bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐J‐213: Comparison of the Effect of Metal Implants in Three Different Radiation Therapy Modalities; IMXT, Tomotherapy, and Proton Therapy
Authors: Chung K , Cho S , Min B , Lim Y , Shin D , Lee S , Park S , Cho K
Published: June 2012

Purpose: To compare and evaluate the dosimetric effect of metal implants in 3 different radiation therapy modalities, photon IMRT, Tomotherapy, and Proton Therapy. Methods: We have constructed two sets of PMMA phantom with and without titanium alloy orthopaedic implants embedded. Combinations of the two phantom sets provide a hole in the middle for a dose measurement using ion chambers and also a space in which an EBT film can be inserted for a measurement of dose distribution in that plane. The phantoms have been CT‐scanned separately and will be planned in 3 different treatment modalities (IMXT, Tomotherapy, and Proton therapy in passive scattering mode). In each plan, we make an assumption that the beam path(s) should include the metal implants so that we can compare the dosimetric effect in different modalities directly. The plan comparison between the three modalities will be made. In addition, point doses and dose distributions of the plans will be measured using ion chambers and EBT films and compared to the Result of the plan study. Results: The plan outcome of three different modalities shows different properties as expected. The uncertainty due to the presence of high‐density metal is not only coming from the metal artifacts in CT numbers but also coming from the distinct physical property of the metal especially for the proton therapy. The most suitable treatment modality among the three treatment methods in various aspects will be detemined. Conclusions: The comparison Result would provide a guideline for the selection of radiation therapy modalities in the presence of metal implants and help to make decisions based on quantitative supporting data. In addition, the Result would be a basis for the further study to effectively avoid the undesired effects of the metal implants in radiation therapy especially for the proton therapy.
SU‐E‐T‐06: A Mathematical Explanation to Tumor's Response to Perfusion and Hypoxic Fraction after Radiation
Authors: Yan Y , Kissick M , Bussink J , Jacques S , van der Kogel A , Campos D , Zhao D
Published: June 2012

Purpose: To develop a dynamic model that explains oxygen dynamics between the microvascular perfusion and the hypoxic cell population inside a tumor. Methods: Bussink et al (Radiat Res 153(4), p.398 (2000)) observed fast oxygen dynamics, faster than cell‐death. Based on a simplified three‐compartment‐model: the microvasculature, well‐oxygenated, and hypoxic tumor cell populations. We applied a first‐order differential model for the tumor's transient response as a function of oxygen content within the blood vessels. The sink terms in our model for each compartment are fast changing parameters because radiation rapidly changes the oxygen consumption of the tumor cell in a time scale which is much faster than the population changes of the tumor. Transportation balance condition is also applied for each compartment. Results: Our simulation results can explain the experimental data in Bussink et al's (Radiat Res 153(4), p.398 (2000)) paper. We provide an explanation for the relative complex behavior of the microvascular perfusion after radiation that emphasizes the role of dynamic metabolic changes in addition to population changes. Conclusions: A newly developed dynamic model leads our understanding to the interrelationship between microvascular oxygen content within the blood vessels and the hypoxia state of the tumor to a deeper level, which has the potential to provide the theoretical foundation for the patient' specific adaptive radiotherapy.
SU‐E‐T‐09: Empirical Evidence for Decreased Cell Survival in Flattening Filter Free Beams
Authors: Langner U
Published: June 2012

Purpose: To present empirical evidence for a decrease in cell survival with the introduction of markedly increased average dose rates in flattening filter free beams. Methods: Data of (Bewes et al., 2008) are used to calculate a modification factor for cell survival curves, using the linear quadratic model, that is inversely proportional to the reciprocal of the average dose rate. Results: The survival curve becomes steeper as the average dose rate increases above a ∼600 cGy/min threshold. For average dose rates below this threshold, no significant effect or even an increase in cell survival was observed (as explained by the Lea‐Catcheside dose protraction factor). For a maximum assumed dose modification factor of 1.2 a decrease in cell survival of 3.5% for a dose of 2Gy and 36% for a dose of 8 Gy is predicted for a MM576 cell line. For a maximum assumed dose modification factor of 1.4 a decrease in cell survival of 60% for a dose of 2Gy and 1 order of magnitude for a dose of 8 Gy is predicted for a NCI‐H460 cell line. For a dose modification factor of 2.65 with a dose rate of 24 Gy/min, a decrease in cell survival of 1 order of magnitude at 2Gy and 5 orders of magnitude at 8 Gy is predicted for a NCI‐H460 cell line if it is assumed that the inverse proportionality is true. Conclusion: Although no effects on cell survival was seen previously with clinically achievable dose rates (except for enhanced cell survival through repair for increased treatment times), it might become more evident with the increase of the average dose rate in flattening filter free beams. This will necessitate revision of fractionation schemes and maximum tolerable doses for organs at risk for stereotactic body radiation therapy.
SU‐E‐T‐18: Output Dependence on Dose Rate for Uniform Scanning Proton Beams
Authors: Ramirez E , Zheng Y
Published: June 2012

