bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐304: Proposed Design and Shielding Requirements of a Mini Linear Accelerator Treatment System
Authors: Eslick E , Fitzsimmons K , Keall P
Published: November 2011

Purpose: The global cost burden of radiotherapy is $5 billion annually, with vault design, construction and linacs being major components. To maintain state‐of‐the‐art technologies and simultaneously reduce costs, we propose a novel mini linac system utilizing a vertically mounted fixed beam 6MV linac, a rotating couch, and IGRT. The aim of this study was to design and quantify the building size, concrete requirements and cost of the proposed mini linac system compared with conventional systems (6MV gantry mounted linacs in average vault size) and compact linacs (6MV gantry mounted in small vault). Methods: The inner vault footprint was calculated based on the equipment configuration in the room and the necessary treatment and clearance distances. Utilizing shielding equations, we determined the necessary thicknesses of barriers in the mini system vault, and the amount of concrete required. In determining cost estimates of linac systems/vaults, values were obtained from the literature and shielding specialists. We compared the vault footprint and the total costs of linac systems/vaults. Results: The proposed mini system vault footprint is 20m2 compared to 85m2 and 110m2 for compact systems and conventional systems respectively. The mini system only requires secondary shielding. Parametric studies revealed that leakage radiation was dominant, thus allowing the use of an ultra low leakage ratio mini linac. We determined that the amount of concrete required in the shielding of the mini system is 50m3 compared with 150m3 and 216m3 for the compact and conventional vaults respectively. The total costs of the proposed mini system and vault was estimated at US$1,200,000, which was lower than the compact and conventional linacs/vaults, US$1,500,000 and US$3,000,000 respectively. Conclusions: We are able to substantially reduce the cost of high quality radiotherapy using a vertically mounted 6MV fixed beam mini linac housed in small vault.
SU‐E‐T‐312: Development of a Rat Model of Radiation‐Induced Heart Disease Using SACRTD
Authors: Sharma S , Sridharan V , Boerma M , Corry P , Moros E
Published: November 2011

Purpose: In radiotherapy treatment of thoracic and chest wall tumors, the heart may be included (partly/fully) in the radiation field, and as a result patients may develop radiation‐induced heart disease (RIHD) several years after irradiation. There is no method to prevent or reverse RIHD, and biological mechanisms are largely unknown. We developed a model of conformal local heart irradiation in rats and present the first results. Method and materials:A small animal conformal radiation therapy device (SACRTD) was used to irradiate experimental animals. Adult male Sprague‐Dawley rats were exposed to localized heart irradiation with a total dose of 18 Gy. For this purpose, rats were anesthetized with isoflurane and placed in a custom made vertical rat holder. The heart was exposed to 3 fields of 6 Gy each, one AP field and 2 lateral fields. For each field, the heart was visualized with the digital X‐ray imager and placed in the isocenter of the 1.8 cm diameter beam. Hearts and lungs were isolated at time points from 2 hours to 4 days after irradiation, fixed in formalin and processed. Immunohistochemistry was performed to show γH2Ax, a marker for DNA damage caused by oxidative stress, and nitrotyrosine, a marker for protein nitration caused by nitrosative stress.Results: Both γH2Ax staining and nitrotyrosine staining were observed throughout the irradiated hearts, indicating that the whole heart was exposed to radiation. γH2Ax was most prominent at 2 hours after irradiation, as DNA repair is known to start immediately after exposure. Increased nitrotyrosine staining was observed mainly in cardiomyocytes, at all time points after irradiation. Minimal γH2Ax and nitrotyrosine were observed in the lung samples Conclusions: The results showed that the SACRTD is an excellent tool for in‐vivo radiobiological studies. Studies on functional and structural changes in the rat heart after localized irradiation are ongoing.
SU‐E‐T‐314: Challenges in Radiochromic Film Dosimetry Applied to Microbeam Radiation Therapy (MRT)
Authors: Anderson D , Warkentin B , Siegbahn E A , Fallone B G
Published: November 2011

Purpose: Uncertainties in radiochromic film dosimetry applied to microbeam radiation therapy (MRT) dosimetry at the Canadian Light Source (CLS) are investigated. A dose‐dependent change in film response as a function of microbeam width is characterized and its impact on peak‐to‐valley dose ratios (PVDRs) evaluated. Variation across the MRT collimator and inhomogeneity in film construction are also studied. Methods: Gafchromic® EBT2 film was used to measure doses from a filtered synchrotron source on the surface of solid water for microbeams ranging from 10 micrometers to 1 mm and doses between 0.4 – 74 Gy. The microbeams were generated using a variable slit or a MRT collimator (50 micrometer slits, 400 micrometer spacing). The film was scanned with an Epson Perfection V700 Photo digital scanner (4800 dpi). Corresponding Monte Carlo (PENELOPE) simulations were performed.Results: A dose‐dependent reduction in film response with decreasing microbeam width was observed. The measured ratio of dose for a 50 micrometer slit beam relative to a 1 mm beam ranged from 0.15 and 0.56, decreasing as the dose is increased. A corresponding simulation predicts a ratio of 0.91. Depending on dose, measured PVDRs ranged from 7 to 64. Variability in measured doses between different slits of the MRT collimator is also problematic, with deviations from the average of up to 26% in the microbeam path, and over 70% between two microbeams. Inherent inhomogeneity in optical density of the unexposed EBT2 film may also result in uncertainties of approximately 0.15 Gy. Conclusions: Although the high resolution of radiochromic film makes it a potentially attractive two‐dimensional dosimeter for MRT, large uncertainties arise from variations in dose‐response as a function of microbeam size, and spatial inhomogeneity of the response. These problems must be addressed before film can be considered for accurate measurement of absolute doses, and even relative metrics.
SU‐E‐T‐315: In Silico, in Vitro, and in Vivo Quantification of Tungsten and Iodine in Dose Enhanced RT (DERT)
Authors: Nelson G , Bazalova M , Chang K , Ackerman N , Colomer M Vilalta , Graves E
Published: November 2011

