bluuebunny/crossref_metadata_2025_split huggingface dataset:
SU‐E‐T‐31: Alternative VMAT Technique Reduces Total Monitor Units for Lung SBRT
Authors: Happersett L , Mechalakos J , Kuo L , Zhang P , Rimner A 
Published: July 2014

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.
SU‐E‐T‐32: An Application of GafChromic RTQA2 Film to the Patient Specified Quality Assurance
Authors: Peng J , Hu W 
Published: July 2014

Purpose:GafChromic RTQA2 film is known as a quality assurance (QA) tool for light field and radiation field verification. This study is attempted to apply the RTQA2 film to the patient specified quality assurance.Methods:Pre‐irradiated and post‐irradiated RTQA2 films were scanned in a reflection mode using a flatbed scanner. A plan‐based dose calibration method utilized the mapping information of calculated dose image and measured film image to create a dose vs. pixel value calibration model. This model was used to calibrate the measured film image from the pixel value (gray value) image to the dose image. The dose agreement between calculated and measured dose images were analyzed using the gamma analysis. To evaluate the feasibility of this method, three clinical approved RapidArc cases (one abdomen cancer and two head‐and‐neck cancer patients) were tested. The tolerance of 3% dose difference and 3 mm distance to agreement (DTA) and gamma index ≤ 1 were set for the analysis.Results:The calibrated film dose image from measurement was successfully compared to the predicted dose image from the commercial treatment planning. The gamma analysis results showed good consistency. Gamma passing rates were 99.02%, 94.84%, and 98.33% for the three patients, respectively.Conclusion:The plan based calibration method has the feasibility for dose verification without shortages of film batch and development time variation.
SU‐E‐T‐37: An Optical Investigation Into the Polarization and Scattering Effects Underlying the Artifacts of Radiochromic Film Dosimetry with Commercial Flatbed Scanners
Authors: Schoenfeld A , Poppinga D , Harder D , Doerner K , Poppe B 
Published: July 2014

Purpose:This study aims to investigate the optical properties of radiochromic EBT3 films on exposure to polarized incident light.Methods:An optical table setup was used to investigate the properties of exposed and unexposed EBT3 films. The films were placed with their long side horizontally and illuminated with polarized incident white light. The polarization of light with the electrical vector pointing vertically is referred to as 0°, accordingly horizontal orientation corresponds to 90°. The light transmission was measured depending on the polarization angle of the incident light and the polarization of a polarizer in front of the detector. Secondly, the scattering properties of exposed and unexposed films were measured by placing a plane convex lens behind the films and a screen in its focal plane. Thereby, the distribution of the scattering angles appears as an intensity map on the screen. The distributions of scattering angles caused by EBT3 films and by neutral density filters were compared.Results:EBT3 films show a strong dependence of the light transmission on the polarization of the incident light. With both polarizers parallel, a peak transmission was found at 90° orientation of the polarizers. With the rear polarizer at right angles with the front polarizer, peak transmissions were found at front polarizer orientations 45° and 135°. The scattering appears to be anisotropic with a preference direction parallel to the long side of the film. The portion of scattered light and the half value scattering angle both increase with the dose on the film.Conclusion:EBT3 films show dose dependent changes in polarized light transmission and anisotropic light scattering. These effects impair the light absorption measurements on exposed films performed with commercial flatbed scanners and are causing the well‐known artifacts of radiochromic film dosimetry with flatbed scanners, the “orientation effect” and the “parabola effect”.
SU‐E‐T‐44: Angular Dependence of Surface Dose Enhancement Measured On Several Inhomogeneities Using Radiochromic EBT3 Films
Authors: Jansen A , Schoenfeld A , Poppinga D , Chofor N , Poppe B 
Published: July 2014