Purpose: Accurate determination of proton output is critical to ensure accurate proton dose delivery. It was found that the proton output factor was decreased when the dose rate was low. The purpose of the study is to investigate the dose rate effects on proton dose output factors. Methods: We systematically measured output (nC/MU) of the proton beam as a function of dose rate for various beam conditions. We first measured dose rate effects for proton beams of three range and modulation combinations: range 16 cm and modulation 10 cm, range 5 cm and modulation 3 cm, and range 30 and modulation 15 cm. We then investigated the dose rate effects for two scanning field sizes, the standard 18×18 cm̂2 and the largest available 25×25 cm̂2. Results: The output factor varied with dose rate, increasing more than 6% when the dose rate increased from 0.1 Gy /minutes to about 2.5 Gy/minute. The output variation on dose rate seems to depend on only the dose rate itself. The effect of proton energy and scanning field size on the dose rate dependence was minimal. Conclusions: The proton dose output factor depends on the dose rate at our facility, and can be large (>5%). This dependence needs to be taken account in output modeling for more accurate proton dose delivery.
SU‐E‐T‐19: Fitting a Multiple Source Photon Model for Monte Carlo Treatment Plan Verification
Authors: Ventura A , Shih J , Svoboda J
Published: June 2012

Purpose: To assess and reduce the difficulty of fitting a multiple source photon model for monte carlo treatment plan verification. Methods: The EGS4 user code MCSIM, from Fox Chase Cancer Center, was chosen for its support of a multiple source photon model, of which the point and secondary (extrafocal) photon sources were utilized. A described method of fitting the secondary source to in‐air output factors was implemented. Additionally, a method to fit the point source to a single large field dose distribution was explored. The point source fitter utilizes a database of pre‐simulated mono‐energetic fanlines to build distributions from arbitrary spectra. Perturbations are made to fanline spectra to reduce the errors along them. In this study the energy spectrum for each fanline has been limited to the log‐normal distribution, which reduces the number of parameters for each to two. Results: It was found that one spectral parameter could be set to a constant for all fanlines and the other restricted to linearity with respect to off‐axis position. The model matched the outputs and distributions in non‐superficial areas to within 2% for 6MV and 15MV Varian iX field sizes between 4 and 40 cm. Various types of treatment plans were then successfully verified, including 3D, VMAT, IMRT, and an iPlan Monte Carlo stereotactic lung to within 3% (tumor dose). Conclusion: With such tools it is practical for a non‐research physicist to fit a two source photon model for the purpose of monte carlo treatment plan verification. The only commissioning data needed are in‐air output factors, a single large field dose distribution, and the usual machine parameters provided by LINAC vendors for clinical second check programs. Even when only photons are simulated and spectra are greatly simplified it is possible to achieve acceptable results for non‐superficial tumors. Furthermore, this is achieved without proprietary machine specifications.
SU‐E‐T‐21: Modeling a MLC Scatter Source for In‐Air Output Factors
Authors: Park S , Kim S , Park Y , Park J , Kim J , Kim H , Choi C , Ye S
Published: June 2012

Purpose: Scattered radiation from multi‐leaf collimators (MLCs) is no longer negligible for calculating in‐air output ratio, Sc for small and irregular fields often used in intensity‐modulated radiation therapy (IMRT). An extra‐focal source model for scattered radiation from MLCs, namely MLC scatter source, has been developed to improve the accuracy of the Sc calculation. Methods: A conventional dual‐source model was made by using Sc data that were measured for collimator‐defined fields of Varian Clinac IX linear accelerator. Then, an MLC scatter source at the center of the MLC position of the linear accelerator was assumed in the model. The MLC scatter source model consisted of two Gaussian functions of which parameters were iteratively optimized against the Sc data measured for different MLC fields with fixed collimator sizes. To evaluate the effectiveness of the developed source model, measurements were made for various MLC‐defined irregular or square fields. The calculated Sc data by using (1) the developed source model and (2) the conventional dual source model were compared with the measured data. Results: The mean discrepancy between the measured Sc and calculated Sc from the developed source model was 0.08+‐0.28%, while one from the conventional source model was 0.44+−0.39%. Conclusions: The developed MLC scatter source model in conjunction with the dual source model could improve the accuracy of the Sc calculation in IMRT fields.
SU‐E‐T‐25: Real Time Simulator for Designing Electron Dual Scattering Foil Systems
Authors: Carver R , Hogstrom K , Price M , Leblanc J , Harris G
Published: June 2012

Purpose: To create a user friendly, accurate, real time computer simulator to facilitate the design of dual foil scattering systems for electron beams on radiotherapy accelerators. The simulator should allow for a relatively quick, initial design that can be refined and verified with subsequent Monte Carlo (MC) calculations and measurements. Methods: The simulator consists of an analytical algorithm for calculating electron fluence and a graphical user interface (GUI) C++ program. The algorithm predicts electron fluence using Fermi‐Eyges multiple Coulomb scattering theory with a refined Moliere formalism for scattering powers. The simulator also estimates central‐axis x‐ray dose contamination from the dual foil system. Once the geometry of the beamline is specified, the simulator allows the user to continuously vary primary scattering foil material and thickness, secondary scattering foil material and Gaussian shape (thickness and sigma), and beam energy. The beam profile and x‐ray contamination are displayed in real time. Results: The simulator was tuned by comparison of off‐axis electron fluence profiles with those calculated using EGSnrc MC. Over the energy range 7–20 MeV and using present foils on the Elekta radiotherapy accelerator, the simulator profiles agreed to within 2% of MC profiles from within 20 cm of the central axis. The x‐ray contamination predictions matched measured data to within 0.6%. The calculation time was approximately 100 ms using a single processor, which allows for real‐time variation of foil parameters using sliding bars. Conclusions: A real time dual scattering foil system simulator has been developed. The tool has been useful in a project to redesign an electron dual scattering foil system for one of our radiotherapy accelerators. The simulator has also been useful as an instructional tool for our medical physics graduate students.
SU‐E‐T‐91: Validation of Geant4 Physics for Ionization Chamber Calculations in Radiotherapy Photon Beams
Authors: Chequers MH , Sawakuchi GO
Published: June 2012

