was simulated using the EGSnrc user code DOSXYZnrc. MC geometries of the treatment couch, cylindrical phantom, and chest phantom were developed by thresholding CT data sets using MATLAB™. The FSD was modeled as water. The measured and simulated dose distributions were normalized to the median dose within the FSD. Results: Using an electron beam with a mean energy of 6.05 MeV, a Gaussian radial-intensity distribution with a full width at half maximum of 1.5 mm, and a divergence of 0°, the measured and simulated dose profiles agree within 1.75% and 1 mm. Measured and simulated dose distributions within both the cylindrical and chest phantoms agree within 3% over 94% of the FSD volume. The overall uncertainty in the FSD measurements is 3.1% (k=1). Conclusion: MC simulations agree with FSD measurements within measurement uncertainty, thereby verifying the accuracy of the linear accelerator model for the simulation of IMRT treatments of static geometries. The experimental verification will be extended to treatments of targets undergoing three-dimensional motion.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-394: The Use of Jaw Tracking in Intensity Modulated and Volumetric Modulated Arc Radiotherapy for Spine Stereotactic Radiosurgery

Purpose: To evaluate the potential advantages of jaw tracking for intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) in spine radiosurgery. Methods: VMAT and IMRT plans were retrospectively generated for ten patients. Six plans for each patient were created in the Eclipse treatment planning system for a Varian Truebeam equipped with a Millennium 120 MLC. Plans were created to study IMRT and VMAT plans with and without jaw tracking, as well as IMRT plans of different flattening filter free (FFF) energies. Plans were prescribed to the 90% isodose line to 16 or 18 Gy in one fraction to cover 95% of the target. Planning target volume (PTV) coverage, conformity index (CI), dose to spinal cord, distance to fall off from the 90% to 50% isodose line (DTF), as well as delivery time were evaluated. Ion chamber and film measurements were performed to verify calculated and measured dose distributions. Results: Jaw tracking decreased the spinal cord dose for both IMRT and VMAT plans, but a larger decrease was seen with the IMRT plans (p=0.004 vs p=0.04). The average D10% for the spinal cord was least for the 6MV FFF IMRT plan with jaw tracking and was greatest for the 10MV FFF plan without jaw tracking. Treatment times between IMRT and VMAT plans with or without jaw tracking were not significantly different. Measured plans showed greater than 98.5% agreement for planar dose gamma analysis (3%/2 mm) and less than 2.5% for point dose analysis compared to calculated plans. Conclusion: Jaw tracking can be used to help decrease spinal cord dose without any change in treatment delivery or calculation accuracy. Lower dose to the spinal cord was achieved using 6 MV beams compared to 10 MV beams, though 10 MV may be justified in some cases to decrease skin dose.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-396: Dosimetric Accuracy of Proton Therapy for Patients with Metal Implants in CT Scans Using Metal Deletion Technique (MDT) Artifacts Reduction

Purpose: To evaluate the dosimetric accuracy for proton therapy patients with metal implants in CT using metal deletion technique (MDT) artifacts reduction. Methods: Proton dose accuracies under CT metal artifacts were first evaluated using a water phantom with cylindrical inserts of different materials (titanium and steel). Ranges and dose profiles along different beam angles were calculated using treatment planning system (Eclipse version 8.9) on uncorrected CT, MDT CT, and manually-corrected CT, where true Hounsfield units (water) were assigned to the streak artifacts. In patient studies, the treatment plans were developed on manually-corrected CTs, then recalculated on MDT and uncorrected CTs. DVH indices were compared between the dose distributions on all the CTs. Results: For water phantom study with 1/2 inch titanium insert, the proton range differences estimated by MDT CT were with 1% for all beam angles, while the range error can be up to 2.6% for uncorrected CT. For the study with 1 inch stainless steel insert, the maximum range error calculated by MDT CT was 1.09% among all the beam angles compared with maximum range error with 4.7% for uncorrected CT. The dose profiles calculated on MDT CTs for both titanium and steel inserts showed very good agreements with the ones calculated on manually-corrected CTs, while large dose discrepancies calculated using uncorrected CTs were observed in the distal end region of the proton beam. The patient study showed similar dose distribution and DVHs for organs near the metal artifacts recalculated on MDT CT compared with the ones calculated on manually-corrected CT, while the differences between uncorrected and corrected CTs were much pronounced. Conclusion: In proton therapy, large dose error could occur due to metal artifact. The MDT CT can be used for proton dose calculation to achieve similar dose accuracy as the current clinical practice using manual correction.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-413: Experience-Based VMAT Plan Quality Database

Purpose: To quantify VMAT plan quality using a retrospective study of over 200 clinical treated VMAT plans created using the Eclipse Treatment Planning System to create benchmarks of plan quality for a few categories of treatment sites. Methods: Using a controlled phantom geometry, various dosimetric indices were investigated to quantify dosimetric plan quality as a function of isocenter displacement from center of mass, average path length, number of arcs and PTV proximity to critical structures. Beginning with published dosimetry indices from SRS and SBRT evaluations, UDI (Unified Dosimetry Index) and modified UDI were tested before creating a new factor VMAT-DI. VMAT-DI was developed within boundaries of this project and it includes renormalized factors of conformity index, coverage index, modified gradient index and homogeneity index as well as indices based on routine clinical practice such as absolute dose max index. The plans were then evaluated using the VMAT-DI such that benchmarks for planning could be created. Results: The majority of the plans evaluated could be assigned VMAT-DI values within a range for each treatment site. However, the outliers were results of difficult planning parameters such as very irregular targets, inhomogeneities or difficult to achieve critical structure constraints. To effectively use VMAT-DI for guidance, especially for prediction of the plan quality for body sites new to the practice, VMATDI database needs to be subdivided by target complexity and by body site index/average path length factor. Conclusion: An experienced-based VMAT-DI database can be used to help analyze plans before evaluation by the physician to show that it adheres to the clinical standards of previously treated VMAT plans which will make a guideline for concluding the optimization. The introduction of institution-wide clinical planning protocols, standardizing OAR naming and constraints will make it possible to incorporate a cumulative critical structure dosimetry index such as NTCP.

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