Purpose:The quantification of the relative surface dose enhancement in dependence on the angle of incidence and the atomic number Z of the surface material.Methods:Experiments were performed with slabs made of aluminum, titanium, copper, silver, dental gold and lead. The metal slabs with equal sizes of 1.0×8.0×8.8mm3 were embedded in an Octavius 4D phantom (PTW Freiburg, Germany). Radiochromic EBT3 films were used to measure the surface dose for angles of incidence ranging from 0° to 90°. The setup with the metals slabs at the isocenter was irradiated with acceleration voltages of 6MV and 10MV. Water reference measurements were taken under equal conditions.Results:The surface dose enhancement is highest for angles of incidence below 30° and drops significantly for higher. The surface dose enhancement produced by lead and dental gold at 6MV showed a peak of 65%. At 90°, the surface dose enhancement dropped to 15% for both materials. The surface dose enhancements for silver, copper, titanium and aluminum were 45%, 32%, 22% and 12% at 0°, respectively. At an angle of incidence of 80°, the values dropped to 22%, 18%, 12% und 6%. The values for 10MV were very similar. Lead and dental gold showed peaks of 65% und 60%. Their values dropped to 18% at an angle of 90°. The surface dose enhancements for silver, copper, titanium and aluminum were 45%, 30%, 20% and 8% at 0°. At 80° the values dropped to 30%, 20%, 12% and 5%. A dependence of the magnitude of the surface dose enhancement on the atomic number of the surface material can be seen, which is in consistence with literature.Conclusion:The results show that the surface dose enhancements near implant materials with high Z‐values should be taken into consideration in radio therapy, even when the angle of incidence is flat.
SU‐E‐T‐49: Automatic Beam Angle Determination for Lung IMRT Planning Using a Beam Configuration Atlas
Authors: Yuan L , Ge Y , Yin F , Li Y , Sheng Y , Wu Q J. 
Published: July 2014

Purpose:To present a technique to automatically determine beam angle configurations for lung IMRT planning based on the patient‐specific anatomy and tumor geometry.Methods:The relationship between individual patient anatomy and proper beam configurations was learned from high quality clinical plans in three steps. First, a beam configuration atlas was obtained by classifying 60 lung IMRT plans into 6 beam configuration clusters based on a dissimilarity measure defined between different beam configurations. A beam configuration template was extracted from each cluster to form an atlas. Second, a beam efficiency index map (EI map) was constructed to characterize the geometry of the tumor relative to the lungs, the body and other OARs along each candidate beam direction. Finally, the EI maps of the clinical cases and the cluster assignments of their beam configurations were paired to train a Bayesian classification model. This technique was validated by leave‐one‐out cross validation with 16 cases randomly selected from the original dataset. An IMRT plan (autobeam plan) for each test case was generated using the beam configuration template according to the cluster assignment given by the model and was compared with the corresponding clinical plan.Results:The dosimetric parameters (mean±S.D. in percentage of prescription dose) in the auto‐beam plans and in the clinical plans, respectively, and the p‐values by a paired ttest (in parenthesis) are: lung Dmean: 16.3±9.3, 18.6±7.4 (0.48), esophagus Dmean: 28.4±18, 30.7±19.3 (0.02), Heart Dmean: 21.5±17.5,21.1±17.2 (0.76), Spinal Cord D2%: 48±23, 51.2±21.8 (0.01), PTV dose homogeneity (D2%–D99%): 22±27.4, 20.4±12.8 (0.10). The dose reductions by the autobeam plans in esophagus Dmean and cord D02 are statistically significant but the differences (<4%) may not be clinically significant. The other dosimetric parameters are not statistically different.Conclusion:Plans generated by the automatic beam angle determination method can achieve dosimetric quality equivalent to that of clinical plans.Partially supported by NIH/NCI under grant #R21CA161389 and a master research grant by Varian Medical System.
SU‐E‐T‐235: Monte Carlo Analysis of the Dose Enhancement in the Scalp of Patients Due to Titanium Plate Backscatter During Post‐Operative Radiotherapy
Authors: Hardin M , Wolf E , Elson H , Warnick R , Lamba M 
Published: July 2014