Purpose: To investigate tungsten nanoparticles (WNP) and conventional iodine contrast as potential vehicles for dose enhancers (DE) for dose enhanced radiation therapy (DERT). Methods: We systematically evaluated DERT in silico, in vitro, and in vivo by quantifying dose enhancement, cell kill, and tumor control. Because of variations in the K‐edge and toxicity of these different nanoparticles we expect to see differences in their viability and effective dose boosting. We used Na[M3‐W3(O2)(OAc)a] as our WNP, and iohexol as our iodine agent. Monte Carlo (MC) simulations using the EGSncr codes were performed to quantify the macroscopic dose enhancement as a function of DE concentration from both DEs. The DEs were used in 7, 15, and 30 mg/ml concentrations in vitro with AA8 CHO cell lines and were irradiated with varying doses using a 150 kV beam to establish cell survival curves. Balb‐c mice were injected with the same cell lines used in vitro to grow as subcutaneous tumors. The tumors were directly injected with the DE formulations and a series of fluoroscopic x‐ray images were acquired to determine the biological half‐life of the DEs in the tumor. Mouse tumors with and without DE were irradiated with a 150 kVp beam and tumor regrowth delay was measured. Results: MC simulations showed macroscopic dose enhancement varying from 1.8 to 3.8 for iodine and from 1.9 to 4.0 for tungsten when irradiated with a 150 kV beam. The tungsten in vitro study indicated dose enhancement of a factor of 2.2 for a concentration of 30 mg/mL. The biological half‐life of tungsten in the tumor injected with 200 mg/mL was shown to be 20 minutes. Conclusion: Preliminary in silico, in vitro, and in vivo studies of tungsten nanoparticles and iodine contrast showed the potential for their use in dose enhanced radiation therapy.
SU‐E‐T‐318: Using Monte Carlo in the Design of Small Animal Irradiator Collimators
Authors: Han E , Sharma S , Yan Y , Moros E , Corry P
Published: November 2011

Purpose: For preclinical research using small animals, a dedicated image‐guided irradiation system has been developed with CBCT imaging and conformal radiation delivery capabilities. A special collimator for a rat model of radiation induced heart disease is desirable. This study aims to design a new primary and secondary collimator system using a Monte Carlo based code. Methods: At an SSD of 32.5 cm, the required maximum field size (FWHM) at 1.0 cm depth in water was 2.0 cm, which is comparable to the size of a normal ratˈs heart. For simulations, we installed the BEAMnrc (GUI version, revision 2.0) and Dosxyznrc (GUI version, revision 1.1) Monte Carlo based codes on a 64 bit Intel quad‐core processor 8×2.0 GHz running the Linux operating system. The simulation model included the X‐ray tube (225kVp), with a Be inherent filter, an external Cu filter (0.5 mm thick), brass primary and secondary collimators and a 10×10×10 cm3 solid water phantom. 1) We began with a simple geometrical design, 2) we expanded the aperture of the primary collimator for the desired FWHM of the beam, and 3) we trimmed the beam by reducing the aperture size of the bottom part of the secondary collimator to eliminate low energy photons and sharpen the penumbra. Results: Monte Carlo simulations successfully accomplished a beam 2.0 cm (FWHM) in diameter at 1.0 cm depth with a sharp penumbra. A new collimator will be constructed based on the final simulated design. We will then verify the Monte Carlo generated depth dose distribution and dose profiles. Conclusions: Monte Carlo based modeling is an effective tool to create new designs for small animal conformal radiotherapy systems. The Beamnrc and Dosxyznrc codes will also be utilized further as the treatment planning engine for the localized heart irradiation project.
SU‐E‐T‐323: Investigation of the Effect of the Gold‐Tissue Interface on the Radiation Dose Enhancement Due to Gold Nanoparticles
Authors: Paudel N , Shvydka D , Parsai E
Published: November 2011