Purpose: To determine the accuracy of the GEANT4 Monte Carlo toolkit for ionization chamber calculations in radiotherapy photon beams. Methods: First, we used the Fano cavity example included in the GEANT4 distribution to validate calculations under Fano conditions. We determined a combination of parameters and physics list that provided results consistent within +/− 0.5% with the Fano theorem. Next we performed simulations to investigate the accuracy of using GEANT4 for ionization chamber calculations. Eight ionization chambers were modeled using detailed manufacturer specifications including A1, A1SL, NE2571, PTW30010, PTW30012, PTW31010, PTW31014 and PTW31016. The absorbed dose to water for a cylindrical water cavity and the absorbed dose to air in the ionization chambers' cavities were scored for 1.25 MeV photons. The ratio of these quantities was then compared to values from EGSnrc simulations. Results: Simulations using the Fano cavity example yielded results within +/− 0.5% with the Fano theorem across 1.25, 3 and 4 MeV incident photon energies. The most accurate and consistent results were obtained using the G4eIonisation ionization model and G4GoudsmitSaundersonMscModel multiple scattering (MS) model with a maximum step size limitation of 0.001 mm, which yielded results accurate to +/− 0.3% for all energies. This set of parameters and physics processes as well as the G4UrbanMscModel93 MS model were used for the ionization chamber calculations. The calculated quantities were compared to those used in Muir and Rogers 2010 (Med. Phys. 37: 5939–5950) and agreed to within sub‐percentage differences for most chambers. Conclusions: The GEANT4 toolkit can achieve sub‐percentage accuracy for ionization chamber calculations in radiotherapy photon beams. This is achieved by using either the G4GoudsmitSaundersonMscModel or G4UrbanMscModel93 MS models. Although less accurate (+/− 0.5%), simulations employing the G4UrbanMscModel93 MS model are on average two orders magnitude faster than that of the G4GoudsmitSaundersonMscModel MS model (+/− 0.3%). Natural Sciences and Engineering Research Council of Canada
SU‐E‐T‐93: High Dynamic Range Scanning for Optical‐CT in 3D Dosimetry
Authors: Niebanck M , Newton J , Juang T , Oldham M
Published: June 2012

Purpose: The recent emergence of powerful, fast and high‐resolution 3D dosimetry techniques brings exciting potential to radically strengthen the foundations of quality‐assurance in radiation therapy, and hence treatment efficacy. This work presents a new acquisition method for 3D dosimetry by optical‐computed‐tomography (optical‐CT), which can improve accuracy when imaging strongly attenuating dosimeters (e.g. large or heavily dosed). Methods: The method involves acquiring optical‐CT projection images where the intensity of the light source is increased as much as possible without causing detector saturation within the dosimeter. This improves signal‐to‐noise in dark regions of the dosimeter, but flood‐field and regions outside the dosimeter are unuseable due to saturation. The problem of acquiring a useable flood is solved by acquisition at a reduced shutter opening time. The non‐saturated flood is then scaled up for use with projections, by the ratio of shutter times. The method relies on linearity of signal with shutter time, which is investigated here. The method is evaluated by application to a range of dosimeters with varying degrees of strong attenuation. Results: The relationship between signal and shutter time in the flood was found to be highly linear, a key enabling result for this method. When applied to moderately attenuating dosimeters, the new method agrees with the standard method to a high degree (<1% deviation on average). For very dark dosimeters, the new method was found to have greatly improved signal‐to‐noise (a factor of 2‐3 times better) in the darkest regions and eliminated streak artifacts present in reconstructions using the standard acquisition method. Conclusions: A new optical‐CT acquisition method is presented which yields improved signal‐to‐noise for dosimetry measurements in strongly attenuating dosimeters. A further important advantage is the method does not require any tinting of the refractively‐matched fluid in the optical‐CT water‐bath, which greatly increases convenience and practicability. Grant ‐ R01CA100835
SU‐E‐T‐95: Investigation of 3D Dosimetry for an Anthropomorphic Spine Phantom
Authors: Grant R , Ibbott G , Yang J , Adamovics J , Followill D
Published: June 2012

