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DOE OSTI2014

SU-E-J-53: Dosimetric Evaluation at Volumetric Modulated Arc Therapy for Treatment of Prostate Cancer Using Single Or Double Arcs

Purpose: Evaluate and compare retrospective prostate treatment plan using Volumetric Modulated Arc Therapy (RapidArc™ - Varian) technique with single or double arcs at COI Group. Methods: Ten patients with present prostate and seminal vesicle neoplasia were replanned as a target treatment volume and a prescribed dose of 78 Gy. A baseline planning, using single arc, was developed for each case reaching for the best result on PTV, in order to minimize the dose on organs at risk (OAR). Maintaining the same optimization objectives used on baseline plan, two copies for optimizing single and double arcs, have been developed. The plans were performed with 10 MV photon beam energy on Eclipse software, version 11.0, making use of Trilogy linear accelerator with Millenium HD120 multileaf collimator. Comparisons on PTV have been performed, such as: maximum, minimum and mean dose, gradient dose, as well as the quantity of monitor units, treatment time and homogeneity and conformity index. OARs constrains dose have been evaluated, comparing both optimizations. Results: Regarding PTV coverage, the difference of the minimum, maximum and mean dose were 1.28%, 0.7% and 0.2% respectively higher for single arc. When analyzed the index of homogeneity found a difference of 0.99% higher when compared with double arcs. However homogeneity index was 0.97% lower on average by using single arc. The doses on the OARs, in both cases, were in compliance to the recommended limits RTOG 0415. With the use of single arc, the quantity of monitor units was 10,1% lower, as well as the Beam-On time, 41,78%, when comparing double arcs, respectively. Conclusion: Concerning the optimization of patients with present prostate and seminal vesicle neoplasia, the use of single arc reaches similar objectives, when compared to double arcs, in order to decrease the treatment time and the quantity of monitor units.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-J-58: Dosimetric Verification of Metal Artifact Effects: Comparison of Dose Distributions Affected by Patient Teeth and Implants

Purpose: Implant-supported dentures seem particularly appropriate for the predicament of becoming edentulous and cancer patients are no exceptions. As the number of people having dental implants increased in different ages, critical dosimetric verification of metal artifact effects are required for the more accurate head and neck radiation therapy. The purpose of this study is to verify the theoretical analysis of the metal(streak and dark) artifact, and to evaluate dosimetric effect which cause by dental implants in CT images of patients with the patient teeth and implants inserted humanoid phantom. Methods: The phantom comprises cylinder which is shaped to simulate the anatomical structures of a human head and neck. Through applying various clinical cases, made phantom which is closely allied to human. Developed phantom can verify two classes: (i)closed mouth (ii)opened mouth. RapidArc plans of 4 cases were created in the Eclipse planning system. Total dose of 2000 cGy in 10 fractions is prescribed to the whole planning target volume (PTV) using 6MV photon beams. Acuros XB (AXB) advanced dose calculation algorithm, Analytical Anisotropic Algorithm (AAA) and progressive resolution optimizer were used in dose optimization and calculation. Results: In closed and opened mouth phantom, because dark artifacts formed extensively around the metal implants, dose variation was relatively higher than that of streak artifacts. As the PTV was delineated on the dark regions or large streak artifact regions, maximum 7.8% dose error and average 3.2% difference was observed. The averaged minimum dose to the PTV predicted by AAA was about 5.6% higher and OARs doses are also 5.2% higher compared to AXB. Conclusion: The results of this study showed that AXB dose calculation involving high-density materials is more accurate than AAA calculation, and AXB was superior to AAA in dose predictions beyond dark artifact/air cavity portion when compared against the measurements.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-J-75: Importance of 4DCT for Target Volume Definition in Stereotactic Lung Radiotherapy