Purpose: Injection of gold nanoparticles into a tumor site has been found promising in achieving selective radiation dose enhancement within the tumor due to goldˈs high atomic number Z=79, and favorable biological compatibility. So far the amplitude of the dose enhancement has been modeled in an approximation of average distribution of separate gold atoms rather than nanoparticles, thus neglecting the effect of the gold‐tissue interface. Evaluation of the dose deposition at the interface allowed us to estimate the dose enhancement more adequately. Method and Materials: Monte Carlo N‐Particle radiation transport code version 5 (MCNP5) was used to study the dose enhancement in a 1‐dimensional approximation, with a layer of gold placed within tumor. Dimension of gold along the beam direction was set to 100 nm, while in the transverse directions to 10×10 cm2. Mono‐energetic photon sources, from 20 keV to 4 MeV, were directed onto the tumor, and dose enhancements relative to the uniform all‐tumor case were obtained in the vicinity of the gold layer. Results: The dose deposited in the tumor in the vicinity of the gold layer varies with the energy of the incoming photon beam. The highest enhancement, up to several hundred percents, was obtained for the lowest energy; with much more modest enhancement of 10 to 20% at MeV energies. The region of the dose enhancement fall‐off around the particle extends to ∼3microns, therefore for realistic nanoparticle concentrations ∼1% dose enhancement regions will overlap. Conclusions: Our approach of evaluating the gold‐tissue interface around one particle allowed for a simple way of obtaining dose distributions and more adequate values of dose enhancement factors. Use of gold particles can be justified even for MeV photon therapies where only modest, 10 to 20 % dose enhancement ratios are achieved, if a tumor site is close to a critical structure.
SU‐E‐T‐325: Development of a Radioscandium Immunoconjugate for Radioimunotherapy
Authors: Moghaddam L , Jalilian A
Published: November 2011

Purpose: Developing monoclonal antibodies with beta‐emitters has led to the introduction of important agents in radioimmunotherapy. In this work, Sc‐46 chloride was obtained by thermal neutron flux (4 × 1013 n.cm‐2.s‐1) of natural metallic scandium sample followed by dissolution in acidic media (radiochemical purity ITLC, >99%) as a substitute for 47Sc in radiolabeling studies. Methods: 46Sc was produced with a specific activity of approximately 7.0–7.5 mCi/mg and radionuclidic purity of >98% by irradiation. NHS DOTA was freshly prepared and kept under a blanket of dry N2. Rituximab (MabThera) was used without further purification. Whatman No. 1 paper was obtained from Whatman (Maidstone, UK) for instant thin layer chromatography (ITLC). Radio‐chromatography was performed by using a bioscan AR‐2000 radio TLC scanner instrument (Bioscan). A high purity germanium (HPGe) detector coupled with a Canberra™ (model GC1020‐7500SL) multichannel analyzer and a dose calibrator ISOMED 1010 were used for counting distributed activity in rat organs. Results: The liver is the major target organ due to the presence of metaloproteins, apart from the liver, the spleen is the second major accumuation target.Thus, transferrin is principally responsible for scandium metabolism in vivo, while binding with ferritin is negligible []. Slight differences in the reported biodistribution data and the present work are possibly because of using rats instead of mice. Conclusions: As an immunoconjugate model, rituximab was conjugated with DOTA chelating agent, followed by 46Sc‐radiolabeling. The biodistributions of radiopharmaceutically acceptable 46ScCl3 and 46Sc‐DOTA‐rituximab formulations were checked in wild‐type rats up to 72 hours post injection. The biodistribution of 46ScCl3 among the tissues were in agreement with the former reports, while 46Sc‐DOTA‐rituximab accumulated mostly in the liver and spleen as shown in many other radiolabeled anti‐CD20 immunoconjugates. The development of other 47Sc‐labeled‐monoclonal antibodies for ultimate radioimmunotherapy based on this study using long‐half life, 46Sc radionuclide is possible.
SU‐E‐T‐326: Investigating the Use of Tantalum Mesh Bolus in Electron Beams
Authors: St. Aubin J , Villarreal‐Barajas J , Newcomb C
Published: November 2011

Purpose: In order to achieve a higher surface dose for electron beams of less than 15 MeV, tantalum wire mesh bolus has been investigated in the past. Despite an increase in surface dose as seen by a depth dose (DD) curve, these investigations did not quantify variation in surface dose over the radiation field. Thus quantifications of surface dose variations are given along with an investigation into the effect of randomized mesh placements over several treatment fractions. Method and Materials:A tantalum wire mesh of 0.02 inch (0.51 mm) thickness and a mesh spacing of eight meshes per inch was used as bolus for a 6 MeV electron beam. DD curves were measured using a Scaditronix EFD3G electron diode in a water tank and profiles were measured with EBT2™ Gafchromic films in Solid Water™. The EBT2™ films were scanned using an Epson Expressions 10000XL flatbed scanner correcting for scanner non‐uniformities. Results: Despite a very flat surface dose as seen by one DD measurement, another DD measurement taken with the electron diode at a lateral offset of 2mm showed a 17% increase in surface dose. In general, high spatial frequency variations in the surface dose of nearly 70% were seen in the EBT2™ film due to the scattering and absorption of electrons in the tantalum mesh bolus. However, through the randomized placement of the tantalum mesh bolus on the surface over the course of ten fractions, the variations in surface dose were reduced to around 34%.Conclusion: Tantalum wire mesh bolus has been shown to increase the surface dose of 6 MeV electrons, but with significant variations over the full radiation field. By randomly placing the mesh bolus on the surface, the large surface dose variations can be reduced to around 34%.
SU‐E‐T‐328: Evaluation of Radiation Dosimetry of Bolus Electron Conformal Therapy (ECT)
Authors: Toramatsu C , Furuya T , Sugimoto S , Kitou S , Kurokawa C , Ozawa S , Karasawa K , Sasai K
Published: November 2011

