bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐380: Treatment Time Reduction for Proton Modulated Scanning Beams Using a Ridge Filter
Authors: Durgin J , Dolney D , McDonough J
Published: August 2010

Purpose: Proton modulated beam scanning has the potential to significantly reduce dose to normal tissue, especially proximal to the target. However, the relatively long delivery time creates intrafractional motion uncertainty and limits the number of patients who could potentially benefit. To address this problem, a device that reduces the number of required energy layers by spreading the beam longitudinally has been designed. Materials and Methods: Using Geant4 code developed for scanning beam commissioning, a device was modeled within IBA's universal nozzle. Through Monte Carlo results, the shape, material, and location of the device were optimized for distal falloff preservation, dose uniformity, and minimized lateral spreading. Monte Carlo data was then used to commission a commercial treatment planning system. Results: The final design of the device resembled a bar ridge filter. The design is mostly air to preserve distal falloff. A low density plastic was selected for reduced neutron production and large‐angle scatter compared to higher Z materials. The ridge filter was positioned at 50 cm from the isocenter for clinical feasibility, although lateral penumbra decreases at closer distances. Commissioning the test system showed calculated dose delivered to a sphere (d=1.5cm) with the device remained within 3% when compared to an unobstructed beam while the number of energy layers decreased from 14 to 5. Because time switching between energy layers is a major contribution to the overall time, this reduction could approximately double treatment speed. Conclusion: Through Monte Carlo simulations, a device has been designed that significantly reduces the number of energy levels required for modulated beam scanning. The device could potentially increase treatment speed, which would decrease intrafractional motion margins and expand the number of patients being treated.
SU‐GG‐T‐386: Monte Carlo Study of Absorbed Dose to Solid Water for External Auditing
Authors: Kim J , Park J , Park Y , Choi C , Choi Y , Kim H , Kim H , Park S , Cho W , Ye S
Published: August 2010

Purpose: An external audit for radiotherapy has been performed by the third parties to maintain a uniform quality of patient care among different facilities. Among a lot of auditing items, we developed a method to determine absorbed dose to water from Farmer‐type ion‐chamber measurements in solid water within the context of AAPM TG‐51 protocol. The AAPM T‐51 protocol has liquid water as a phantom material for clinical reference dosimetry. Instead of liquid water, the use of a solid phantom is convenient for an external auditing of busy clinics. Method and Materials: Due to different compositions and densities of solid water, correction factor for irradiation geometry and chamber responses are needed to convert measurements in solid water into absorbed dose to water. To avoid the complexity of measurement we kept the reference condition of solid water identical to that of water. The compositions and homogeneity of commercial solid water are varied among different manufactures and not consistent with the compositions provided by manufacturers. Therefore, the compositions of solid water used in this study were experimentally determined by using an electron probe micro‐analyzer (EPMA). The absorbed dose conversion factors for the solid water phantom were measured and, calculated by using the EGSnrc Monte Carlo code system. Results: Measured and calculated conversion factors under the reference condition were in a range of 1.00 – 1.005 for 6–15 MV photon beams, and 1.001 – 1.021 for 6–20 MeV electron beams. The total uncertainty of TG‐51 protocol measurement using solid water was determined to be ±1.5%. Conclusion: The measurement time (including setup time of a solid water phantom and a chamber for several photon and electron beams) was typically less than 30 minutes for external auditing. Conflict of Interest: Research sponsored by Korea Institution of Nuclear Safety Corporation
SU‐GG‐T‐387: Matching of Flattening‐Filter‐Less (FFL) to Standard (STD) Linac Beams Based on Monte Carlo Simulations
Authors: Tsiamas P , Bhagwat M , Seco J , Lyatskaya Y , Kappas K , Theodorou K , Marcus K , Zygmanski P
Published: August 2010

Purpose: Flattening‐Filter‐Less (FFL) linacs increase the dose rate and therefore decrease treatment time, which may minimize the possibility of intra‐fraction motion. However, up till now only a few FFL linac studies have been reported and FFL beam characteristics are not completely known. Method&Materials: This study tries to investigate the impact of the MLC‐modulation for the same prescribed dose using FFL instead of STD linac, by matching two linac penetration (PDD) and dose output (calibration dose per MU for given reference conditions) and by determining the field size and depth dependence of the relative beam intensities across the field and relative MLC properties for the two linacs. We commissioned Monte Carlo (MC) dose calculation for Varian21EX(6MV) standard machine by matching the golden beam dose data (GBD). The electron beam energy was determined to be ESTDEGBD=6.5MeV. Next, beam properties were determined for FFL(EFFL=ESTD). Linac matching procedure was then performed by first matching the PDD by the increasing the energy, and by calculating the MU‐correction factor FMU equal to dose ratio FFLmatch/STD. Next, relative beam intensities FOAR(x,d,c)=OARSTD(x,d,c)/OARFFLmatch(x,d,c) were determined for open beams and parameterized for different field sizes c and depths d. Similar properties were determined for sweeping gap dynamic MLC motion tests for various gap sizes (G=1,5,10,20mm) and other more complex MLC tests. Finally, relative MLC transmission properties were determined FMLC=TSTD/TFFLmatch. Additional beam data was generated for IMRT‐relevant symmetric and asymmetric rectangular fields for open and MLC‐modulated beams for both linacs. Results: Adjusted electron energy for FFL was 8.0MeV to 8.5MeV, depending of the field size. Monitor Unit correction factor was FMU≈1.8 and 3.3 for FFL and FFLmatch linacs correspondingly. The other factors were: FMLC=1.2, FOAR(x,40)=1.003+0.003⋅x2 − 0.000003⋅x4+0.1⋅Erf[0.2⋅x]2 for c=40cm and FOAR(x,c)≈FOAR(x⋅(1+40/c),40) for other field sizes c. Conclusion: Presented parameterization of beam data is essential in analysis of factors influencing IMRT on FFL linacs.
SU‐GG‐T‐393: Optimization Platform Based on Monte Carlo Dose Calculation
Authors: D'Amours M , Dagnault A , Pouliot J , Verhaegen F , Beaulieu L
Published: August 2010

