bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐183: Measuring the Spot Size of a Proton Therapy Scanning Nozzle
Authors: Bues M , Ciangaru G , Gaza R , Matsuda K , Gillin M , Smith A
Published: July 2008

Purpose: Spot size, i.e. the lateral extent of the proton pencil beams used to compose a patient treatment field, is an important characteristic of a proton therapy scanning nozzle. Among other things, the spot size determines the lateral penumbra that may be achieved through pencil beam weight optimization. In this paper we describe measurements of spot size made on a synchrotron based proton therapy scanning system located at the University of Texas MD Anderson Cancer Center, in Houston. Method and Materials: Spots size is measured using three different techniques: pinpoint chamber, EBT film, and Lanex screen with CCD camera. Spot size is measured by fitting a Gaussian distribution to the measured data. The Lanex CCD system consists of a Lanex screen, enclosed in a light tight box, which is being observed with a CCD camera by way of a Mylar mirror. (See Figure 1.) Software was written in order to allow for an efficient acquisition of data under the condition of a pulsed proton beam. Results: In all three techniques spot size was measured at isocenter, in air. All three systems were found to give the same result within the margins of uncertainty for each measurement technique. Spot size was found to strongly depend on energy. Furthermore the spot proved not to be circularly symmetric but rather to have a larger extent in the cross plane direction. See Figure 2. We also found a non‐Gaussian component to the beam spot, consistent with Molier's theory of multiple Coulomb scattering. (See Figure 3.) Conclusion: We are able to characterize the spot size of a proton therapy scanning nozzle using three different techniques. The three different techniques differ in signal to noise ratio and in the time it takes to take the measurement.
SU‐GG‐T‐184: Photon Energy Spectrum Prediction of a Medical Linear Accelerator Using Scatter Radiation Analysis
Authors: Hawwari M , Lamba M , Spitz H , Elson H
Published: July 2008

Purpose: To evaluate a system to determine the photon spectrum from a 6 MV linear accelerator using narrow beam scatter measurements, Monte Carlo simulations of the scatter, and linear algebra. Method and Materials: The relative number of photons that scatter at specific angles and distances from a polyenergetic narrow beam of photons striking a scattering medium is a small but measurable quantity. The relative number of photons from a monoenergetic beam of photons scattering in similar geometry can be modeled using Monte Carlo techniques. A system of linear equations can be established and solved for the photon energy spectrum if there are an equal number of scatter angle measurements and Monte Carlo modeled photon energies. Measurements were performed using a Farmer chamber and electrometer with an acrylic scatterer on a 6 MV linear accelerator at six scatter angles with a 1.5×1.5 cm field. Monte Carlos simulations were performed at six energies with equivalent geometry. A linear system of the form b=Ax was solved where (b) is the measured data vector, (A) is a mathematical operator matrix that consist of calculated mono‐energetic scatter fractions, and x is the unknown flux vector. Results: The operator matrix was found to be an ill‐posed matrix, so the eigenvalues of the matrix were evaluated and the significantly small eigenvalues were ignored to enhance the accuracy of the results. Errors were estimated by solving the systemfor b−Ax=0 and Ab−A∧2x=0, which theoretically should give a zero answer. The results, while coarse with only six energy bins, are consistent with published data. Conclusion: This technique produced reproducible results that can be refined with more scatter angles and energy bins. The results can potentially be applied to any poly‐energetic beam.
SU‐GG‐T‐248: Determination of Proton Stopping Power Using MV CT, KV CT & Proton Radiographic Images
Authors: Kwak J , Lee S , Park S , Shin J , Park S , Shin D , Yoon M , Lim Y , Kim D , Kim J , Cho K
Published: July 2008

The most of radiotherapy planning systems are operated with the calibration curves of KV CT (Kilo‐Voltage Computational Tomography) images. However, the proton stopping power curve used in proton radiation treatment planning system is optimized for organic materials rather than for metals and other inorganic matter, since the proton stopping power curves obtained with a KV X‐ray beam don't provide one‐to‐one corresponding relation between HU number and Proton stopping power of general materials. At Proton Therapy Center, National Cancer Center(NCC) in Korea, proton treatments started on March 19, 2007. This proton treatment facility is equipped with a cyclotron (Proteus235,IBA). The Proton Therapy Center also has a Tomotherapy machine, 4 LINACs, 2 CT simulator, a conventional simulator. MV CT(Mega‐Voltage Computational Tomography) images provided by the Tomotherpay machine, conventional KV‐CT images taken with conventional CT's and proton radiographic images with Proteus235 have been used to obtain the general proton stopping power relation. We have taken the radiographic images of GAMMAX and CRIS CT calibration phantoms, contrast agents, metals and plastics using various beams. Specially designed Beam Current Modulation of the proton beam provides the beam eye's view of proton. We can use the special beam condition to verify the CT calibration of the Radiation Treatment Planning system and to examine the proton stopping power of various materials for example, a aperture structure, specially, a compensator.
SU‐GG‐T‐256: Measurement and Evaluation of Inhomogeneity Corrections and Monitor Unit Verification for Treatment Planning
Authors: Saxena R , Higgins P
Published: July 2008