Purpose: To evaluate the radiation dosimetry of bolus electron conformal therapy (ECT) using new commercially available bolus fabrication software.Methods: A customized electron bolus was designed using 3D planning system (TPS) Eclipse Ver.8.6.15 (Varian) and p.d ECT (.decimal) software. Computed tomography (CT) images of tough water was obtained and the planning target volume (PTV) was assumed. The proximal surface of the bolus was designed to conform to the 90% isodose line to the distal surface of the PTV using p.d ECT. Dose was calculated with a pencil beam algorithm for review with TPS. Based on structure set from TPS, bolus was fabricated using a computer‐controlled milling machine. To evaluate the dose distribution, CT images with the bolus on the tough water were acquired and final dose distribution was computed. Gafchromic EBT 2 film (Lot A09171002, ISP) were sandwiched between tough water at 1, 2 and 3 cm depth from the bottom of bolus and irradiated at same geometry with planning. Film was scanned by EPSON GTX970 scanner and that 2D dose distribution is compared with calculation. Results: 2D dose distribution showed qualitative agreement between calculation and measurement, absolute dose value is corresponded within 3% in the region of PTV. The pass rates of the gamma test were greater than 90% at 3 cm depth Conclusions: In this study the dose distribution for bolus ECT was measured by EBT 2 film. The 3D electron bolus which is fabricated by p.d ECT software reproduce the accurate dose distribution that is calculated with TPS.
SU‐E‐T‐331: A Geant4 Simulation Study for the Yields of Positron‐Emitting Nuclei (10C, 11C, and 15O) Induced by Protons and Carbon Ions
Authors: Lau A , Ahmad S
Published: November 2011

Purpose: To investigate the yields and the yield‐depth distributions of positron‐emitting nuclei 10C, 11C, and 15O induced by protons and carbon ions in tissue equivalent material (PMMA) through detection of annihilation gamma‐rays using Geant4 Monte Carlo Toolkit for dose verification. Methods: An application utilizing various physics packages (low and standard electromagnetic, parameterized and Binary Cascade inelastic and parameterized elastic) was constructed with the Geant4 Monte Carlo Toolkit. A phantom (9 cm × 9 cm × 30 cm) consisting of PMMA (C5H8O2, density 1.18 g/cm3) was irradiated with 140, 175 MeV protons and 259.5 A, 306 A, 343.46 A MeV carbon‐ions. The beam parameters mimic a pencil like beam with position resolution of 10 mm FWHM and a Gaussian energy spread 0f 0.2% FWHM. In each simulation the energy deposited was recorded for every 0.1 mm increment of depth in the phantom. The resulting positron‐emitting nuclei and their yield were recorded at the production point and the point of decay in increments of 1 mm. Results: The overall percentage yields and the yield‐depth distributions of positron emitting nuclei per incident particle for both protons and carbon ions of multiple energies were obtained. Conclusions: Our yields of positron emitting nuclei are in excellent agreement with MCHIT simulation and existing experimental data. However, we found in our simulation overestimation of the percentage yields of 10C with incident carbon ions when compared to existing experimental data. The application built in this study thus can predict the amount of positron emitting nuclei as well as their yield‐depth distributions in hadron induced radiation therapy.
SU‐E‐T‐333: Validation of Align RT System for Breast Radiation Therapy with Deep Inspiration Breath Hold (DIBH) Technique
Authors: Lyatskaya Y , Cormack R , Bellon J
Published: November 2011

Purpose: To characterize performance and accuracy of 3‐camera Align RT system (London, UK) in application to DIBH technique. Methods: Align RT system consisting of 3 cameras was installed in a linac room and used for the current study. Ten breast cancer patients were enrolled in an IRB‐approved prospective protocol to evaluate the accuracy of Align RT for determining the position of the patient with respect to the treatment isocenter. 3D shifts as detected by Align RT were recorded weekly, on the same day conventional port films were also obtained. Shifts based on port films were compared with those determined by Align RT. The accuracy of Align RT system was determined for different regions of interest (ROI), including the entire chest surface, the ipsilateral breast and the sternum. Results: 25 separate recordings were available for analysis. For the first 3 patients, Align RT images were not always available due to failure of synchronization of images from multiple cameras; this was corrected by a subsequent software upgrade. The average (SD) differences in shifts from Align RT and port‐films were 1.5mm (4mm), 2mm (5mm) and 0.1mm (3mm) in lateral, longitudinal and vertical directions, with maximum deviations of 10, 12 and 6 mm respectively. There was no dependence of the accuracy of vertical shift definition on ROI. Conclusions: Despite initial difficulty with calibration, Align RT proved accurate and reproducible. When compared with conventional port‐films, Align RT was accurate within standard deviation of 3mm in the vertical plane, which may be the most relevant direction for DIBH. These results are independent of ROI. This accuracy, combined with the ease of use and lack of radiation exposure, makes Align RT a promising tool for DIBH.
SU‐E‐T‐336: Quantification of the 3D Dose Distributions Around Small Metal Objects Using Polymer Gel Dosimeter
Authors: Warmington L , Watanabe Y
Published: November 2011