Purpose: Dosimetry and planning for brachytherapy relies on the TG43 formula. This formalism, like any homogeneous dose calculation method, neglects the various effect of heterogeneities such as tissue composition, air, applicators and shielding. This study aims to incorporate Monte Carlo (MC) dose calculation within an inverse planning algorithm in order to improve the dose conformity, thus increasing the quality of the treatment. Material an Methods: Dose calculation and optimization is based on pre‐calculated Dose Distribution Kernel (DK). A DK represent the dose distribution produced by the irradiation of a single source located at one dwell position. The DKs are calculated using an automated CT based Monte Carlo software using the Geant4 toolkit. A research version of the inverse planning software IPSA was modified in order to read the set of DKs and use these dosimetric informations instead of the TG43 formula to generate the plan. This method was tested for two HDR brachytherapy sources: the microSelectron 192Ir source and the Axxent electronic source at 50 kVp. Results: The impact of the water approximation is found to depend on photon energy, with higher dose attenuation for the lower energies of the Axxent source in comparison with 192Ir with an underdosage of 5.4 % versus 2 % on the CTV V100. Incorporating Monte Carlo DK into the planning is found to correct the dose coverage for both energies. Conclusion: A new method to optimize high dose rate (HDR) brachytherapy plans that uses MC dosimetric information was developed. Including CT‐based information in MC dosimetry in the inverse planning process is shown to take into account the full range of scatter and heterogeneity conditions. This method can therefore predict and improve the dose coverage and homogeneity of clinical cases.
SU‐GG‐T‐398: Monte Carlo Commissioning of BrainLab SRS Cones on a NovalisTX
Authors: Stathakis S , Mihailidis D , Esquivel C , Gutiérrez A , Mavroidis P , Shi C , Papanikolaou N
Published: August 2010

Purpose: To commission a Monte Carlo model of the high dose rate 6MV photon beam of the Varian NovalisTX linear accelerator for patient dose calculations. Methods and Materials: The high dose rate 6 MV beam on the Varian NovalisTX accelerator is utilized for stereotactic radiosurgery (SRS) treatment. The SRS beams are confined by cone collimators and the available collimator sizes range from 5 and 10 to 30 mm, in every 2 mm increment. Furthermore, the linear accelerator is equipped with multileaf collimator with MLC leaf width of 2.5mm. It is necessary to investigate an accurate and detector independent method for the narrow beam output factor measurement. In this study, the beam data was collected using a microLion chamber (vol. 0.002cc) and used to commission the linac model and all the SRS cones. Film measurement were used to measure the MLC transmission and leakage. Results: Monte Carlo results are found to be in excellent agreement with the measurements for the open, and cone beam fields. Measured output factors were in agreement with Monte Carlo results but there was a discrepancy against the the VARIAN recommended output factors measured directly by Wellhöfer CC01 chamber and Scanditronix photon field diode which are consistently higher for all the cones. Conclusions: A Monte Carlo model of the NovalisTX high dose rate SRS beam has been created that is able to simulate the beam with high accuracy. Beam parameters that are otherwise difficult or detector dependent can be determined using the Monte Carlo method.
SU‐GG‐T‐401: The Effects on Parameters of Bragg Peak Curve by Inhomogeneities Location
Authors: Park S , Rah J , Park S , Suh T , Jung W , Shin J
Published: August 2010

Purpose: To describe the effect on the Bragg peak curve by inhomogeneous materials placed in entrance Plateau and Bragg peak region, respectively. Method and Materials: We evaluated Bragg peak position, range, penumbra and doses by using the Geant4, version4.9.02, simulation toolkit. The bone and adipose tissue were inserted in entrance Plateau and Bragg peak region. Positions of inhomogeneities were selected about proximal 36% (entrance region of Plateau) and 50% (entrance region of Bragg peak) point of maximum dose. The thickness of each material was varied with range from 0.1 and 1.0 cm. And also the initial energy of proton beams was changed from 108 MeV to 220 MeV with an order of 30 MeV. Physics models of Geant4 consist of electro‐magnetic process and nuclear process. The electro‐magnetic model bases on the ICRU49 and nuclear process was applied by the theory‐driven G4PreCompound model. Results: The dose perturbation was shown in Bragg peak and inhomogeneous material regions concerning the location of inhomogeneities. There were, especially, a great differences when material of high density was inserted within homogeneous structure. However, there were not significantly variations of other evaluated parameters concerned with inserted location of each material. Conclusion: Effects of location of inhomogeneous material were principally shown to the dose variation. Our study suggests that the location of heterogeneous materials mainly affects dose variation when material of high density is inserted within homogeneous structure. These highlight the need for accurate location relations between materials with high density and target within treatment path.
SU‐GG‐T‐402: On Monte Carlo Modeling of Megavoltage Photon Beams: A Revisited Study on Beam Parameters Sensitivity
Authors: Chibani O , Moftah B , Ma C
Published: August 2010