Purpose: We aimed to investigate the importance of 4DCT for lung tumors treated with SBRT and whether maximum intensity projection (MIP) and free breathing (FB) images can compansate for tumor movement. Methods: Six patients with primary lung cancer and 2 patients with lung metastasis with a median age of 69.5 (42–86) were included. Patients were positioned supine on a vacuum bag. In addition to FB planning CT images, 4DCT images were obtained at 3 mm intervals using Varian RPM system with (Siemens Somatom Sensetion 64). MIP series were reconstructed using 4DCT images. PTV-FB and PTV-MIP (GTV+5mm) volumes were contoured using FB and MIP series, respectively. GTVs were defined on each of eight different breathing phase images and were merged to create the ITV. PTV-4D was generated with a 5 mm margin to ITV. PTV-MIP and PTV-4D contours were copied to FB CT series and treatment plans for PTV-MIP and PTV-FB were generated using RapidArc (2 partial arc) technique in Eclipse (version 11, AAA algorithm). The prescription dose was 5600cGy in 7 fractions. ITV volumes receiving prescription dose (%) and V95 for ITV were calculated for each treatment plan. Results: The mean PTV-4B, PTV-MIP and PTV-FB volumes were 23.2 cc, 15.4cc ve 11cc respectively. Median volume of ITV receiving the prescription dose was 34.6% (16.4–70 %) and median V95 dose for ITV was 1699cGy (232cGy-5117cGy) in the plan optimized for PTV-FB as the reference. When the plan was optimized for PTV-MIP, median ITV volume receiving the prescription dose was 67.15% (26–86%) and median V95 dose for ITV was 4231cGy (1735cGy-5290cGy). Conclusion: Images used in lung SBRT are critical for treatment quality; FB and MIP images did not compensate target movement, therefore 4DCT images should be obtained for all patients undergoing lung SBRT or the safety margins should be adjusted.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-J-80: Interplay Effect Between VMAT Intensity Modulation and Tumor Motion in Hypofractioned Lung Treatment, Investigated with 3D Pressage Dosimeter

Purpose: To demonstrate an embedded tissue equivalent presage dosimeter for measuring 3D doses in moving tumors and to study the interplay effect between the tumor motion and intensity modulation in hypofractioned Volumetric Modulated Arc Therapy(VMAT) lung treatment. Methods: Motion experiments were performed using cylindrical Presage dosimeters (5cm diameter by 7cm length) mounted inside the lung insert of a CIRS thorax phantom. Two different VMAT treatment plans were created and delivered in three different scenarios with the same prescribed dose of 18 Gy. Plan1, containing a 2 centimeter spherical CTV with an additional 2mm setup margin, was delivered on a stationary phantom. Plan2 used the same CTV except expanded by 1 cm in the Sup-Inf direction to generate ITV and PTV respectively. The dosimeters were irradiated in static and variable motion scenarios on a Truebeam system. After irradiation, high resolution 3D dosimetry was performed using the Duke Large Field-of-view Optical-CT Scanner, and compared to the calculated dose from Eclipse. Results: In the control case (no motion), good agreement was observed between the planned and delivered dose distributions as indicated by 100% 3D Gamma (3% of maximum planned dose and 3mm DTA) passing rates in the CTV. In motion cases gamma passing rates was 99% in CTV. DVH comparisons also showed good agreement between the planned and delivered dose in CTV for both control and motion cases. However, differences of 15% and 5% in dose to PTV were observed in the motion and control cases respectively. Conclusion: With very high dose nature of a hypofraction treatment, significant effect was observed only motion is introduced to the target. This can be resulted from the motion of the moving target and the modulation of the MLC. 3D optical dosimetry can be of great advantage in hypofraction treatment dose validation studies.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-J-113: The Influence of Optimizing Pediatric CT Simulator Protocols On the Treatment Dose Calculation in Radiotherapy