Purpose: Patients are increasingly undergoing radiotherapy procedures, in which small metals are implanted in the body for target localization in IGRT or dose enhancement. The purpose of this study is to investigate the feasibility of 3D dose measurements in the vicinity of a thin metallic objects using a polymer‐gel dosimeter. Methods: A MAGIC gel dosimeter was manufactured in‐house. The gel was poured into 20‐ml glass vials, then, it was cooled in a refrigerator for about 30 mins until the gel had a high viscosity. Tweezers were used to carefully insert lead, gold, or aluminum foils (1cm × 1cm × ∼0.5mm) into the gel with minimal disturbance to the surrounding area. Finally, the gels were returned to the refrigerator and allowed to solidify for at least 24 hours. The samples were irradiated with 18MV and 250kVp photon beams such that the foils were perpendicular to the beam direction. T2‐weighted images were acquired after 24 hours with a 3T‐MRI scanner using a turbo spin‐echo sequence. Imaging parameters were as follows: field‐of‐view = 180mm × 180mm, pixel‐size = 0.35mm × 0.35mm, and slice‐thickness = 5mm. Dose distributions were calculated by using a two‐point method from the MR images taken at two echo times (20 and 101 ms). Results: There was no signal from the metal foil allowing ease of identification. All 18MV samples and the 250kVp aluminum sample did not show any appreciable change in the dose distribution. The 250kVp samples for lead and gold, however, had a significant decrease in absorbed dose downstream of the foil. The dose profiles along the beam direction revealed dose reductions of 18% and 16% for lead and gold, respectively. Conclusion: We demonstrated that polymer‐gel dosimetry can be used to measure the fine dosimetric structures around a small metallic object although its accuracy still must be assessed.
SU‐E‐T‐337: Monte‐Carlo Study of Dose Perturbation Effects in External Beam Therapy of the Prostate Caused by the Presence of Post‐Implant Brachytherapy Seeds
Authors: Bassalow R , Sabbas A , Delamerced M , Trichter S , Kulidzhanov F , Nedialkova L , Nori D , Chao K , Chang J
Published: November 2011

Purpose: Previously it has been reported that brachytherapy dose distribution can be affected by seed self‐attenuation. We investigate the magnitude of dose perturbation in external beam radiation therapy caused by the presence of remaining brachytherapy seeds in the prostate. Methods: A phantom from a patient CT data was generated. A custom Python code was developed to modify the phantom by replacing selected voxels with the Cs‐131 seed material and density. 3D dose distributions and mean dose in the volume containing seeds for different seed arrangements were calculated using Monte‐Carlo EGSnrc dosxyznrc code and compared to the dose distributions without the seeds for a typical 5‐field treatment using 6MV and 18MV beams. Seeds were arranged symmetrically around the isocenter in a rectangular grid pattern with varying number of seeds along the x‐, y‐ and z‐ axes. In calculating mean dose and dose difference maps, seed voxels were excluded from calculation. Results: Difference in mean dose in the volume containing seeds and the same volume without seeds, was calculated for 6 and 18 MV beams. Mean dose increase is linear with the number of seeds and ranges from 0.7% for 75 seeds to 1.4% for 175 seeds for 6 MV and from 1.8% for 75 seeds to 4.6% for 18 MV for 175 seeds. Conclusions: In the external beam radiotherapy of the prostate, that contains seeds from the previous LDR seed implant, the mean dose in the prostate is increased proportionally to the number of seeds and the external beam energy used.
SU‐E‐T‐343: Monte Carlo Study on a Novel Direction‐Selective Flattening Filter System
Authors: Chofor N , Harder D , Willborn K , Poppe B
Published: November 2011

Purpose: Monte Carlo evaluation of a newly‐designed narrow steel cone flattening filter, which functions via the direction‐selective attenuation of the photons produced in the target. Methods: Using BEAMnrc and DOSXYZnrc (EGSnrc), novel 6 and 15 MV bremsstrahlung beams were modeled for a Siemens Primus linac. Under flattening filter free (FFF) conditions, the classical flattening filter was omitted. For the direction‐selective filter (DSF) configuration, an axis‐centered bullet‐shaped narrow steel cone filter is now mounted directly below the graphite electron absorber of the tungsten target. The neutron source strength was modeled by assessing photoneutron production at all tungsten surfaces hit by photons, assuming total photon absorption and using the macroscopic photoneutron cross section of tungsten. Results: Photon spectra for FFF and DSF conditions are less filtered compared to classical flattening filter conditions, while depth dose curves are very similar. Under DSF conditions, field flatness has been achieved for a circle of diameter 15 cm at 100 cm SSD, and the normalized peripheral dose was reduced by factor 1.5. For FFF and DSF, the dose rate at 10 cm water phantom depth was enhanced by factors 2 and 1.7 (6 MV) and 4 and 2.5 (15 MV) respectively, while the corresponding beam head photon leakage dose remained unchanged. The photoneutron source strength produced/Gy was reduced by factors of 3.5 (FFF) and 2.8 (DSF). Conclusions: With the DSF system, the simplicity of treatment techniques and workflow is maintained, because field flatness up to 15 cm field diameter is achieved at all depths, while the advantages characteristic for FFF conditions, the dose‐rate enhancement, the reduction of photon and neutron leakage from the beam head/Gy in the treatment volume, and the reduction of the normalized dose levels in the far periphery are maintained.
SU‐E‐T‐344: A Study of True Beam Linac Operation in a Varying Magnetic Field Environment
Authors: Santoro J , Furhang E , Kim C , Whittum D , Van Heteren J , Potter R
Published: November 2011