Purpose:To quantify the clinically observed dose enhancement adjacent to cranial titanium fixation plates during post‐operative radiotherapy.Methods:Irradiation of a titanium burr hole cover was simulated using Monte Carlo code MCNPX for a 6 MV photon spectrum to investigate backscatter dose enhancement due to increased production of secondary electrons within the titanium plate. The simulated plate was placed 3 mm deep in a water phantom, and dose deposition was tallied for 0.2 mm thick cells adjacent to the entrance and exit sides of the plate. These results were compared to a simulation excluding the presence of the titanium to calculate relative dose enhancement on the entrance and exit sides of the plate. To verify simulated results, two titanium burr hole covers (Synthes, Inc. and Biomet, Inc.) were irradiated with 6 MV photons in a solid water phantom containing GafChromic MD‐55 film. The phantom was irradiated on a Varian 21EX linear accelerator at multiple gantry angles (0–180 degrees) to analyze the angular dependence of the backscattered radiation. Relative dose enhancement was quantified using computer software.Results:Monte Carlo simulations indicate a relative difference of 26.4% and 7.1% on the entrance and exit sides of the plate respectively. Film dosimetry results using a similar geometry indicate a relative difference of 13% and ‐10% on the entrance and exit sides of the plate respectively. Relative dose enhancement on the entrance side of the plate decreased with increasing gantry angle from 0 to 180 degrees.Conclusion:Film and simulation results demonstrate an increase in dose to structures immediately adjacent to cranial titanium fixation plates. Increased beam obliquity has shown to alleviate dose enhancement to some extent. These results are consistent with clinically observed effects.
SU‐E‐T‐237: Monte Carlo Dosimetric Characterization of the Mobetron Mobile Linac
Authors: Garcia F , Granero D , Vijande J , Perez‐Calatayud J , Ballester F 
Published: July 2014

Purpose:The aim of this work is to characterize dosimetrically a clinical intraoperative electron beam accelerator, Mobetron (IntraOp Medical, Inc.) in clinical use in our Hospital. Once this first step is completed our purpose is to evaluate shielding requirements for such a device by preparing adequate phase space files.Methods:It is known that electron beam simulation parameters required for state‐of‐the‐art Monte Carlo codes to obtain a good match with measured data, like the mean energy or the FWHM, may not be code‐independent due to the different set of process simulated and formalisms involved. Then, to cross‐check our results against any issue in the simulation we have compared experimental data (PDD and profiles for electrons in the range 4 to 12 MeV) with simulations performed independently using both Penelope2011 and Geant4 codes. To do so, the geometry and materials of the head of the accelerator have been fully characterized following information provided by the manufacturer.Results:Both simulations agree with experimental data within experimental uncertainties (±1 mm displacement), although small variations (less than 10%) in the mean energy and FWHM are required to match measured values depending on the code used.Conclusion:Independent Monte Carlo simulations were used to obtain an excellent match to measured electron dose distributions. This opens the road to use such data for evaluating shielding requirements which is the main objective of this project.
SU‐E‐T‐238: Monte Carlo Estimation of Cerenkov Dose for Photo‐Dynamic Radiotherapy
Authors: Chibani O , Eldib A , Price R , Mora G , Ma C 
Published: July 2014