Purpose: To investigate the possibility of applying optimized scanning protocols for pediatric CT simulation by quantifying the dosimetric inaccuracy introduced by using a fixed HU to density conversion. Methods: The images of a CIRS electron density reference phantom (Model 062) were acquired by a Siemens CT simulator (Sensation Open) using the following settings of tube voltage and beam current: 120 kV/190mA (the reference protocol used to calibrate CT for our treatment planning system (TPS)); Fixed 190mA combined with all available kV: 80, 100, and 140; fixed 120 kV and various current from 37 to 444 mA (scanner extremes) with interval of 30 mA. To avoid the HU uncertainty of point sampling in the various inserts of known electron densities, the mean CT numbers of the central cylindrical volume were calculated using DICOMan software. The doses per 100 MU to the reference point (SAD=100cm, Depth=10cm, Field=10X10cm, 6MV photon beam) in a virtual cubic phantom (30X30X30cm) were calculated using Eclipse TPS (calculation model: AcurosXB-11031) by assigning the CT numbers to HU of typical materials acquired by various protocols. Results: For the inserts of densities less than muscle, CT number fluctuations of all protocols were within the tolerance of 10 HU as accepted by AAPM-TG66. For more condensed materials, fixed kV yielded stable HU with any mA combination where largest disparities were found in 1750mg/cc insert: HU{sub reference}=1801(106.6cGy), HU{sub minimum}=1799 (106.6cGy, error{sub dose}=0.00%), HU{sub maximum}=1815 (106.8cGy, error{sub dose}=0.19%). Yet greater disagreements were observed with increasing density when kV was modified: HU{sub minimum}=1646 (104.5cGy, error{sub dose}=- 1.97%), HU{sub maximum}=2487 (116.4cGy, error{sub dose}=9.19%) in 1750mg/cc insert. Conclusion: Without affecting treatment dose calculation, personalized mA optimization of CT simulator can be conducted by fixing kV for a better cost-effectiveness of imaging dose and quality especially for children. Unless recalibrated, kV should be constant for all anatomical sites if diagnostic CT scanner is used as a simulator. This work was partially supported by Capital Medical Development Scientific Research Fund of China.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

SU-E-J-192: Verification of 4D-MRI Internal Target Volume Using Cine MRI

Purpose: To investigate the accuracy of 4D-MRI in determining the Internal Target Volume (ITV) used in radiation oncology treatment planning of liver cancers. Cine MRI is used as the standard baseline in establishing the feasibility and accuracy of 4D-MRI tumor motion within the liver. Methods: IRB approval was obtained for this retrospective study. Analysis was performed on MR images from four patients receiving external beam radiation therapy for liver cancer at our institution. Eligible patients received both Cine and 4D-MRI scans before treatment. Cine images were acquired sagittally in real time at a slice bisecting the tumor, while 4D images were acquired volumetrically. Cine MR DICOM headers were manipulated such that each respiratory frame was assigned a unique slice location. This approach permitted the treatment planning system (Eclipse, Varian Medical Systems) to recognize a complete respiratory cycle as a “volume”, where the gross tumor was contoured temporally. Software was developed to calculate the union of all frame contours in the structure set, resulting in the corresponding plane of the ITV projecting through the middle of the tumor, defined as the Internal Target Area (ITA). This was repeated for 4D-MRI, at the corresponding slice location, allowing a direct comparison of ITAs obtained from each modality. Results: Four patients have been analyzed. ITAs contoured from 4D-MRI correlate with contours from Cine MRI. The mean error of 4D values relative to Cine values is 7.67 +/− 2.55 %. No single ITA contoured from 4D-MRI demonstrated more than 10.5 % error compared to its Cine MRI counterpart. Conclusion: Motion management is a significant aspect of treatment planning within dynamic environments such as the liver, where diaphragmatic and cardiac activity influence plan accuracy. This small pilot study suggests that 4D-MRI based ITA measurements agree with Cine MRI based measurements, an important step towards clinical implementation. NIH 1R21CA165384-01A1.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-J-207: Assessing the Validity of 4D-CT Based Target Volumes and Free Breathing CBCT Localization in Lung Stereotactic Ablative Radiation Therapy (SABR)