Purpose: To quantify the effects of transient, DC magnetic fields on the normal operation of a True Beam linac and to verify normal and accurate treatment delivery capability. This is necessary to ensure normal linac operation and accurate treatment delivery in such an environment. Methods: A data acquisition system was constructed consisting of a Lake Shore vector magnetometer and 3‐axis probe including custom software to monitor and record the real‐time vector magnetic field value at an in‐air reference point near the linac. This data was correlated with various dosimetric and beam quality measurements taken simultaneously. Long duration measurements (2–5 minutes) of the four steering servo currents (Asymmetry Radial (AR), Position Radial (PR), Asymmetry Transverse (AT), and Position Transverse (PT)) were recorded and correlated with magnetic field measurements. Measurements of machine output, dose rate, and beam asymmetry were also recorded and correlated with magnetic field readings. These measurements were repeated for various combinations of gantry orientation and photon and electron beam energies. Results: Linac output, dose rate, and beam asymmetry versus magnetic field demonstrated no dependence on magnetic field. Correlation plots of the four steering servo currents versus magnetic field indicated a strong dependence (correlation coefficient close to ±1.0) for 3 of the 4 steering servos at a gantry angle of 0° at a photon energy of 6MV. This dependence weakened for one of the servos (AT) and strengthened for one (AR) as the gantry angle was changed to 270°. Extrapolation of the correlation plots of each servo to the engineering limits were used to obtain a maximum permissible magnetic field value for the room Conclusions: Effects of magnetic field on linac operation was systematically quantified. Results indicate that the linac is fully capable of normal and accurate treatment delivery despite the ambient magnetic field environment in the room.
SU‐E‐T‐348: Commissioning of An Independent Monitor Unit Verification Program (RadCalc Version 6.1) in Photon Point Dose Calculation
Authors: Changkaew P , Stansook N , Khachonkham S , Piriyasang D , Sakulsingharoj S , Tangboonduangjit P
Published: November 2011

Purpose: To commissioningRadCalc program for secondary point dose check in conventional 2D and 3DCRT calculation techniques. Methods: Test plans for 6 MV and 10 MV photon beams with conventional and 3DCRT planwere created inRadCalc program.Field was varied with a range of field sizes and modifier techniques referred to IAEA‐TECDOC‐1583 with central axis and off central axis calculation points. The measured dose for each field was then compared to the calculated dose (RadCalc program and Eclipse treatment planning system). Results: In conventional field validation,RadCalc shows good agreement with measured doses.For open and almost all wedge fields, RadCalccalculationagree with dose measurement within +/−1%. Differences of over 1% were found in maximum wedge fields and some half fields. However, the dose difference between calculation and measurementwas within +/−2% in all tests. In 3DCRT plan validation, the central axis point dose between calculation and measurement in all tests are agree within the IAEA‐TECDOC‐1583 criteria. When measured/ calculated dose points were in non‐tissue equivalent such as lung and bone which were off central axis situation, dose difference is more than +/−10%. Unfortunately if measured/ calculated dose points areoutside the field (scatter dose), the result cannot be validated. Part of this large difference between two calculated and measured can be the volume effect of the chamber.In comparison between dose calculated by RadCalc and Eclipse, overall results show dose difference within +/−5%. Conclusions: RadCalc program can be used as a point dose calculation check in conventional 2D and 3DCRT techniques.
SU‐E‐T‐351: Dosimetric Impact of Periodic and Random Motion on Proton Double‐Scattering and Uniform‐Scanning Deliveries
Authors: Su Z , Lopatiuk‐Tirpak O , Zeidan O , Slopsema R , Flampouti S , Meeks S , Li Z
Published: November 2011

Purpose: To experimentally investigate target motion impact on proton dosimetry and the spatiotemporal interplay in double scattering (DS) and uniform scanning (US) deliveries using 3D polymer gel dosimeters and a programmable motion platform. Methods: A one beam proton plan with 16cm range and 6cm modulation (total 13 energy layers) in both DS and US modes was generated in the Eclipse treatment planning system (TPS) to be used for gel irradiation in static, periodic and random motion. The periodic motion was a single sinusoidal trace along the SI direction with 2‐cm peak‐to‐peak amplitude and 0.25Hz frequency. The random motion trace was an actual patient prostate trajectory from a treatment fraction. All irradiated gel dosimeters were setup on the platform in an identical fashion. The dose distribution of static gel dosimeter was compared to those of treatment plan. Dose distributions were compared between DS and US in both periodic and prostate motion conditions. 3D gamma analysis (4%, 4mm criterion) was calculated against planned dose distribution for all the gel dosimeters. Results: There was an overall good agreement in dose distributions between stationary DS delivery and TPS plan. 93% of voxels has gamma less than 1. 3D gamma failure maps were significantly different between DS and US periodic motion gels. There were 88% and 91% of voxels has gamma less than 1 for DS and US, respectively. In prostate motion cases, even though about 70% voxels has gamma larger than 1 for both DS and US deliveries, significant gamma failure map difference was still observed. Conclusion: Good dose agreement between TPS plan and stationary gel dosimeter demonstrated the validity of using gel dosimeter study target motion effect and its spatiotemporal interplay with proton irradiation. Isodose and 3D gamma comparisons between DS and US for both periodic and prostate motion confirmed their dosimetric impact.
SU‐E‐T‐354: Pairs of Sloped Spread‐Out Bragg Peaks Improve Normal Tissue Sparing
Authors: Veltchev I , Fourkal E , Ma C
Published: November 2011