Purpose: To evaluate 3D dosimetry for a spinal cord treatment plan delivery using the Radiological Physics Center's (RPC) anthropomorphic spine phantom. Methods: The RPC's spine phantom currently uses radiochromic film and thermoluminescent dosimeters (TLD) to evaluate spinal metastases treatments. A second dosimetry insert for the phantom was created to hold a PRESAGE® 3D dosimeter which matched the location of the TLD and film in the original insert. The phantom was CT imaged with each insert and an IMRT treatment plan was developed. The IMRT plan was delivered to the phantom twice; once with each insert. The film and PRESAGE® were scanned on a CCD microdensitometer and optical‐CT system, reconstructed to a 2 mm slice width, respectively. The measured dose distributions were compared to the treatment plan calculated dose distribution using RPC in‐house developed software or the Computational Environment for Radiotherapy Research (CERR). Film and PRESAGE® dose profiles were taken across several planes and compared for agreement. The distance to agreement (DTA) between the measured data and treatment plan, within the high dose gradient region, was quantified. Results: The PRESAGE® and plan dose profiles agreed to within 2and 1 mm in the AP and SI directions, respectively. The film and plan also agreed to within 2 mm across all profiles. Conclusions: The PRESAGE® 3D dosimeter, based on these preliminary data, shows potential as a dosimeter for the RPC's phantom irradiation studies. Future work will add markers to the PRESAGE® insert to allow for a reproducible registration in CERR and a an optical‐CT system, reconstructed to a 2 mm slice width dose calibration protocol will be created. CA 100835
SU‐E‐T‐97: Intra and Inter Variability in Beam Data Commissioning among Water Phantom Scanning Systems
Authors: Das I , Chu C , Conlon J , Neck D , Gibbons J
Published: June 2012

Purpose: There are many water‐phantom scanning systems with advanced features to collect accurate commissioning data. However the intra‐ and inter‐variability of commissioning data has not been reported which is attempted in this study. Methods: Four vendors with modern water‐phantom scanning systems; PTW, Sun Nuclear (SN), Standard Imaging (SI) and IBA were invited to an institution to demonstrate beam data collection. Each system was used to collect percent depth dose (PDD) and profiles several times in a day with their choice of detector for four different machines for photon and electron beam commissioning. This provided information on intra‐variability. At the end, each vendor was allowed to setup and collect data on a single unit for inter‐variability. All data were sent to a central location for analysis and evaluation. Results: The depth dose and profiles for 2×2cm2and 10×10cm2 fields were analyzed for intra‐ and inter‐variability. With repeated measurements, the intra‐variability provided a detailed degree of fidelity of data collection. This was shown to be with (± 0.1%) among all vendors. Ignoring data in buildup region and comparing with one system (PTW), the PDDs variability were slightly larger 0.02±0.17%, 0.14±0.21%, 0.17±0.2%, for SI, SN and IBA, respectively. The profiles inter‐variability in central region were <0.1 %, however in penumbra up to ± 4.8%were observed. The polarity effect was also noted up to 3% which was depth and detector dependent. Conclusions: Intra‐ and inter‐variability among various scanning system are very small indicting that all modern systems if used properly could collect data within±0.2% accuracy. The selection of device should be based on institutional comfort and personal preference of software and hardware. This study provides unique opportunity to compare data among systems which is otherwise not possible.
SU‐E‐T‐99: Small Field Output Factor Measurement Using MAGIC Gel Dosimeter in 3T MRI
Authors: Ding X , Bourland J , Dolesh W , Best R , Mcgowin I , Liu J
Published: June 2012

Purpose: Small field dosimetry is very important because of radiation therapy techniques that use small fields such as IMRT, gamma and body radiosurgery, cyberknife and tomotherapy. We investigated use of a MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) gel dosimeter to quantitatively measure small field output factors (OFs) for 6MV x rays. Methods: In this work, MAGIC (Gelatin 9%; Methacrylic acid 4%; CuSO4 0.1mM; Ascorbic ascid 2mM; Glucose 10%) gel phantoms were developed to measure the 6MV x‐ray output factors for 1×1 up to 10×10 cm2 square fields (Varian 2100C linear accelerator). For comparison, 3 ion chambers (PTW:TN30013, Exradin A12, Capintec PR‐05P), Gafchromic film (EBT2), and TLD (LiF‐100) were used to measure the small field OFs under identical experimental conditions: 6cm depth (solid water), SAD=100cm, SSD = 94cm, 6MV, 512 MUs per irradiation. Relative OFs were normalized to a reference field (10×10 cm2 @ SAD =100cm). MAGIC gel dosimeters were scanned in a 3T GE signa® EXCITETM clinical scanner using a Spin Echo pulse sequence for dose distribution readout (pixel size = 0.4mm, slice thickness 3mm, TR = 4000ms, TE = 10ms and 110ms, respectively). Gel dose distributions were then calculated using custom Matlab code. Results: 6MV x‐ray OFs versus field size for all detectors were graphically compared. The MAGIC polymer gel dosimeter OF for a 1×1 cm2 field is 0.612 (+/− 5%), approximately 2% different from the OFs measured using small volume dosimeters (TLD, EBT2 film and the Capintec PR‐05P ion chamber). Larger ion chamber (PTW:TN30013 and Exradin A12, both ∼ 0.6cc) OFs were low (OF = 0.26 for 1×1cm2) due to nonequilibrium and partial volume conditions. Conclusions: MAGIC gel dosimeter with 3T MRI scanner as a read‐out makes it an ideal tool for small field dosimetry.
SU‐E‐T‐102: Evaluation of the Characteristics of TLD LiF:Mg.Ti‐100 Powder: A Measure of Consistency between Multiple Batches of Powder
Authors: Alvarez P , Aguirre J , Smith S , Followill D
Published: June 2012