Purpose: Four-dimensional-computed-tomography(4D-CT) imaging for target-volume delineation and cone-beam-tomography(CBCT) for treatment localization are widely utilized in lung-SABR.Aim of this study was to perform a quantitative-assessment and inter-comparison of Internal-targetvolumes( ITV) drawn on various phases of breathing-cycle 4D-CT-scans, Maximum-intensity-projection(MIP), average-intensity-projection(AIP)and static CT-scans of lung-motion-phantom to simulate lung-SABR patient geometry. We also analyzed and compared the ITVs drawn on freebreathing- CBCT. Materials and Methods: 4D-CT-scans were acquired on Philips big-bore 16slice CT and Bellows-respiratory monitoring-system using retrospective phase-binning method. Each respiratory cycle divided into 10-phases. Quasar-Phantom with lung-inserts and 3cm-diameter nylonball to simulate tumor and was placed on respiratory-motion-platform for 4D-CT and CBCT-acquisition. Amplitudes of motions: 0.5,1.0,2.0,3.0cm in superior-inferior direction with breathing-cycle time of 6,5,4,6sec, respectively used.4D-CTs with 10-phases(0%to90%)for each excursion-set and 3D-CT for static-phantom exported to iPlan treatment-planningsystem( TPS).Tumor-volumes delineated in all phases of 4D-CT, MIP,AIP,CBCT scans using fixed-HU-threshold(−500to1000)values automatically.For each 4D-dataset ITV obtained by unifying the tumorcontours on all phases.CBCT-ITV-volumes were drawn in Eclipse-TPS. Results: Mean volume of tumor contours for all phases compared with static 3D-CT were 0.62±0.08%, 1.67±0.26%, 4.77±0.54% and 9.27±1.23% for 0.5cm,1cm,2cm,3cm excursions respectively. Differences of mean Union-ITV with MIP-ITV were close(≤2.4%).Mean Union-ITV from expected-theoretical values differed from −4.9% to 3.8%.Union-ITV and MIP-ITV were closer within 2.3%. AIP-ITVs were underestimated from 14 to 32% compared to union-ITV for all motion datasets. Differences of −5.9% to −44% and −5% to 6.7% for CBCT-ITV from MIP-ITV and AIP-ITV respectively.Motion excursions and centroid positions were within 2mm for 4D-CT and CBCT-ITVs to that of expected values. Conclusion: 4D-CT MIP-ITV and Union-ITV showed very good agreement that validates that ITV can be fast contoured on MIP.Contouring ITV in AIP must be avoided as it significantly underestimates the volume with all excursions. Free breathing CBCT-ITV showed good agreement with AIP-ITV but underestimated the MIP-ITV. Estimation of excursions and centroid values for 4D-CT and CBCT were in good agreement with expected values.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-J-263: Dosimetric Analysis On Breast Brachytherapy Based On Deformable Image Registration

Purpose: To quantitatively compare and evaluate the dosimetry difference between breast brachytherapy protocols with different fractionation using deformable image registration. Methods: The accumulative dose distribution for multiple breast brachytherapy patients using four different applicators: Contura, Mammosite, Savi, and interstitial catheters, under two treatment protocols: 340cGy by 10 fractions in 5 days and 825cGy by 3 fractions in 2days has been reconstructed using a two stage deformable image registration approach. For all patients, daily CT was acquired with the same slice thickness (2.5mm). In the first stage, the daily CT images were rigidly registered to the initial planning CT using the registration module in Eclipse (Varian) to align the applicators. In the second stage, the tissues surrounding the applicator in the rigidly registered daily CT image were non-rigidly registered to the initial CT using a combination of image force and the local constraint that enforce zero normal motion on the surface of the applicator, using a software developed in house. We calculated the dose distribution in the daily CTs and deformed them using the final registration to convert into the image domain of the initial planning CT. The accumulative dose distributions were evaluated by dosimetry parameters including D90, V150 and V200, as well as DVH. Results: Dose reconstruction results showed that the two day treatment has a significant dosimetry improvement over the five day protocols. An average daily drop of D90 at 1.3% of the prescription dose has been observed on multiple brachytherapy patients. There is no significant difference on V150 and V200 between those two protocols. Conclusion: Brachytherapy with higher fractional dose and less fractions has an improved performance on being conformal to the dose distribution in the initial plan. Elongated brachytherapy treatments need to consider the dose uncertainty caused by the temporal changes of the soft tissue.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-09: A Dosimetric Analysis of Various Clinically Used Bolus Materials