Purpose: To investigate 3D optimization with parallel‐opposed proton beams and show that the solution space can be greatly reduced, resulting in a dramatically shortened optimization time. In passive scattering proton treatments the target is covered with spread‐out Bragg peaks (SOBP) produced by modulating the proton spectrum to deliver a constant dose at depth. Larger targets necessitate the use of wider SOBP with wider spectrum, which inevitably leads to dramatic increase of the surface dose to a level comparable to the target dose. Therefore, often two parallel‐opposed beams are used to remedy the surface dose increase.Methods: Using an analytical approach we prove that in a parallel‐opposed setup there exist a unique optimum solution that leads to lowest normal tissue dose. The results are further validated in an optimization routine operating on a set of FLUKA‐generated Bragg peaks.Results: In a test case, two opposed optimum SOBP beams create homogeneous target coverage while the dose outside the target is reduced by 22% as compared to the case of two flat SOBPs. We note that the single‐beam optimum dose distribution inside the target resembles linear distribution. Therefore, one can seek a sloped spread‐ out Bragg peak (SSOBP) with weights obtained through optimization regardless of the density fluctuations in the phantom. There is a unique solution when each beam is optimized separately and the optimizer is shown to always converge to the desired weight distribution. Conclusions: The parallel‐opposed technique with two SSOBPs is the ultimate distal edge tracking, for it employs only half of the spectrum necessary to cover the target and results in maximum normal tissue sparing. Its linear modification automatically ensures both homogeneous target coverage and reduced dose to the organs at risk and normal tissue. Full 3D optimization involving this technique will result in faster computation times.
SU‐E‐T‐365: Dosimetric Study of An HDR Applicator of SAVI for Partial Breast Irradiation
Authors: Dou K , Jacobs M , Ottinger M , Seidel M , Reynolds S
Published: November 2011

Purpose: To investigate the dosimetric effect by experimentally simulating the situation with and without tissue invagination and the potential translation and rotation of the SAVI applicator for partial breast irradiation. Methods: The SAVI applicator with the cavity filled with air and water was merged into a water phantom and the delivered dose was measured using an ion chamber and film. The measurements were compared with a homogenous dose calculation by the treatment planning system. A dose variation from the SAVI translational shift was measured by moving an ion chamber along with the central axis direction. A dose change from the SAVI rotation was measured by placing an ion chamber at a fixed point while rotating the SAVI device about the central axis. Results: The dosimetric effects for the SAVI device were found to be related to the cavity dimension, source arrangement, and dwell times. For the single dwell source placed in the center of the applicator, the maximum difference of the dose with the air cavity at 1cm away from the air‐water boundary is about 7% higher than that with water filling in the cavity. But the measurements with nearly fully loaded multi‐sources for the same situation show a difference of less than 3%. The 3% dose variation in average was found from either the 3 mm translation or 3 degree rotation of the SAVI applicator. Conclusions: The maximum dosimetric effect of an air cavity is 7% off compared with a water filled cavity when a single dwell source position is used in the center of the central catheter. Multiple catheters of the SAVI applicator with a nearly fully loaded dwell source position produce the discrepancy of less than 3% and allow for optimal and conformal dose distribution to a lumpectomy cavity while minimizing the dose to adjacent normal structures.
SU‐E‐T‐366: Dosimetric Characterization of Brachytherapy Surface Applicators
Authors: Kennedy R , Micka J , DeWerd L
Published: November 2011

Purpose: Conical surface applicators with diameters ranging from 10mm to 45mm have been developed by Varian Medical Systems, Inc. These applicators are designed to be used with the GammaMedplus iX and VariSource iX high‐dose rate Ir‐192 afterloaders, allowing for conformal dose delivery for the treatment of surface lesions. Clinical use of these applicators has become widespread, yet no peer‐reviewed method of output verification exists. Traditional brachytherapy dosimetry protocols (AAPM TG‐43U1) cannot be applied directly, creating a gap in the chain of calibration traceability. The ultimate goal of this work is the creation of a dosimetry protocol for the calibration of applicators like those under investigation. An initial step towards this includes Monte Carlo and experimental assessment of the dose‐rate and distributions for each applicator. Methods: Each applicator was modeled in MCNP5 based on specifications provided by the manufacturer. A collision kerma tally was used to determine the kerma distributions at the surface and at depth in water. Surface dose profiles and fractional depth dose curves in water were generated for each applicator. Results: Initial assessment of the surface dose distributions show a peak at the center of the applicator with a rapid fall off to the edges. A prominent dip in the surface dose profile is observed with the vertical entry applicators due to the shorter source‐to‐surface (SSD) distance. The penumbra for both the horizontal and vertical applicators was 1.04cm, despite differences in the SSD. Conclusions: The surface dose distributions and fractional depth dose curves generated from this initial Monte Carlo investigation show characteristics typical for applicators of this type. This work expands the knowledge about output characteristics of these kinds of applicators, leading to the ultimate goal of a clinically relevant dosimetry protocol for clinicians. Conflict of Interest: Varian Medical Systems, Inc. provided sources and applicators.
SU‐E‐T‐370: Dosimetric Comparison Between Single‐Lumen and Two Types of Multi‐Lumen Catheters Used for Partial Breast Brachytherapy
Authors: Liu B , Jiang H , Zhou S , Kampp T , Astrahan M
Published: November 2011