Purpose: To commission Monte Carlo beam models for 5 Varian megavoltage photon beams (4, 6, 10, 15 and 18 MV). The goal is to match measured dose distributions in a water phantom within 1% local difference for a wide range of field sizes. Another objective is to re‐investigate the sensitivity of dose distributions to the primary electron beam parameters. Material and Methods: The GEPTS Monte Carlo code is used for photon beam simulation and dose calculations. Linac geometry models are based on (i) manufacturer specifications, (ii) corrections made by Chibani and Ma (Med. Phys. Vol. 34, 2007), and (iii) recent drawings of the W/Pb shields. Measurements were performed using pinpoint and Farmer ionization chambers depending on the field size. In addition to the three commonly used primary electron beam parameters (EAV: average energy, FWHM: energy spectrum broadening, and R: beam radius), we also included the primary electron angular divergence (q). Results: We were able to match within 1% local difference measured depth dose distributions and lateral dose profiles (within 1%/1mm in penumbra), at any depth beyond 1 cm. Varying primary beam parameters away from the solution can lead to 5% differences with measurements for small (e.g. 2×2 cm2) and large (e.g. 35×35 cm2) field size WHILE a perfect agreement still stands for 10×10. Very small and very large field sizes prove to be more sensitive to FWHM, R and q variations than the 10×10 field size. Conclusion: GEPTS beam models reproduce measured dose distributions within 1% local difference and simultaneously for field sizes varying from 2×2 to 35×35. Achieving 1% agreement between measured and calculated dose distributions for the 10×10 field size is relatively easy. However, this does not guaranty a similar agreement for other field sizes. Therefore, Monte Carlo beam models need to be commissioned for multiple field sizes..
SU‐GG‐T‐404: Demonstration of the Ability to Morph an Existing Whole‐Body Phantom to a Different Individual
Authors: Na Y , Ding A , Caracappa P , Xu X
Published: August 2010

Purpose: Although patient‐specific partial‐body phantoms can be created from CT data, whole‐body phantoms are desired for the assessment of organ doses in CT, internal nuclear medicine and external‐beam radiation treatment procedures. This study proposes an innovative strategy for developing patient‐specific whole‐body phantoms by morphing a pair of deformable phantoms. Adjustments were made to the original data to make these phantoms compatible with anatomical parameters and the tissue/organ surfaces matching the values specified by patient‐specific image data. Method and Materials: In order to match tissue/organ surfaces to the patient‐specific data, three different adjustment approaches to the mesh deformation are considered as follows: (1) uniform scale factor adjustment: displacement of each vertex point by a uniform scale factor in relation to the organ centroid; (2) unique scalar factor adjustment: translation of each vertex point along the direction of vertex normals by a unique scalar factor; (3) mesh‐based deformation: tracking of mesh surface geometry by a union of direction vectors at the center of each vertex of the polygon mesh surfaces using ray casting algorithm. This approach is mainly used for deforming the primary organ meshes to patient‐specific organs collected from scanned image data through the general radiological modalities. The new mesh‐based phantoms need to be converted to voxels with correct tissue density and elemental composition information for Monte Carlo dose calculations. Results: The final organ volume and mass values of the RPI patient‐specific phantoms in mesh representation have an acceptable relative error, less than 0.5%, according to the acceptance criteria in mesh deformation algorithms. To accomplish acceptable results, several iterations were required for some of the internal organs. Conclusion: This study has demonstrated patient‐specific computational phantom modeling methods with the feasibility of the size‐adjustable RPI‐AM and RPI‐AF base phantoms. The anatomical details were accurately and consistently persevered through this approach.
SU‐GG‐T‐419: Modeling Multiple Coulomb Scattering in Therapeutic Proton Beams: A Comparison of Two Algorithms
Authors: Rechner L , Mirkovic D , Fontenot J , Newhauser W
Published: August 2010

Purpose: Accurate modeling of multiple Coulomb scattering (MCS) of heavy charged particles is imperative to achieve accurate dose distributions for proton therapy simulations. The Gaussian approximation based on Rossi's theory used in the Monte Carlo N Particle extended (MCNPX) code overestimates the characteristic scattering angle for thin materials. The purpose of this research was to evaluate a new, more accurate MCS algorithm based on Moliere theory that has been implemented in MCNPX. Method and Materials: Simulations of a proton beam with a range in water of 16.1 cm incident on steel spheres of diameters 6.4 mm, 9.5 mm, and 15.9 mm were run and compared to previous simulations completed with the old algorithm and to measured data. Simulated dose was tallied in air at the locations of the film measurements, which were 2 cm, 15 cm, 30 cm, and 63 cm distal to the spheres. Results: When compared to the old MCS algorithm, the new MCS algorithm more accurately modeled the maximum dose enhancement due to scattering 58% of the time and the maximum dose shadow due to scattering 83% of the time. Visual inspection of the dose profiles distal to the steel spheres reveals the consistent improvement in the dose distribution resulting from the new algorithm. Conclusion: The new MCS algorithm appears to be more accurate than the old algorithm. In order to further characterize the algorithm's performance, additional simulations with a simpler geometry and comparisons with measurements are required.
SU‐GG‐T‐421: Study of Response of Plastic Scintillation Detectors in Small Field 6 MV Photon Beams
Authors: Wang L , Beddar S
Published: August 2010