Purpose: To evaluate the dosimetric effect of various clinically used bolus materials. Methods: Materials investigated include solid water, superflab, wet gauze, wet sheets, Play-Doh{sup ™}, and gauze embedded with petroleum jelly. Each bolusing material was scanned in a Philips CT to determine the Hounsfield unit (HU) and to verify uniformity throughout the material. Using the corresponding HU, boluses of 0.5 cm and 1.0 cm thicknesses were created in the Eclipse treatment planning system (TPS) on a solid water phantom. Dose was calculated at various depths for beam energies 6 MV, 6 MeV, 9 MeV, and 12 MeV to determine the effects of each material on deposition of dose. In addition, linac-based measurements at these energies were made using a farmer chamber in solid water. Wet sheets and wet gauze were measured with various water content to quantify the effects on dose. Results: Preliminary CT scans find a range in HU of bolus materials from −120 to almost 300. There is a trend in the dose at depth based on the HU of the material; however inconsistencies are found when the bolus materials have a negative HU value. The measured data indicates that there is a linear relationship between the mass of water in a material and the dose reading, the slope of which is material dependent. Conclusion: Due to the variation in HU of the bolus materials studied, it is recommended that any new bolus be evaluated before clinical use to determine physical and dosimetric properties. If possible, patients should have bolus included in their CT scans; or if the bolus is created in the TPS, the HU should correspond to the material used. For water-soaked materials, once the bolus material is selected (gauze or sheet), the bolusing effect is only dependent on the amount of water applied to the material.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-28: A Treatment Planning Comparison of Volumetric Modulated Arc Therapy Vs. Proton Therapy for Post-Mastectomy Radiotherapy

Purpose: The delivery of post-mastectomy radiotherapy (PMRT) can be challenging for patients with left-sided breast cancer due to the PTV size and proximity to critical organs. This study investigates the use of protons for PMRT in a clinically-representative cohort of patients, and quantitatively compares volumetric modulated arc therapy (VMAT) to proton therapy to have an evidence-based rationale for selecting a treatment modality for these patients. Methods: Eight left-sided PMRT patients previously treated at our clinic with VMAT were identified for the study. PTVs included the chest wall and regional lymph nodes. Passively scattered (PS) and intensity modulated proton therapy (IMPT) plans were constructed using the Eclipse proton planning system. The resulting plans were compared to the original VMAT plan on the basis of PTV coverage; dose homogeneity index (DHI) and conformity index (CI); dose to organs at risk (OAR); tumor control probability (TCP), normal tissue complication probability (NTCP) and secondary cancer complication probability (SCCP). Differences were tested for significance using the paired Student's t-test (p<0.01). Results: All modalities produced clinically acceptable PMRT plans. The comparison demonstrated proton treatment plans provide significantly lower NTCP values for the heart and the lung while maintaining significantly better CI and DHI. At a prescribed dose of 50.4 Gy (RBE) in the PTV, the calculated mean NTCP value for the patients decreased from 1.3% to 0.05% for the whole heart (cardiac mortality) and from 3.8% to 1.1% for the lungs (radiation pneumonitis) for both proton therapy plans from VMAT plans. Both proton modalities showed a significantly lower SCCP for the contralateral breast compared to VMAT. Conclusion: All three plans (VMAT, PS, and IMPT) provide acceptable treatment plans for PMRT. However, proton therapy shows a significant advantage over VMAT with regards to sparing OARs and may be more advantageous for patients with unfavorable anatomy.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-31: Alternative VMAT Technique Reduces Total Monitor Units for Lung SBRT

Purpose: To investigate an alternative approach to VMAT optimization for hypofractionation lung treatment which increases average aperture opening and results in lower total Monitor Units (MU) without significantly sacrificing plan quality. Methods: Benchmark Volumetric Modulated Arc Therapy (bVMAT) plans were generated for 10 lung Stereotactic Body radiotherapy (SBRT) cases using Eclipse Version 11.0.42 (Varian Medical Systems) without a maximum MU constraint. Prescriptions ranged from 40 to 54Gy in 3 to 5 fractions. AAA dose calculation and PRO fluence based optimization was utilized. Two comparison VMAT plans were generated for each case, one that forced an initial “open” mlc aperture conformal to the tumor as a starting condition (oVMAT) with similar optimization parameters and arc geometries, and one that repeated the bVMAT optimization but added a maximum MU constraint (muVMAT). All plans used two arcs with lengths between 168 to 230 degrees. PTV D 95% and Dmean, lung V20 Gy, chest wall V30 Gy, average aperture opening and MU's were compared. Statistical significance was evaluated using Wilcoxon signed rank test. Results: Average PTV D(95), PTV mean and lung V20Gy over all plans was 99.2 ± 1.7%, 103.3 ± 0.6% and 7.8 ± 2.4% respectively. The average chest wall V30Gy was 61 ± 61 cc and ranged between 0 to 166 cc. There were no significant differences between the three techniques for the dosimetric quantities. MUs were reduced by 11 ±11% (p<0.01) and 25 ± 5% (p<0.01) and the average aperture size was increased by 13.7 ± 14% (p=0.02) and 35.8 ± 10% (p<0.01) with muVMAT and oVMAT, respectively, compared to bVMAT. Conclusion: oVMAT and muVMAT techniques were both able to increase average aperture size and reduce total MU compared to the benchmark VMAT plan, but the magnitude of the changes observed for oVMAT was larger.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-50: Automatic Validation of Megavoltage Beams Modeled for Clinical Use in Radiation Therapy