Purpose:Estimation of Cerenkov dose from high‐energy megavoltage photon and electron beams in tissue and its impact on the radiosensitization using Protoporphyrine IX (PpIX) for tumor targeting enhancement in radiotherapy.Methods:The GEPTS Monte Carlo code is used to generate dose distributions from 18MV Varian photon beam and generic high‐energy (45‐MV) photon and (45‐MeV) electron beams in a voxel‐based tissueequivalent phantom. In addition to calculating the ionization dose, the code scores Cerenkov energy released in the wavelength range 375–425 nm corresponding to the pick of the PpIX absorption spectrum (Fig. 1) using the Frank‐Tamm formula.Results:The simulations shows that the produced Cerenkov dose suitable for activating PpIX is 4000 to 5500 times lower than the overall radiation dose for all considered beams (18MV, 45 MV and 45 MeV). These results were contradictory to the recent experimental studies by Axelsson et al. (Med. Phys. 38 (2011) p 4127), where Cerenkov dose was reported to be only two orders of magnitude lower than the radiation dose. Note that our simulation results can be corroborated by a simple model where the Frank and Tamm formula is applied for electrons with 2 MeV/cm stopping power generating Cerenkov photons in the 375–425 nm range and assuming these photons have less than 1mm penetration in tissue.Conclusion:The Cerenkov dose generated by high‐energy photon and electron beams may produce minimal clinical effect in comparison with the photon fluence (or dose) commonly used for photo‐dynamic therapy. At the present time, it is unclear whether Cerenkov radiation is a significant contributor to the recently observed tumor regression for patients receiving radiotherapy and PpIX versus patients receiving radiotherapy only. The ongoing study will include animal experimentation and investigation of dose rate effects on PpIX response.
SU‐E‐T‐239: Monte Carlo Modelling of SMC Proton Nozzles Using TOPAS
Authors: Chung K , Kim J , Shin J , Han Y , Ju S , Hong C , Kim D , Kim H , Shin E , Ahn S , Chung S , Choi D 
Published: July 2014

Purpose:To expedite and cross‐check the commissioning of the proton therapy nozzles at Samsung Medical Center using TOPAS.Methods:We have two different types of nozzles at Samsung Medical Center (SMC), a multi‐purpose nozzle and a pencil beam scanning dedicated nozzle. Both nozzles have been modelled in Monte Carlo simulation by using TOPAS based on the vendor‐provided geometry. The multi‐purpose nozzle is mainly composed of wobbling magnets, scatterers, ridge filters and multi‐leaf collimators (MLC). Including patient specific apertures and compensators, all the parts of the nozzle have been implemented in TOPAS following the geometry information from the vendor. The dedicated scanning nozzle has a simpler structure than the multi‐purpose nozzle with a vacuum pipe at the down stream of the nozzle.A simple water tank volume has been implemented to measure the dosimetric characteristics of proton beams from the nozzles.Results:We have simulated the two proton beam nozzles at SMC. Two different ridge filters have been tested for the spread‐out Bragg peak (SOBP) generation of wobbling mode in the multi‐purpose nozzle. The spot sizes and lateral penumbra in two nozzles have been simulated and analyzed using a double Gaussian model. Using parallel geometry, both the depth dose curve and dose profile have been measured simultaneously.Conclusion:The proton therapy nozzles at SMC have been successfully modelled in Monte Carlo simulation using TOPAS. We will perform a validation with measured base data and then use the MC simulation to interpolate/extrapolate the measured data. We believe it will expedite the commissioning process of the proton therapy nozzles at SMC.
SU‐E‐T‐240: Monte Carlo Modelling of SMC Proton Nozzles Using TOPAS
Authors: Mora G , Chibani O , Li J , Eldib A , Ma C 
Published: July 2014

Purpose:To simulate the 60Co beam from a novel Gamma—Tomo SBRT system and to calculate the output factors and dose rates for different source configurations and collimator sizes.Methods:The BEAM‐Monte Carlo code is used to realistically model the system geometry, including 32 source capsules, source housing, primary collimator and 4 different changeable collimators. The sources (6.22 mm of diameter) are located in 2 rows with different angles and distances to the longitudinal axis and the beams can be collimated to obtain four different circular field sizes at the isocenter (35 mm, 16 mm, 7mm, and 3.5mm). Since the dose distributions will be determined in a spherical polystyrene phantom of 8cm radius centered at the isocenter, a new geometry module is designed for the dose calculation.Results:: The caracteristics of the particle spectra emerging from each source are determined and the influence of the source position on the spectra reaching the isocenter is found to be minimal (<2%). We found that the scattered photons represent about 15% of the total number of photons reaching the scoring plane immediately (1 mm) below the source and primary collimator. The radial photon fluence does not vary significantly inside the collimator opening and decreases quickly beyond the 3cm radius (projected at the isocenter plane).Conclusion:Our preliminary results indicate that the position of the source and primary collimator assembly does not affect the energy spectra and fluence distributions reaching the isocenter significantly. This allows the use of a well‐represented source model for all the sources to perform the dose calculations for all the changeable collimators for this novel Gamma‐Tomo SBRT system.
SU‐E‐T‐242: Monte Carlo Simulations Used to Test the Perturbation of a Reference Ion Chamber Prototype Used for Small Fields
Authors: Vazquez Quino L , Calvo O , Huerta C , DeWeese M 
Published: July 2014