Purpose: The plastic scintillation detector (PSD) has many advantages over other detectors in small field dosimetry due to its high spatial resolution, excellent water equivalency and instantaneous readout. This study investigates the relative response of PSDs in a 6 MV photon beam for various field sizes by Monte Carlo simulations. Method and Materials: Two PSDs are modeled: BC‐400 (f1mm×4mm) and BCF‐12 (f1mm×2mm). A plastic optical fiber stem of length 3 cm coupled to the detectors is included in the simulation. Two orientations of the PSD are studied when positioning the detector at 10 cm depth in the 6 MV beam: (1) the axis of the PSD is perpendicular to the incident beam; (2) the axis of the PSD is parallel to the incident beam and the tip of the detector is pointing to the radiation source. The response of the detectors is calculated as the detector dose per unit water dose for various field sizes ranging from 10×10 cm2 down to 0.5×0.5 cm2. With the parallel orientation of the PSD, the off‐axis dose profiles in the 0.5×0.5 cm2 photon beam are calculated from the simulation results of the PSDs and compared to the dose profile from a CC01 chamber (cavity only) simulation and the dose profile calculated in water only. Results: In perpendicular orientation, the response of BCF‐12 shows only 0.5 % variation from 10×10 cm2 to 0.5×0.5 cm2, while the response of BC‐400 shows more than 3 % deviation at 0.5×0.5 cm2 due to longer sensitive region. In parallel orientation of PSD, the response of both PSDs varies less than 0.4 % for the same range of field sizes. The dose profiles from PSD simulations (parallel orientation) are very close to that of water. Conclusion: PSDs are excellent dosimeters for small field measurement. Supported by the NCI (1R01CA120198‐01A2)
SU‐GG‐T‐422: Evaluation of the Variability in Peripheral Dose between Different Linacs for 6MV Beams and Comparison with an Existing Monte Carlo Linac Model
Authors: Joosten A , Levi F , Baechler S , Mirimanoff R , Bochud F , Moeckli R
Published: August 2010

Purpose: To determine the variability in peripheral dose due to different linac head designs for a given beam energy and to compare it with an existing Monte Carlo (MC) model. Method and Materials: Peripheral doses were measured for four Linacs with different head designs at 6 MV. Peripheral doses were measured for open square fields ranging from 5×5 cm2 up to 20×20 cm2 at 10 cm depth. The variability in measured peripheral doses was determined as a function of field size and distance from the beam edge. An existing MC model simulating the Siemens Primus Linac was used to simulate the same measurements. The behavior of the MC doses with respect to the minimum and maximum measured doses for a given field size and distance from the beam edge were determined. Results: The variability in peripheral dose ranged from 20% for the 20×20 cm2 field up to over 100% for the 5×5 cm2 field. The MC doses were mostly within the measured profiles for field sizes ≥ 10×10 cm2. However, for small field sizes and large distances from the beam edge (>10 cm), Monte Carlo leans towards the lowest measured doses. This is due to incomplete modeling of head leakage which dominates for small fields and large distances from the beam edge. Conclusion: Uncertainties in MC doses due to the variability in peripheral dose between different linacs operating at 6MV were derived. MC tended towards the lowest measured doses and some fine tuning of the MC model will be needed to fit the average of the measured linacs.
SU‐GG‐T‐424: Domain‐Division Monte Carlo Dose Calculation Method for Particle Therapy
Authors: Ishikawa KL , Niita K , Takeda K , Fukunishi N , Takagi S
Published: August 2010

Purpose: To enable dose calculations with huge voxel data a limited amount of memory. To accomplish this, we develop a Monte Carlo method using domain division. Method and Materials: We divide the entire geometry into several domains whose data size is such that Monte‐Carlo simulations can be performed with a given amount of memory. We first perform a simulation for the domain to which the particle beam is incident and dump the information of exiting particles to files. Then, we perform simulations for the neighboring domains using the dump files as a source and, again, dump the information of exiting particles. Then, again, we perform simulations for the neighboring domains using the dump files as a source. We repeat similar procedures till there are no dumped particles left. We developed and parallelized an external module to the Monte Carlo particle and heavy ion transport code PHITS that tallies the dose by repeating these procedures. Results: We are evaluating the performance of the domain‐division Monte Carlo method using human voxel phantoms. We have confirmed that the present method obtains the same results as the conventional Monte Carlo method for the entire geometry. The increase in calculation time due to dumping and re‐reading is only 8 %, which is acceptably small. We have also obtained excellent scaling up to 200 processors so far. Conclusion: The method we have developed is expected to open a way to the realization of large‐scale Monte Carlo dose calculations with huge voxel data on general‐purpose supercomputers and in a grid environment, where only a limited amount of memory is available, instead of on specialized large‐memory machines.
SU‐GG‐T‐426: Feasibility of an Independent Dose Calculation for VMAT with Monte Carlo
Authors: Quino L Vazquez
Published: August 2010