Purpose: To automatically validate megavoltage beams modeled in XiO™ 4.50 (Elekta, Stockholm, Sweden) and Varian Eclipse™ Treatment Planning Systems (TPS) (Varian Associates, Palo Alto, CA, USA), reducing validation time before beam-on for clinical use. Methods: A software application that can automatically read and analyze DICOM RT Dose and W2CAD files was developed using MatLab integrated development environment.TPS calculated dose distributions, in DICOM RT Dose format, and dose values measured in different Varian Clinac beams, in W2CAD format, were compared. Experimental beam data used were those acquired for beam commissioning, collected on a water phantom with a 2D automatic beam scanning system.Two methods were chosen to evaluate dose distributions fitting: gamma analysis and point tests described in Appendix E of IAEA TECDOC-1583. Depth dose curves and beam profiles were evaluated for both open and wedged beams. Tolerance parameters chosen for gamma analysis are 3% and 3 mm dose and distance, respectively.Absolute dose was measured independently at points proposed in Appendix E of TECDOC-1583 to validate software results. Results: TPS calculated depth dose distributions agree with measured beam data under fixed precision values at all depths analyzed. Measured beam dose profiles match TPS calculated doses with high accuracy in both open and wedged beams. Depth and profile dose distributions fitting analysis show gamma values < 1. Relative errors at points proposed in Appendix E of TECDOC-1583 meet therein recommended tolerances.Independent absolute dose measurements at points proposed in Appendix E of TECDOC-1583 confirm software results. Conclusion: Automatic validation of megavoltage beams modeled for their use in the clinic was accomplished. The software tool developed proved efficient, giving users a convenient and reliable environment to decide whether to accept or not a beam model for clinical use. Validation time before beam-on for clinical use was reduced to a few hours.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-52: Beam Data Comparison for 20 Linear Accelerators in One Network

Purpose: To compare photon beam data for the 20 Varian linear accelerators (TrueBeam, iX, and EX models) in use at five centers in the same network with the intent to model with one set of beam data in Eclipsec. Methods: Varian linear accelerators, TrueBeam (3), 21 EX, iX, and Trilogy (14), and 6 EX (3), installed between 1999 and 2014 have their 6 MV and 15 MV x-ray beams reevaluated. Full commissioning, including output factors (St), percent depth doses (PDD), and off-axis profiles, was recently performed for a TrueBeam with a cc04 ion chamber in an IBA Blue phantom. Similarly, a subset of beam data for each of the other accelerators was measured recently as follows: for 3×3, 10×10, and 30×30 cm{sup 2} field sizes, flatness and penumbra (80–20%) were measured at dmax and 10 cm depths, PDD were measured at 10 and 20 cm depths, and St were measured at 5 cm depth. Measurement results for all machines were compared. Results: For 15 high-energy (6 and 15 MV) and 3 low-energy machines (6MV only): 1) PDD agreed within 1.4% at 10 and 20 cm depths; 2) penumbra agreed within 1.0 mm at dmax and 10 cm depths; 3) flatness was within 1.3% at dmax and 10 cm depths; and 4) with exception of the three low energy machines, output factors were within 1.1% and 0.5% for 3×3 and 30×30 cm{sup 2}, respectively. Measurement uncertainty, not quantified here, accounts for some of these differences. Conclusion: Measured beam data from 15 high-energy Varian linacs are consistent enough that they can be classified using one beam data set in Eclipse. Two additional high-energy machines are removed from this group until their data are further confirmed. Three low-energy machines will be in a separate class based upon differences in output factors (St)

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

SU-E-T-66: Characterization of Radiation Dose Associated with Dark Currents During Beam Hold for Respiratory-Gated Electron Therapy