Purpose: Analyze the characteristics of TLD LiF‐100 powder between multiple LiF crystal batches. Methods: The RPC used TLD LiF‐100 encapsulated powder to verify the output for photon and electron beams for 4 to 23 MV X‐ray beams and 6 to 23 MeV electron beams, respectively, from the past 15 years. During that time period, the RPC commissioned more than 15 batches of TLD powder. Commissioning of each batch of powder encompassed determining the system sensitivity (dose response), linearity, energy and fading characteristics of each batch of powder to determine the correction factors for the calculation of dose. The system sensitivity is the signal/mg per unit known dose of 60Co for each reading session. Other correction factors account for the loss of signal (fading) between the irradiation and read dates, supralinearity of the dose response and energy differences as compared to the 60Co irradiated standards. Results: More than 15 batches of TLD were commissioned to determine correction factors for the calculation of dose. The correction for fading, a characteristic of the LiF crystal, varied by ±1% between the multiple batches. The linearity correction, between 25 and 600cGy, normalized to 300cGy, showed a maximum variation of ±3% between batches. The energy correction factors, as defined for the RPC beam output audit system varied within ±1.7% (one std dev.) for the 15 batches. The system sensitivity is highly dependent on the LiF crystal grown for each batch, specific TLD reader and reading session conditions. The system sensitivity, while keeping the readers and reading sessions constant, varied by as much as 20% between batches. Conclusions: Each batch of LiF‐100 TLD powder showed variability in their powder characteristics such that calculation of dose accurately, with minimal uncertainty, requires a new commissioning. Work supported by PHS CA010953 awarded by NCI, DHHS
SU‐E‐T‐105: The LET Dependence of Liquid Ionization Chambers (LICs) in High‐LET Beams
Authors: Tegami S , Gomez F , Gónzales‐Castaño D , Jaekel O , Pardo‐Montero J , Holzscheiter M
Published: June 2012

Purpose: LICs are novel detectors for radiotherapy: the higher density of the medium allows to build them with a smaller sensitive volume, making them appealing in particle therapy. With RBE varying along the depth dose curve (DDC) and with the rising interest in dose/LET‐painting, verifying the LET is becoming more important. Nevertheless, while the LET distributions for different ionizing particles have been calculated, they have never been directly measured in realistic therapeutic beams. Our interest in LICs is based on the characterization of the beam quality in terms of LET. It has been shown in earlier works that the extrapolation of the linear portion of the voltage curve yields an intercept with the x‐axis that depends on LET. The quantitative establishment of this method, however, depends on how accurately recombination effects are taken into account. Methods: Due to the higher density of charge carriers produced in the liquid, LICs have high recombination effects: general recombination effects, involving pairs belonging to different tracks (dose rate dependent), and initial recombination between ion‐electron pairs belonging to the same incident particle event (LET dependent). To perform this study we propose a two‐dimensional array of LICs, composed by a 16×8 matrix of 2×2 mm2 pixels, which gives a fine spatial resolution on the plane. Results: Voltage curves have been measured for proton, carbon and oxygen beams available at the HIT facility in Heidelberg for different energies and dose rates. After correcting the curves for general recombination losses using the Three Voltage Method, we have indications of dose rate independence, indicating successful correction. Conclusions: Further investigations are foreseen to quantify the LET dependence along the DDC, where different LET values are expected. A comparison with simulated dose averaged LET values will give quantitative information about 2D LET distributions for different beam species.
SU‐E‐T‐108: 3D Measurement of Neutron Dose from a Novel Neutron Imaging Technique
Authors: Kapadia A , Crowell A , Fallin B , Howell C , Agasthya G , Lakshmanan M , Newton J , Juang T , Oldham M
Published: June 2012

Purpose: We have been developing a fast‐neutron spectroscopic technique to quantitatively image the distribution of elements in the body using quasi‐monochromatic neutron beams. Previously, we demonstrated the ability of the technique to quantify specific elements in the liver and breast while limiting radiation dose to clinically acceptable levels. Here we present the results of a physical dose measurement performed through neutron irradiation of 3D PRESAGE dosimetry phantoms. Methods: Two PRESAGE optical‐CT dosimeters were placed inside a physical phantom of the human torso and irradiated with 8 MeV neutrons produced via the 2H(d,n) reaction using a tandem Van‐de‐Graaff accelerator. The dosimeters, measuring 10 cm and 4 cm in diameter, were located in regions corresponding to the liver (10 cm), and the kidney (4 cm). Irradiation was performed with the neutron beam incident directly on the larger dosimeter. Cumulative neutron fluence incident upon each dosimeter was determined using an aluminum‐foil activation technique. Following irradiation, the change in optical density in both dosimeters was measured to determine the relative irradiation and dose distribution in each volume. Results: Both PRESAGE dosimeters exhibited detectable changes in optical density corresponding to the dose deposited in the volume. The two dosimeters registered doses of 8.5 Gy (direct incidence, 4.5 hour irradiation) and 0.25 Gy (off‐axis, 20 hour irradiation), respectively. The larger dosimeter showed highest intensity at the entry point of the beam with exponential drop‐off along the beam direction. The smaller dosimeter registered a more uniform change in intensity, consistent with the higher incidence of scattered neutrons at this location. Conclusion: The results demonstrate the utility of PRESAGE dosimeters in measuring dose from neutron irradiation and highlight the difference in relative doses between primary and proximal organs when exposed to neutron beams. This work was supported by the United States Department of Energy, Office of Nuclear Physics under Grant No. DE‐FG02‐97ER41033, the National Cancer Institute under grant R01CA100835, and by the Department of Defense under award W81XWH‐09‐1‐0066.
SU‐E‐T‐109: An Evaluation of a Newly Available Portal Dosimetry QA Tool for SRS Dynamic Arc and VMAT
Authors: Luo G , Crass J , Morales M , Ding G
Published: June 2012