Purpose: To investigate the feasibility of a Monte Carlo based independent dose calculation for VMAT in a Novalis Tx linear accelerator equipped with high definition MLC. Method and Materials: Percent depth dose curves were calculated and compared against measured ones in water to commissioning the linear accelerator built on BEAMNRC. From the DICOM containing the information of the VMAT plan all the parameters needed for the simulation were extracted using a MATLAB in‐house software, which with this data create the files for the Monte Carlo simulation and is exported to a LINUX environment where the simulation is done in MCSim egs4 Monte Carlo code. MCSim egs4 software was used to simulate the VMAT treatment plans, in which a cylindrical phantom (15 cm radius and 18 cm depth) is used to compute the dose. In‐house software using a MATLAB platform was utilized to convert the 3D dose file obtained from the Monte Carlo simulation to a file which can be read using commercially‐available software for dose visualization (RIT software), in which can be compared with the treatment plan exported form the TPS. Results: Several simulations in the MCSim egs4 platform were performed and good results have been observed. The accuracy of the calculation is still low but the main purpose of this preliminary work is just for demonstrating the feasibility of the VMAT calculation in Monte Carlo software. Conclusion: Based on the results, the Monte Carlo based NovalisTx linear accelerator and the MCSim simulations of treatment on a cylindrical phantom it was demonstrate the feasibility of VMAT independent dose calculation in Monte Carlo.
SU‐GG‐T‐430: Normalized Bragg Peak Curves for Various Proton Energies in a Cylindrical Water Phantom: A Simulation with MCNPX and GEANT4 Monte Carlo Codes
Authors: Handley S , Chen Y , Ahmad S
Published: August 2010

Purpose: To simulate proton interaction (energy from 60 to 250 MeV) in water using MCNPX version 2.7B Monte Carlo code to determine accurately proton ranges from Bragg peak. Method and Materials: A cylindrical water phantom (100cm length, 30cm radius), composed of 1000 packed circular sensitive detector discs (2cm radius, 1 mm thick) was used. A pencil beam source was incident on the surface of the cylinder in z direction. Tabular nuclear data from LA150N and LA150H libraries were used for neutrons and protons with <150 MeV. For >150 MeV, the LAHET physics code was used and the hadronic interactions were handled using the Bertini cascade model. The Vavilov model was used to account for charged‐particle straggling. Normal transport was performed for neutron, proton, photon, electron, deuteron, triton, helium‐3, alpha, and heavy ions. Light ion recoil was accounted for in the simulations and the cutoff energy for these particles was set to zero to prevent early termination of their transport. A 5cm × 5cm field size beam was also simulated. A total of 80,000 source histories were run for each simulation. Eleven source energies were tested for the pencil beam and three source energies were studied for the 5cm × 5cm field size beam. Results: Our MCNPX results were compared with GEANT4 results as well as the Continuing Slowing Down Approximation (CSDA) data from the United States National Institute of Standards and Technologies (NIST). The deviation in Bragg peak range was <1% of that calculated with the GEANT4 and <0.6% of the CSDA data. The entrance to peak dose ratios between the different codes were comparable for 150 and 200 MeV though larger difference was noted for 250 MeV beam. Conclusions: Our proton Bragg peak simulation results using MCNPX are in excellent agreement with GEANT4 as well as NIST data.
SU‐GG‐T‐431: Design of a Graphical User Interface Aiding Tool for Monte Carlo Simulation Tuning Process for Electron Beam Modeling
Authors: Eldib A , Moez MA , Fan J , Ma C
Published: August 2010

Purpose: It is usually difficult to obtain all the geometric details of a clinical accelerator necessary for accurate modeling of the treatment head. This work is aimed at designing a program that could provide, based on measurements, possible reasons for the dosimetric discrepancy and possible solutions based on empirical equations that are implemented in the program. Material and Methods: In this work, Monte Carlo simulations were performed for all available electron energies. MCBEAM and MCSIM Monte Carlo codes were used for treatment head simulation and phantom dose calculation. As the incident electron energy is the primary tuning parameter for an electron beam several monoenergies were simulated using Monte Carlo. A series of empirical equations were created that govern several beam parameters (R90%, R80%, R50%, R20%, width at 80% of maximum dose) as a function of energy. The empirical equations were then incorporated in a graphic user interface (GUI) program, which allows for the input of the measured and calculated percentage depth dose curves and profiles. The program could then suggest what should be the next step in the tuning process. By analysis of the measured curve, the program could predict the need for a spectrum to represent the electron beam source. An iterative search was then done to find the best spectrum that would represent the electron source. Results: Applying the equation for a measured 6MeV beam predicted that a spectrum of energies is needed to represent the electron source. The iterative search was able to predict the energy spectrum with great accuracy. Conclusion: Our GUI‐based program can be a helpful tool for Monte Carlo beam modeling to facilitate the clinical implementation of Monte Carlo simulations for radiotherapy treatment planning and dosimetry verification. It can reduce the modeling time significantly for less experienced Monte Carlo users
SU‐GG‐T‐439: Linac Waveguide Performance in the Presence of Parallel MR Fringe Fields
Authors: Santos DM , St. Aubin J , Fallone B , Steciw S
Published: August 2010