Purpose: The main objective of this study was to estimate the radiation dose contributed by dark currents associated with the respiratory-gated electron therapy during beam hold. The secondary aim was to determine clinical benefits of using respiratory-gated electron therapy for left-sided breast cancer patients with positive internal mammary nodes (IMN). Methods: Measurements of the dark current-induced dose in all electron modes were performed on multiple Siemens and Varian linear accelerators by manually simulating beam-hold during respiratory gating. Dose was quantified at the machine isocenter by comparing the collected charge to the known output for all energies ranging from 6 to 18 MeV for a 10cm × 10cm field at 100 SSD with appropriate solid-water buildup. Using the Eclipse treatment planning system, we compared the additional dose associated with dark current using gated electron fields to the dose uncertainties associated with matching gated photon fields and ungated electron fields. Dose uncertainties were seen as hot and cold spots along the match line of the fields. Results: The magnitude of the dose associated with dark current is highly correlated to the energy of the beam and the amount of time the beam is on hold. For lower energies (6–12 MeV), there was minimal dark current dose (0.1–1.3 cGy/min). Higher energies (15–18 MeV) showed measurable amount of doses. The dark current associated with the electron beam-hold varied between linear accelerator vendors and depended on dark current suppression and the age of the linear accelerator. Conclusion: For energies up to 12 MeV, the dose associated with the dark current for respiratorygated electron therapy was shown to be negligible, and therefore should be considered an option for treating IMN positive left-sided breast cancer patients. However, at higher energies the benefit of respiratory gating may be outweighed by dose due to the dark current.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-67: Clinical Implementation and Evaluation of the Acuros Dose Calculation Algorithm

Purpose: The main aim of the current study is to present a detailed description of the implementation of the Acuros XB Dose Calculation Algorithm, and subsequently evaluate its clinical impacts by comparing it with AAA algorithm. Methods: The source models for both Acuros XB and AAA were configured by importing the same measured beam data into Eclipse treatment planning system. Both algorithms were evaluated by comparing calculated dose with measured dose on a homogeneous water phantom for field sizes ranging from 6cm × 6cm to 40cm × 40cm. Central axis and off-axis points with different depths were chosen for the comparison. Similarly, wedge fields with wedge angles from 15 to 60 degree were used. In addition, variable field sizes for a heterogeneous phantom were used to evaluate the Acuros algorithm. Finally, both Acuros and AAA were tested on VMAT patient plans for various sites. Does distributions and calculation time were compared. Results: On average, computation time is reduced by at least 50% by Acuros XB compared with AAA on single fields and VMAT plans. When used for open 6MV photon beams on homogeneous water phantom, both Acuros XB and AAA calculated doses were within 1% of measurement. For 23 MV photon beams, the calculated doses were within 1.5% of measured doses for Acuros XB and 2% for AAA. When heterogeneous phantom was used, Acuros XB also improved on accuracy. Conclusion: Compared with AAA, Acuros XB can improve accuracy while significantly reduce computation time for VMAT plans.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-71: Commissioning and Acceptance Testing of a Commercial Monte Carlo Electron Dose Calculation Model (eMC) for TrueBeam

Purpose: To provide commissioning and acceptance test data of the Varian Eclipse electron Monte Carlo model (eMC v.11) for TrueBeam linac. We also investigated the uncertainties in beam model parameters and dose calculation results for different geometric configurations. Methods: For beam commissioning, PTW CC13 thimble chamber and IBA Blue Phantom2 were used to collect PDD and dose profiles in air. Cone factors were measured with a parallel plate chamber (PTW N23342) in solid water. GafChromic EBT3 films were used for dose calculation verifications to compare with parallel plate chamber results in the following test geometries: oblique incident, extended distance, small cutouts, elongated cutouts, irregular surface, and heterogeneous layers. Results: Four electron energies (6e, 9e, 12e, and 15e) and five cones (6×6, 10×10, 15×15, 20×20, and 25×25) with standard cutouts were calculated for different grid sizes (1, 1.5,2, and 2.5 mm) and compared with chamber measurements. The results showed calculations performed with a coarse grid size underestimated the absolute dose. The underestimation decreased as energy increased. For 6e, the underestimation (max 3.3 %) was greater than the statistical uncertainty level (3%) and was systematically observed for all cone sizes. By using a 1mm grid size, all the calculation results agreed with measurements within 5% for all test configurations. The calculations took 21s and 46s for 6e and 15e (2.5mm grid size) respectively distributed on 4 calculation servants. Conclusion: In general, commissioning the eMC dose calculation model on TrueBeam is straightforward and thedose calculation is in good agreement with measurements for all test cases. Monte Carlo dose calculation provides more accurate results which improves treatment planning quality. However, the normal acceptable grid size (2.5mm) would cause systematic underestimation in absolute dose calculation for lower energies, such as 6e. Users need to be cautious in this situation.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-97: Dependence Of Optically Stimulated Luminescent Dosimeter (OSLD) Out Of Field Response On Volumetric Modulated Arc Therapy (VMAT) Field Modulation