Purpose:To study the perturbation due to the use of a novel Reference Ion Chamber designed to measure small field dosimetry (KermaX Plus C by IBA).Methods:Using the Phase‐space files for TrueBeam photon beams available by Varian in IAEA‐compliant format for 6 and 15 MV. Monte Carlo simulations were performed using BEAMnrc and DOSXYZnrc to investigate the perturbation introduced by a reference chamber into the PDDs and profiles measured in water tank. Field sizes ranging from 1×1, 2×2,3×3, 5×5 cm2 were simulated for both energies with and without a 0.5 mm foil of Aluminum which is equivalent to the attenuation equivalent of the reference chamber specifications in a water phantom of 30×30×30 cm3 and a pixel resolution of 2 mm. The PDDs, profiles, and gamma analysis of the simulations were performed as well as a energy spectrum analysis of the phase‐space files generated during the simulation.Results:Examination of the energy spectrum analysis performed shown a very small increment of the energy spectrum at the build‐up region but no difference is appreciated after dmax. The PDD, profiles and gamma analysis had shown a very good agreement among the simulations with and without the Al foil, with a gamma analysis with a criterion of 2% and 2mm resulting in 99.9% of the points passing this criterion.Conclusion:This work indicates the potential benefits of using the KermaX Plus C as reference chamber in the measurement of PDD and Profiles for small fields since the perturbation due to in the presence of the chamber the perturbation is minimal and the chamber can be considered transparent to the photon beam.
SU‐E‐T‐247: Multi‐Leaf Collimator Model Adjustments Improve Small Field Dosimetry in VMAT Plans
Authors: Young L , Yang F 
Published: July 2014

Purpose:The Elekta beam modulator linac employs a 4‐mm micro multileaf collimator (MLC) backed by a fixed jaw. Out‐of‐field dose discrepancies between treatment planning system (TPS) calculations and output water phantom measurements are caused by the 1‐mm leaf gap required for all moving MLCs in a VMAT arc. In this study, MLC parameters are optimized to improve TPS out‐of‐field dose approximations.Methods:Static 2.4 cm square fields were created with a 1‐mm leaf gap for MLCs that would normally park behind the jaw. Doses in the open field and leaf gap were measured with an A16 micro ion chamber and EDR2 film for comparison with corresponding point doses in the Pinnacle TPS. The MLC offset table and tip radius were adjusted until TPS point doses agreed with photon measurements. Improvements to the beam models were tested using static arcs consisting of square fields ranging from 1.6 to 14.0 cm, with 45° collimator rotation, and 1‐mm leaf gap to replicate VMAT conditions. Gamma values for the 3‐mm distance, 3% dose difference criteria were evaluated using standard QA procedures with a cylindrical detector array.Results:The best agreement in point doses within the leaf gap and open field was achieved by offsetting the default rounded leaf end table by 0.1 cm and adjusting the leaf tip radius to 13 cm. Improvements in TPS models for 6 and 10 MV photon beams were more significant for smaller field sizes 3.6 cm or less where the initial gamma factors progressively increased as field size decreased, i.e. for a 1.6cm field size, the Gamma increased from 56.1% to 98.8%.Conclusion:The MLC optimization techniques developed will achieve greater dosimetric accuracy in small field VMAT treatment plans for fixed jaw linear accelerators. Accurate predictions of dose to organs at risk may reduce adverse effects of radiotherapy.