Purpose: To investigate the effects of magnetic fields which are parallel to a Linac waveguide and similar to those present in our Linac‐MR system, on a Linac waveguide's performance. Method and Materials: Parallel homogeneous and MR‐like magnetic fields of various strengths (29G – 144G) were applied to a 3D waveguide model emulating the Varian 600C Linac, previously developed by our group. MR‐like fringe fields were approximated using a pair of 3D analytic current loops, whose dimensions and current were optimized to match the field strengths from the MR imager. PARMELA, a particle tracking software, was then used to evaluate the magnetic field effects on the waveguide's performance. The electron beam current, energy, FWHM, and center‐of‐mass (COM) position at the target were investigated as a function of field strength. Results: For parallel fields, an increase in the beam loss in the waveguide was observed. Parallel fields caused an added beam loss of 0.269%/G, while MR‐like fields caused a 0.273%/G beam loss over the field range investigated, where we have demonstrated that most of the beam loss occurs in the electron gun. The energy and COM position of the electrons incident on the target is insensitive to changes in field strength, however there is a large effect on the distribution's FWHM. The FWHM increases from 0.12mm at 0G, to approximately 3.4mm at 125G, at a rate of 0.024mm/G. Conclusions: Our simulations have shown that under large parallel magnetic fields similar to those present in a Linac‐MR system, a Varian 600C Linac is operational. There is however an effect on the percent beam loss and FWHM of the electron beam at the target, where the beam loss occures mostly at the electron gun. Negligable effects are seen on the electron energy and COM position at the target.
SU‐GG‐T‐443: Benchmarking MCNPX for Carbon Ion Beams
Authors: Solberg T , Pompos A
Published: August 2010

Purpose: To investigate the accuracy of carbon ion beam dose deposition simulated with the MCNPX Monte Carlo code. Method and Materials: The capability for heavy ion transport was added to the MCNPX code beginning in v2.6, but to date there has been no benchmarking for use in radiotherapy applications. We have simulated the interactions of 12C6+ ions in the energy range 130 – 430MeV/u using three versions of MCNPX, the 2.7b and 2.7c released versions, and a modified 2.7b_m version. The phantom consisted of a cubic PMMA container, 1.6mm thick, filled with water. The depth dose data obtained were normalized to their respective entrance doses and were compared to published experimental data. Results: We have evaluated the position and height of the Bragg peaks, curve shapes and exit doses. v2.7b did not generate projectile fragments while v2.7b_m did. While v2.7c calculates fragmentation in a manner similar to v2.7b_m, there are still substantial deviations from experimental data in the tail and in the region 6 mm proximal to Bragg peak. The positioning of the Bragg peaks and their heights were found to be acceptable. In the case of the 330MeV/u 12C6+ beam, for example, the calculated peak locations coincided with measurements within available experimental uncertainties, and relative heights differed by less than 10%. At 6mm beyond the peak the predicted dose was almost 50% lower than the measured value and the simulated curve's width was larger by 2mm at the FWHM. Conclusions: The discrepancy in the tail of the distributions indicates that the released transport codes insufficiently model target and/or projectile fragmentation and transport. The discrepancy in FWHM suggests slightly incorrect stopping powers which might be due to the water ionization potential employed in the code. However, this is contraindicated by the good positioning of the peaks and is being investigated.
SU‐GG‐T‐444: Normal Tissue Complication Probability (NTCP) Modeling Using Self‐Organizing Map (SOM)
Authors: Huang E , Bradley J , El Naqa I , Pesce L , Deasy J
Published: August 2010

Purpose: To investigate the possibility using Self‐Organizing Map to predict Normal Tissue Complication Probability. Materials and Methods: As an effective method to project and visual the high‐dimensional data to low‐dimensional data, the SOM is used to predict radiation pneumonitis (RP) risk according to similarity of patient's characteristics. 209 WUSTL lung cancer patients were available for SOM. The SOM models from lung dose volume metrics (SOMlung), heart dose volume metrics (SOMheart), GTV position (SOMGTVposition) and clinical factors (SOMclinical), separately or jointly were built and the model statistical significance was tested using Receiver Operating Characteristics (ROC) curve. The input feature selection is done by comparing difference of resulted ROC areas from including or excluding input feature. Results: Initial input features are selected from hundred dose factors with correlation <0.85 to present selection of multiple highly correlated dose factors as features. Among the input features selected by SOMlung, mean lung dose (MLD) has highest impact for increasing RP risk, area under curve (AUC) of ROC decrement resulted from its exclusion is from 0.8728 to .8539 (p=0.7575) ; for SOMheart, D10heart has highest input impact for increasing RP risk, exclusion will result AUC drop from 0.9198 to 0.8771 (p=0.0442.); for SOMclinical, gtvVol has highest impact for increasing RP risk, AUC drop from 0.7810 to 0.7378 (p=0.00001); for SOMGTVposition, COMSI (GTV inferior‐superior position) has highest input impact for increasing RP risk, AUC drop resulted from its exclusion from 0.8942 to 0.8147 (p= 0.0402.). By adding clinical factor into dose model, the AUC will drop from 0.9027 to 0.8438, implying non‐dose factors don't add important information to predict RP risk. Conclusion: SOM appears to be a robust method to identify predictors of RP risk. Among selected input features, MLD, D10heart, and COMSI are three most important features contributing to increasing RP risk in our institutional dataset.
SU‐GG‐T‐448: Beam Parameters for Gated Irradiations of Moving Targets in Scanned Particle Therapy
Authors: Steidl P , Richter D , Schuy C , Saito N , Naumann J , Brons S , Panse R , Haberer T , Durante M , Bert C
Published: August 2010