Purpose: In our institute, the MatriXX has been used for IMRT, VMAT and dynamic arc QA. However, the small fields (<3cm) shaped by HDMLC for SRS dynamic arc (DA) present QA measurement issues when using the MatriXX due to its limited resolution (7.6 mm). A recently available portal dosimetry (PD) software tool in Eclipse Version 10 is capable of performing QA for arc plans. This study is to evaluate this new tool for QA. Methods: EPID calibration was performed using a Varian Novalis TX unit with HDMLCs for a 6X high dose rate SRS beam. The PDIP algorithm was configured using measured beam data. Brainlab iPlan was used to generate the DA patient treatment plans. The plans were then exported to Aria via DICOM RT. A verification plan was created in Eclipse and the predicted portal dose calculated. The comparison was done between predicted and measured portal dose distributions. Results: Compared with chamber array analysis, the measured portal dose shows significant improvement in correlation with predicted dose. To eliminate the uncertainty of dose normalization, absolute CU is chosen for analysis. Gamma factor was studied. Thecriterion was set at 3%/1mm using a 10% dose threshold such that 95% of the pixels had a gamma <1. QA was performed on 9 plans and the average gamma passing rate is 98.5%, with passing rates ranging from 95% to 100%. Conclusions: With approximately 0.7mm resolution, it is shown that the newly available PD tool is capable of overcoming the limitations of chamber arrays for small field SRS QA and is also suitable for HDMLC QA to provide detailed dose comparison with highly modulated dose profiles within a small treatment field. In addition, the time needed to perform the QA procedure using the PD tool is significantly shortened compared to the MatriXX.
SU‐E‐T‐114: Characterization of the Spatial Response Functions of Ionization Chambers for Photon Beam Dosimetry
Authors: Looe HK , Stelljes TS , Harder D , Poppe B
Published: June 2012

Purpose: The finite extension of an ionization chamber gives rise to a spatial averaging effect, known as the “volume effect”. In order to provide the appropriate corrections, the response functions along its lateral and longitudinal directions are characterized using Gaussian distributions, whose standard deviations slat and slong have been determined for a large set of clinical dosimeters. Methods: Nine cylindrical ionization chambers, two parallel‐plate chambers and two 2D ionization chamber arrays have been examined by scanning rectangular photon fields along their short axes. The true profiles D(x) were known from scans with a small Si diode. The ionization chambers were aligned with their symmetry axes either perpendicular or parallel to the scan direction in order to obtain slat and slong separately. In a search process, D(x) was numerically convolved with normalized one‐dimensional Gaussian kernels K(x) of varying s. The best fit between the convolution product D(x) * K(x) and the measured profile M(x) of the ionization chamber was used to determine parameters slat and slong of the Gaussian kernels. Results: For both the lateral and longitudinal directions, very good agreement was found between M(x) and the convolution products of D(x) with Gaussian kernels K(x). For all chambers, their 2s values are similar to the cavity dimensions, which means that the “tails” of the Gaussian response functions reach into the exterior of the chambers, ‐ an effect of the ranges of the secondary electrons. At higher photon energies response functions K(x) are slightly wider, but no detectable depth dependence has been observed. Conclusions: We have shown that the response functions of ionization chambers can be described by Gaussian distributions, confirming earlier observations, and we determined their standard deviations in both the lateral and longitudinal directions. Using these response functions, appropriate correction methods determined to eliminate the volume effect can be applied.
SU‐E‐T‐123: Understanding the Meaning of IMRT QA Passing Rates with a 2D Diode Array
Authors: Perez‐Andujar A , Morin O , Chuang C , Pouliot J
Published: June 2012