Purpose: To determine the out of field response of Microstar ii OSLDs as a function of field modulation and distance in VMAT plan delivery. This work has potential application in fetal dose monitoring or measurements on cardiac pacemakers Methods: VMAT plans were created in Eclipse and optimized to varying degrees of modulation. Three plans were chosen to represent low, medium and high degrees of modulation (modulation factors as defined by MU/cGy). Plans were delivered to slabs of solid water with dimensions 60cm length, 30cm width, and 10cm height. For each modulation factor, 2 OSLDs were placed at 1cm depth with out of field distances of 1, 2, 3, 5, 8 and 10cm and the plan delivered isocentrically to a depth of 5cm. This technique was repeated for a Farmer Chamber by incrementing the table by the appropriate distance. The charge readings for the Farmer Chamber were converted to dose and the ratios taken as functions of modulation factors and distances out of field Results: Examination of the results as a function of out of field distance shows a trend of increasing OSLD/Farmer Chamber ratios for all modulation factors. The slopes appear to be roughly equivalent for all modulation factors investigated. Results as a function of modulation showed a downward trend for all out of field distances, with the greatest differences seen at 5cm and 8cm Conclusion: This study demonstrates that the response of OSLD dosimeters change as a function of out of field distance and modulation. The differences seen are within the stated accuracy of the system for the out of field distances and modulations investigated. Additional investigation is warranted to see if the OSLD response changes appreciably with longer out of field distances or wider ranges of modulation.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-101: Determination and Comparison of Correction Factors Obtained for TLDs in Small Field Lung Heterogenous Phantom Using Acuros XB and EGSnrc

Purpose: To determine and compare the correction factors obtained for TLDs in 2 × 2cm{sup 2} small field in lung heterogenous phantom using Acuros XB (AXB) and EGSnrc. Methods: This study will simulate the correction factors due to the perturbation of TLD-100 chips (Harshaw/Thermoscientific, 3 × 3 × 0.9mm{sup 3}, 2.64g/cm{sup 3}) in small field lung medium for Stereotactic Body Radiation Therapy (SBRT). A physical lung phantom was simulated by a 14cm thick composite cork phantom (0.27g/cm{sup 3}, HU:-743 ± 11) sandwiched between 4cm thick Plastic Water (CIRS,Norfolk). Composite cork has been shown to be a good lung substitute material for dosimetric studies. 6MV photon beam from Varian Clinac iX (Varian Medical Systems, Palo Alto, CA) with field size 2 × 2cm{sup 2} was simulated. Depth dose profiles were obtained from the Eclipse treatment planning system Acuros XB (AXB) and independently from DOSxyznrc, EGSnrc. Correction factors was calculated by the ratio of unperturbed to perturbed dose. Since AXB has limitations in simulating actual material compositions, EGSnrc will also simulate the AXB-based material composition for comparison to the actual lung phantom. Results: TLD-100, with its finite size and relatively high density, causes significant perturbation in 2 × 2cm{sup 2} small field in a low lung density phantom. Correction factors calculated by both EGSnrc and AXB was found to be as low as 0.9. It is expected that the correction factor obtained by EGSnrc wlll be more accurate as it is able to simulate the actual phantom material compositions. AXB have a limited material library, therefore it only approximates the composition of TLD, Composite cork and Plastic water, contributing to uncertainties in TLD correction factors. Conclusion: It is expected that the correction factors obtained by EGSnrc will be more accurate. Studies will be done to investigate the correction factors for higher energies where perturbation may be more pronounced.

60 APPLIED LIFE SCIENCES↗

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