Purpose: To investigate gating parameters for gated irradiation of moving targets with a scanned ion beam using a 4D motion phantom. Method and Materials: For a scanned particle beam the residual motion within the gating window causes interplay which leads to unacceptable under‐ and overdose. Beam parameters (residual motion, beam spot size, and raster grid spacing) have to be chosen such that the increased overlap (spot size / grid spacing) of neighboring pencil beams mitigates the dosimetric impact of the residual motion. Due to the impossibility of measuring all potential cases that can occur for a given gating parameter set (e.g. different motion amplitudes and starting phases), the optimal parameter set is determined in simulations. The result of the simulations is then validated by a set of representative measurements. We performed experiments with 1D motion and studied the influence of different beam overlaps (spot size vs. grid spacing) with respect to the dosimetric impact of residual motion amplitudes. Films and an array of ionization chambers were used as detectors. As a preparation of measurements with 3D motion, we constructed a thorax‐phantom that simulates 3D internal tumor motion using a robotic arm and further allows independent control of surface motion in order to study robustness against e.g. baseline drifts. Results: A beam overlap of 6.7 was less sensitive to residual motion than an overlap of 3. The robotic phantom moves reproducible with a spatial precision of < 0.3 mm Conclusion: A framework for beam parameter definition for gated treatments with a scanned particle beam has been set up and tested in initial experiments. Conflict of Interest: Research sponsored by German Research Foundation (DFG).
SU‐GG‐T‐514: Dosimetric Measurements of Onyx Embolization Glue for Stereotactic Radiosurgery
Authors: Roberts D , Chaudhary N , Pandey A , Balter J
Published: August 2010

Purpose: Arterio‐venous malformations (AVM) are often treated with a combination of embolization and stereotactic radiosurgery (SRS). Concern has been expressed in the past regarding the dosimetric properties of glues used in embolization and the effects that the introduction of these glues into the brain may have on the quality of the radiosurgery plan. To quantify these effects we have taken large volumes of Onyx©35 and Onyx©18 (ethylene‐vinyl alcohol copolymer doped with tantalum) and measured the attenuation and interface effects of these glues. Methods and Materials: The manufacturer provided large cured volumes (∼28 cc) of both Onyx glues. These samples were 8.5 cm in diameter with a nominal thickness of 5 mm. The samples were placed on a block tray above a stack of solid water with an Attix chamber at a depth of 5 cm within the stack. The Attix chamber was used to measure the attenuation. These measurements were made for 6 MV and 16 MV beams. The interface effects were measured by placing the sample directly on the solid water stack and varying the thickness of solid water between the sample and the Attix chamber. CT numbers of approximately 2700 were measured for both glues. Results: The transmission through the glues relative to solid water was approximately 98% and 97% for 16 MV and 6 MV beams respectively. The interface effect shows an enhancement of approximately 2% and 1% downstream for a 16 MV and 6 MV beams respectively. The largest effect that must be considered is the correction for the apparent attenuation due to the large CT numbers of the glue. Conclusions: The introduction of embolization glues has negligible affect on the dose distribution of MV therapeutic beams. These results are in quantitative agreement with previous Monte Carlo calculations by others. Research sponsored by the MTI Corporation.
SU‐GG‐T‐516: Investigation of Large Discrepancies in Dose‐Rates of Gamma‐Knife Units at Various Institutions
Authors: Shiu A , Drzymala R , Alvarez P , Followill D , Tailor R
Published: August 2010

Purpose: Investigate the culprits responsible for large discrepancies in dose rates of Gamma‐knife units as measured by the RPC‐TLD. Materials and Method: Measurements, conducted at the Advanced‐Medicine Center, Washington University (WU) and MD Anderson (MDA) Cancer Center, involved combinations of different (i) ion‐chamber types, (ii) ion‐chamber planes, and (iii) calibration phantoms/calibration protocols (TG21, TG51). Three ion chambers: Capintec PR‐05 (WU‐Ch‐C), Exradin A‐16 (WU‐Ch‐E), and PTW TN13010 (MDA‐Ch‐P) were employed. Four phantoms were: WU's indigenous water phantom, Elekta AB‐plastic spheres belonging to WU (WU‐Ph‐E) and MDA (MDA‐Ph‐E), and Elekta SW (SW‐Ph‐E) sphere. In addition, RPC‐TLD measurements were performed in the SW‐Ph‐E sphere to provide an independent check of dose rate. Results: All measurements were made with 16‐mm collimator. Dose‐rate comparisons are normalized to the TG21 calibration with the MDA‐Ch‐P in the MDA‐Ph‐E sphere. The TG51 dose‐rate measured with the MDA‐Ch‐P in the SW‐Ph‐E sphere agrees very well with the RPC‐TLD result. Moreover, the TG21 dose rate with the same chamber in the MDA‐Ph‐E sphere provides TG51/TG21 to be 1.009 which is also in good agreement with the established ratio. Surprisingly, the TG21 dose‐rate measured with the WU‐Ch‐C in the WU‐Ph‐E is found to be 2.4% higher than that measured with the MDA‐Ch‐P chamber in the similar phantom MDA‐Ph‐E. This is consistent with the discrepancy seen by the RPC‐TLD in the past. Impact of variation in composition among the two AB‐plastic spheres (WU‐Ph‐E and MDA‐Ph‐E) on dose‐rate measurement is seen to be negligible (<0.5%). Conclusion: The measured 2.4% difference in TG21 dose‐rate appears to be due to combination of difference in ion‐chamber type and its orientation. Compounding this with ∼1% due to the protocol difference (TG51 versus TG‐21) explains the past RPC‐TLD results. Our measurements rule out any significant contribution due to variation in phantom composition.
SU‐GG‐T‐517: Dosimetric Characteristics of the Small Diameter BrainLab Cones for Stereotactic Radiosurgery
Authors: Khelashvili G , Turian J , Chu J
Published: August 2010