Purpose: A lower than ideal tolerance limit is used in intensity‐modulated radiation therapy quality assurance (IMRT QA) with a 2D diode array due to passing rate fluctuations. The objective is to identify patterns in the passing rates to predict sources of uncertainty that can affect treatment delivery, for example, the need to re‐calibrate the multileaf collimator when the passing rates start to decrease. Methods: Five complex clinical prostate IMRT plans were evaluated with a 2D diode array. The QA for each plan was repeated five times during one and a half month period. One of the plans was randomly selected and repeated the same day five consecutive times. The planar doses calculated by the treatment planning system were compared to the measurements of the 2D diode array. The individual passing rates per beam per plan were compared. Results: The average passing rate for each plan ranged from 94% to 97%. While the average percent difference of this ranged between −7.67% to 17.61%. Additionally, the minimum and maximum standard deviation among all beams was 0.13% and 9.63% respectively. We also compared the standard deviation of a plan QA repeated during different days versus a plan QA repeated during the same day. For the former the highest standard deviation was 6.05 % while for the later 0.21%. We noticed that the largest discrepancy between the passing rates was for angles at around 155° and 205°. Conclusion: These results show some inconsistency in the IMRT QA passing rates from one day to the next. Moreover, lower passing rates for a specific angle like the ones shown here can represent possible mechanical or tuning problems with the linear accelerator at these specific locations. Early identification of these sources of uncertainty can greatly improve the precision of the treatment delivery.
SU‐E‐T‐128: Dosimetric Characteristics of Gafchromic EBT3 Films for Megavoltage Photon and Proton Beams
Authors: Hayashi N , Kato H , Yada R , Matsunaga T , Ehara I , Adachi Y
Published: June 2012

Purpose: Gafchromic film for quantitative analysis was renewed from EBT2 to EBT3 film in November 2011. The purpose of this study is to investigate the relevant characteristics of EBT3 film for its application in dosimetric verification for IMRT/VMAT or proton therapy. Method:We investigated the characteristics of EBT3 film with comparison of previous EBT2 film. The experiments in this study composed two categories. At first, the photo spectroscopy for the irradiated film was compared between EBT2 and EBT3. The film 1 day after the irradiation was analyzed by a photo spectrometer (SR520: JASCO Corporation, Japan). Secondly, we investigated several calibration curves which obtained by same batch. The films were calibrated by irradiation the films to 13 dose steps. The irradiated films were scanned by a flatbed scanner (ES‐10000XL, Epson‐Seiko Corporation, Japan). The difference on scan orientation was evaluated alternate portrait and landscape directions. The photon and proton beams were delivered from Clinac 21EX (Varian) and Mitsubishi machine, respectively. Results: The peak absorption wavelength of EBT3 film and its response at all active range were basically same with that of EBT2 film. The peak wavelength of photo absorption in EBT3 was observed at 585 and 634 nm. The fog optical density was increased due to the hazy matte polyester for active layer. However, there is no change the tendency of the calibration curve responding to megavoltage photon and proton beams. The scan orientation dependency of EBT3 film was observed with similar to EBT2 film. The optical density of portrait orientation was 10% higher than that of landscape orientation. Conclusion: The dosimetric characteristics of EBT3 film were basically same with EBT2 film. With regard to the matte polyester, the creation of Newton's rings during scanning procedure was reduced. However, the suitable scan protocol should be used for accurate film dosimetry.
SU‐E‐T‐131: Influence of Scanning Speed on Measurements of Field Flatness and Symmetry of Photon Beams
Authors: Buzurovic I , Gardner S , Studenski M
Published: June 2012

Purpose: The purpose of this study was to investigate influence of different scanning speeds on measurements of photon beam flatness and symmetry. Methods: Commissioning and quality assurance of linear accelerators require extensive beam measurements. To increase efficacy, we evaluated flatness, symmetry and penumbra of 6MV photon beam using the Varian‐TrueBeamTM system. Scanning speeds were 0.3, 0.5, 0.75, 1, 1.5, and 2.5cm/s. Measurements were performed in water phantom (BluePhantom2, IBA‐Dosimetry) at depths of maximum dose, 5,10, and 20cm, for 10×10 cm field size. For each scanning speed and depth, measurements were repeated five times to give results sufficient statistical significance, in both crossline and inline directions. Beam flatness was calculated using variation over mean (80%), whereas symmetry was calculated using point difference quotient (IEC) algorithm. After filed scanning chamber (Wellhofer) was fully stopped, system was paused for stabilization time of 15s to avoid buildup of ripples. Results: It was noticed for all measurements that minimum and maximum flatness and symmetry were recorded when scanning speeds were 0.3cm and 2.5cm, respectively. For depth of maximum dose, maximum flatness and symmetry were 0.82% and 100.58% (crossplane), and 0.94% and 100.96% (inplane). The average was 0.76% and 100.38% (SD 0.04 and 0.12) for crossplane; 0.89% and 100.87% (SD 0.04 and 0.06) for inplane measurements. As the scanning depth increased, flatness and symmetry increased, but SD for all measurements was within the same range (0.04–0.07 and 0.04–0.12). The maximum absolute difference for flatness and symmetry for maximum and minimum speed were 0.16% and 0.34%.However, for scanning speeds from 0.5–1cm/s, results were almost identical with maximum SD 0.03 for both flatness and symmetry. Use of different scanning speeds did not influence penumbra; SD was 0 for all measurements. Conclusions: This study reveals small influence of scanning speed within predefined range. Consequently, difference in measurements does not have clinical significance.
SU‐E‐T‐132: Investigation of Photon and Proton Overlapping Fields in PRESAGE‐ Dosimeters
Authors: Carroll M , Ibbott G , Grant R , Adamovics J , Gillin M
Published: June 2012