Purpose: compare two optimization methods using brachyvision and compare the dosimetry between multimen‐lumen and single‐lumen catheters used for partial breast brachytherapy Methods: Single‐lumen catheter is the Mammosite device. Two multi‐lumen devices are used: contura 5‐lumen and Hologic 4‐lumen devices. The planning system is Brachyvision. PTV is constructed by expanding the balloon by 1cm, and shaping the volume to exclude the pectoralis muscle and leave a 0.5cm margin from the skin. PTV_EVAL is defined as PTV subtracts the balloon. We also define two critical‐organ ROIs: skin region and chest region, where skin region is the skin side to PTV excluding PTV and the chest region is the chest side to PTV excluding PTV. One optimization method is to achieve uniform 3.4 Gy/fx on the PTV surface, and the other method is an inverse optimization method to achieve at least 95% of PTV receive dose higher than 3.4 Gy/fx and less than 10% of PTV receive more than 5Gy/fx with upper bound to skin region and chest region. Patients are grouped into 4 categories: (a) Skin and chest wall are within 1cm from the balloon; (b) only skin (c) only chest wall is within 1cm from the balloon and (d) both skin and chest wall are within 1cm from the balloon. Results: For category (a) patients, both optimization methods give similar satisfactory Results: 98% and 99% of PTV_EVAL receive 3.4 Gy/fx for single‐lumen and multi‐lumen catheters, respectively; while the skin dose are less than 3.4Gy/fx. For patients in categories (b) and (d), the inverse optimization method and multi‐lumen catheter gives significant lower skin dose. Conclusions: single‐lumen, multi‐ lumen device provide similarly satisfactory result for patients with skin and chest at least 1cm from the balloon; for other patients, the inverse optimization method and multi‐lumen catheter device give significantly less skin dose.
SU‐E‐T‐372: A New P‐32 Ophthalmic Applicator and Dosimetry
Authors: Choi C , Kim I , Han H , Park U , Son K , Ye S
Published: November 2011

Purpose: Dosimetric characteristics of new P‐32 ophthalmic applicator as substitute for Sr/Y‐90 irradiation were evaluated. The integrity and safety were confirmed. The feasibility for 32P ophthalmic applicators was evaluated. Method and Materials:According to optimal design of P‐32 ophthalmic applicator, Monte Carlo simulations were performed to calculate dosimetric characteristics: reference dose rate, central axis depth doses, transaxial dose profiles. The reference dose rate at the surface of an ophthalmic applicator is measured by using an extrapolation ionization chamber (EC) and radiochromic film (RCF). Depth dose distributions and dose profiles were measured using RCF. The source non‐uniformity was calculated. The source leakage was examined for the safety. The effective shielding of the applicator was verified. Results: The reference dose rates of EC, RCF was 4.13 ± 0.21 cGy/s and 3.84 ± 0.25 cGy/s, respectively. The axial depth dose rate was reduced into approximately 1/10 as 32P betas penetrate every 2 mm depth. Measured data sets in depths of 1 mm to 3 mm agreed with Monte Carlo data. The dose profiles were not uniform due to non‐uniform activity distribution. At the surface (depth=0.1 mm), source non‐uniformities were 27.8%, 11.3% for film, MCNP, respectively. The exposure rate of the handle grip toward a clinician was 1.4 mR/hour (background: 0.1mR/hour). Conclusion: The 32P applicator with a 20 mCi (740 MBq) activity can deliver the therapeutic doses to the surface of the conjunctiva within 6 min, while sparing the lens better than 90Sr/Y applicators. The integrity and safety of an new P‐32 applicator was confirmed. However, prior to the clinical application of every new applicator, safety, dose uniformity, and absorbed dose rate at the reference point should be carefully evaluated by the method developed in this study.
SU‐E‐T‐377: Evaluation of Calculated and Measured HDR Brachytherapy Doses in the Presence of Heterogeneities
Authors: Libby B , Van Ausdal R , Morrison J , Ter‐Antonyan R , Benedict S
Published: November 2011

Purpose: With the recent introduction of the Grid‐Based Boltzmann Solver (GBBS) in commercial HDR brachytherapy treatment planning (Acuros™, Varian), there is now a rapid and accurate alternative to Monte Carlo calculations to attain improved dosimetric accuracy. The validation of Acuros™ has been performed by comparison to Monte Carlo calculations in water phantom. In this investigation, we compare calculated and measured doses in the presence of both high and low density heterogeneities. Methods: Phantoms were created incorporating both low and high density heterogeneities including representative densities of bone and air cavities. For the comparisons HDR plans were developed to deliver 100 cGy with TG‐43 formalism to optically stimulated luminescence (OSL) detectors placed in or near the heterogeneities. The detectors were calibrated by exposure to a known dose. The plans were then recalculated using the Acuros™ algorithm, and the dose to the detectors was determined both by calculation and measurement. Results: The OSL detectors measured an average dose of 105 cGy when placed after a low density insert in the phantom, as compared to a calculated average dose of 107 cGy. When the OSL detectors were placed after three different higher density inserts ranging from 1.3 to 1.9 g/cm3, the measured doses were 78, 77, and 75 cGy, compared to the calculated doses of 84, 82, and 75 cGy. Conclusions: The Acuros™ algorithm has been demonstrated to perform rapid and accurate dosimetry in homogenous water phantom. In this study we demonstrate that it performs accurately in the vicinity of heterogeneities expected to be encountered in the clinic. This work shows good qualitative agreement between measured and calculated high dose rate brachytherapy doses in the presence of both low and high density heterogeneities, and shows that the confidence‐building step of comparing calculated to measured dose can be readily made with simple phantom geometries.