Purpose: To study the dosimetric characteristics of the small diameter (≤10.0mm) BrainLab cones used for stereotactic radiosurgery (SRS) treatments in conjunction with a Varian Trilogy accelerator. Method and Materials: Required accuracy and precision in dose delivery during SRS can be achieved only when the geometric and dosimetric characteristics of the small radiation fields is completely understood. Although a number of investigators have published the dosimetric characteristics of SRS cones, to our knowledge, there is no generally accepted value for the relative output factor (ROF) for the 5.0mm diameter cone. Therefore we have investigated the dosimetric properties of the small (≤10.0mm) diameter BrainLab SRS cones used in conjunction with the iPlan TPS and a Trilogy linear accelerator with a SRS beam mode. Percentage depth dose (PDD), off‐axis ratios (OAR) and ROF were measured using a SRS diode and verified with Monte Carlo (MC) simulations. The dependence of PDD, OAR and ROF on the alignment of the beam CAX with the detector motion line was also investigated using MC simulations. Results: An agreement of 1% and 1mm was observed between measurements and MC for PDD and OAR. The calculated ROF for the 5.0mm diameter cone was 0.695±0.008 in good agreement with the measured value of 0.711. For 7.5 and 10.0mm diameter cones the agreement was within 0.5%. Simulations of the misalignment of the beam axis and detector motion axis for angles between 0.5°–1.0° show a deviation >2% in PPD beyond certain depth. Conclusions: Monte Carlo calculated ROF values for 5.0 mm diameter cone agrees with our measurement to within 2.3%. Care should be exercised when measuring PDD and OAR for small cones. We recommend the use of MC to confirm the measurement under these conditions.
SU‐GG‐T‐518: Homogeneity and Conformity Comparison of RapidArc and IMRS Radiosurgery for Multiple Brain Metastases
Authors: Wang J , Pawlicki T , Rice R , Mundt A , Murphy K
Published: August 2010

Purpose: This study reports the dosimetric comparison of RapidArc (RA) and IMRS in treating multiple intracranial metastases. The pros and cons of the RapidArc plans were evaluated. Method and Materials: Twelve patients having 2–12 brain metastases treated by IMRS were re‐planned with RapidArc. The prescribed dose (PD) was 18–20 Gy, normalized to 98% of PTV. The IMRS‐plans used 9–11 IMRT beams. The RapidArc plans adopted a 2‐coplanar arc approach and followed systematic strategies in optimization. An optimal RA‐plan was selected from several trials for each patient. The maximal PTV doses (MD) and the volumes V100 and V50 enclosed by 100% and 50% of PD were investigated. Results: Homogeneity index (MDPD), conformity index (CI), and gradient index (GI) were calculated for all plans. The mean dose to normal brain and the maximal doses to brainstem, chiasm, eyes, and optic nerves were analyzed. It is found that the homogeneity of both plans is comparable, while the conformity and gradient indices are slightly higher in RA as compared to those in IMRS plans. The group averages are: MDPD RA, IMRS =1.15, (range 1.1–1.2); CIRA=1.6 (range 1.1–2.0) and CI IMRS= 1.3 (range 1.1–1.4); GIRA = 2.3 (range 1.4–3.8) and GIIMRS = 2.0 (range 1.3–3.2). For organs at risk (OAR), the RA showed slightly larger doses than those in the IMRS plans. Conclusion: Apart from the benefit of MU reduction, RapidArc produced similar dose homogeneity as IMRS, owing to the same emphasis on PTV dose coverage in the optimization. The slightly higher conformity in RA is due to the difference of V100 in two plans, which was not adjustable in the optimization. The same observation is true for the gradient index. The leakage and transmission from the MLCs may contribute to OAR doses in RA, as larger field sizes are used for multiple brain lesions.
SU‐GG‐T‐519: Improving Gamma Knife Plans Using Independent Sector Weighting
Authors: Hwang A , Sneed P , Barani I , Nakamura JL , Barbaro NM , Ma L
Published: August 2010

Purpose: The Gamma Knife Perfexion uses 192 Co‐60 sources divided into eight sectors. For each treatment isocenter, each sector can be either blocked or collimated using one of three collimator sizes, but the beam‐on time is identical for all sectors. We postulated that independently variable sector weighting might confer dosimetric advantages with regard to critical structure sparing. In this work, we investigate the potential advantages of independently varying the weighting of individual source sectors to create sector modulated Gamma Knife (SMoG) plans. Materials and Methods: We simulated independent sector weighting for a clinical Gamma Knife plan for a trigeminal neuralgia case. We first obtained dose matrices corresponding to the contribution of each sector. The sector weighting was varied in eight discrete levels and an optimum combination determined using an optimization algorithm. The resulting sector modulated Gamma Knife plan was then entered into the Gamma Knife treatment planning system by decomposing the weighted sectors into multiple shots at the same location with different weights and blocking. The process was repeated using four levels of sector weighting. Results: Our results showed that the optimized plan reduced the maximum dose to the organ at risk (the brainstem) from 18.6 Gy to 11.8 Gy (with eight levels) compared to the clinical plan. However, the beam‐on time for delivery increased from 32 minutes to 84 minutes. The SMoG plan with four sector weighting levels reduced the maximum brainstem dose to 12.6 Gy and only increased treatment beam‐on time to 42 minutes. Conclusions: We have demonstrated that independent weighting of the individual sectors in the Gamma Knife Perfexion can improve the quality of treatment plans, although at the cost of longer delivery times.