Purpose: To evaluate the effects of overlapping dose volumes for varying field arrangements in two formulations of PRESAGE®: one intended for, and irradiated with, proton beams and the other photon beams. Methods: For each treatment modality (photon, proton), three overlapping field setups were performed. These included a stationary dosimeter irradiated over six fractions, a dosimeter shifted laterally to the field to deliver a dose plateau in two fractions, and a dosimeter rotated on its axis to deliver a two‐field (for protons) and four‐field (for photons) box treatment overlapping in the center of the dosimeter. All subsequent fractions were given within ten minutes and never less than one minute apart. Two cylindrical PRESAGE® dosimeters approximately 7.5 cm in length by 7.5 cm in diameter were irradiated for each setup. The dosimeters were paired, with one dosimeter given total dose by a single fraction while the other followed one of the overlapping field setups. The dosimeters were analyzed using an optical CT scanner and exported to the CERR environment where the doses were compared between paired dosimeters. Results: Dose profile comparisons showed relative dose agreement between paired dosimeters within 5% along the SOBP region of the proton formulation. In the case of the fractionated proton irradiation, there was an over‐response while other setups resulted in under‐responses. Dose agreement between the photon dosimeter treated with six fractions showed a dose under‐response within 11% and never less than 5%. Future measurements will include the remaining field setups. Conclusions: The proton formulation of PRESAGE® showed good dose agreement between single and multiple field irradiations. While the photon formulation had slightly less agreement, additional field setup comparisons may show improved results. These results will aid future measurements of overlapping field treatment plans delivered to PRESAGE® for treatment verification for proton and photon 3D dosimetry.
SU‐E‐T‐136: Comparison of TomoScanner™ 2D Water Phantom versus IBA Helix for Tomotherapy Profile Measurements
Authors: Patel B , Syh J , Durci M , Rosen L , Katz S , Wu H
Published: June 2012

Purpose: To compare the differences in measured data using Tomotherapy's TomoScanner™ 2D Water Phantom versus IBA's Blue Phantom Helix taken during commissioning of the Tomotherapy HD unit. Methods: IBA Helix was used with CC04 ion chamber to measure Inline (jaw‐size width), Crossline (40 cm width) and Percent Depth Dose (PDD). Data was analyzed using IBA Omni‐Pro 7.3 software. Measurements were performed at 85 SSD for field sizes 5×40 cm2, 2.5×40 cm2, and 1.0×40 cm2. All field sizes were measured at 1.5 cm (nominal dmax), 5 cm, 10 cm and 15 cm depths. Scans were performed at a continuous speed setting of 0.5 cm/sec. Setup and data measurements were performed twice on separate occasions for consistency and repeatability. Data measurements were normalized to 10 cm depth and compared to commissioning data taken from TomoScanner™ and Tomotherapy's Twinning Data. Results: For all jaws, the profiles measured at 5 cm, 10 cm, and 15 cm depth using IBA Helix matched the profiles of the TomoScanner™ data within 1%. Profile variance at 1.5 cm depth showed a deviation of up to 3%. For all jaws, the PDD comparisons displayed 3–5% deviation from the surface to 2 cm depth and within 1% deviation at 2 cm to 15 cm depth. Measurements performed using CC04 ion chamber versus A1SL ion chamber (used by Standard Imaging) showed data differences of less than 0.5%. Conclusions: Discrepancies in Tomotherapy beam profiles were observed between different water phantoms. Further investigation is required to determine the cause of variances between IBA and Tomotherapy data sets such as investigating geometrical differences between the water tanks and software dissimilarities in collecting and correcting raw data. It is recommended that independent commissioning data be taken when TomoScanner™ is not the clinical site's standard water phantom.
SU‐E‐T‐137: The Response of TLD‐100 in Mixed Fields of Photons and Electrons
Authors: Lawless M , Junell S , Hammer C , DeWerd L
Published: June 2012

Purpose: Thermoluminescent dosimeters are used routinely for dosimetric measurements of photon and electron fields. However, no work has been published characterizing TLDs for use in combined photon and electron fields. This work investigates the response of TLD‐100 (LiF:Mg,Ti) in mixed fields of photon and electron beam qualities. Methods: TLDs were irradiated in a 6 MV photon beam, 6 MeV electron beam, and a NIST traceable cobalt‐60 beam. TLDs were also irradiated in a mixed field of the electron and photon beams. All irradiations were normalized to absorbed dose to water as defined in the AAPM TG‐51 report. The average response per dose (nC/Gy) for each linac beam quality was normalized to the average response per dose of the TLDs irradiated by the cobalt‐60 standard.Irradiations were performed in a water tank and a Virtual Water™ phantom. Two TLD dose calibration curves for determining absorbed dose to water were generated using photon and electron field TLD response data. These individual beam quality dose calibration curves were applied to the TLDs irradiated in the mixed field. Results: The TLD response in the mixed field was less sensitive than the response in the photon field and more sensitive than the response in the electron field. TLD determination of dose in the mixed field using the dose calibration curve generated by TLDs irradiated by photons resulted in an underestimation of the delivered dose, while the use of a dose calibration curve generated using electrons resulted in an overestimation of the delivered dose. Conclusions: The relative response of TLD‐100 in mixed fields fell consistently between the photon nd electron relative responses. When using TLD‐100 in mixed fields, the user must account for this intermediate response to avoid an over‐ or underestimation of the dose due to calibration in a single photon or electron field.