Purpose: Heterogeneous tissue‐lung and tissue‐bone phantoms have been constructed for the purpose of developing an extended data set of doses measured in different media, particularly at, or near, interfaces. Method and Materials: Data have been acquired with Kodak EDR2 film for a Varian 2300cd (6 and 25 MV) and an Elekta Synergy (6, 10 and 18 MV) linear accelerator. These data have been compiled for use in evaluating different planning system calculation algorithms. Ones we reviewed, as examples, included Pinnacle and the correction‐based, older version of Eclipse. The data were taken with the intent of exploring the limits in the capabilities of planning systems to handle both energy and small‐field effects. As such, data extended from 2×2 to 10×10 cm2 field sizes over the available range of energies. It is also becoming increasingly important to find some method of checking monitor units generated by these planning systems when heterogeneities are included. To this end, we also evaluated the range of performance accuracy for two, familiar 1‐dimensional inhomogeneity correction factors (ICF): a variation on the primary transmission‐based effective attenuation method and the TMR ratio method. Results & Conclusion: We found, as have others, that Pinnacle predicts doses to, generally, within ±5%, even near interfaces over the full range of energies and field sizes tested. Eclipse (pre‐AAA model) Batho and equivalent TAR algorithms were equivalent to, or poorer than the 1‐dimensional ICF corrections, particularly with increasing energy and decreasing field size. The effective attenuation and TMR ratio methods were adequate for 6 MV, even for 2×2 fields and appear to have application for 6×6 fields and larger at all energies. In the interest of patient safety, we recommend that some type of corrected backup calculation be done to verify every treatment plan.
SU‐GG‐T‐263: IMRT Beamlet Match‐Line Dose Heterogeneities Associated with Varian 120 MLCs
Authors: Felty M , Ashburn J , Meigooni A
Published: July 2008

Purpose: To measure the dosimetric consequences of a range of MLC leaf offsets, examining effects of leaf tip transmission and “scallop edge” overshine from rounded Varian MLC leaves. In particular, this investigation focuses on how these effects influence the dose heterogeneity of the match‐line between abutted beamlet fields that are often present in step‐and‐shoot IMRT. Method and Materials: Offset values from 0.0 to 1.0 mm were evaluated. For each offset, two abutted 2×2cm2 MLC‐shaped fields were delivered sequentially using 6 MV photons from a Varian Platinum EX accelerator with a 120 leaf MLC. Four different techniques—Kodak EDR‐2 film, GAFChromic EBT film, CC01 Wellhofer ion chamber (1 mm radius), and 1 mm3 LiF TLD chips—were used to measure delivered dose at depths of 5 to 15 cm. Film data was taken at 100 SAD. TLD and ion chamber data were collected at 400 cm SSD to increase their effective resolution. Results: Increasing the offset from 0.0 to 0.5 mm decreased the average overdose from 30 to 10% in the abutment region. For offsets greater than 0.5 mm, dose at the center of the match line decreased, but dose inhomogeneities became more pronounced due to effects of overshine, scatter, and leaf tip penumbra. At larger offsets, both over‐ and underdoses of 5 to 10% were present in the abutment region. Conclusion: The measured dose distributions suggest that an optimum offset provides, at best, a balance of over‐ and underdosing in abutment regions. Our data show that the choice of offset significantly changes the dose distribution in a region approximately 6 mm wide at the match line. The width of this high‐gradient area is sufficient to influence clinical IMRT doses delivered.
SU‐GG‐T‐265: High Precision GafChromic Based Clinical Dosimetry Using Standard Flat Bed Scanner
Authors: Chung H , Lynch B , Samant S
Published: July 2008

Purpose: To describe the methodology to use a commercially available flatbed document scanner to digitize GafChromic® EBT radiochromic film (RCF) to be used in a clinical environment for highly accurate and reliable all purpose two‐dimensional absolute dosimeter. We propose a methodology that yields high precision dosimetry measurements without significant post‐irradiation correction as has been proposed in the literature. Method and Materials: Epson Expression 1680 Professional scanner was used to digitize the RCF. The scanner was also retrofitted with an inexpensive commercial diffusion glass to minimize the effects of Newton rings. A small strip of films was made from the calibration film and a known dose was delivered. Subsequently, the measurement films were positioned and irradiated with 10 cm and 20 cm build up solid water and 5 cm back scatter solid water. No particular bowing effect correction and/or significant image processing were carried out. The calibration films were used to compute an analytical function to convert optical density to dose. The profiles were compared with CC04 water tank measurements. Additionally, preliminary study of IMRT fields was also done and evaluated using gamma test. Results: The overall agreement of RCF with CC04 is ±3%. For 10×10cm2 field size at 10cm build up, the agreement for central axis percent dose difference and mean of the difference for RCF with respect to CC04 is 2.24% and 1.84%, respectively. For 20cm build up with same field size, the agreement was 0.98% and 1.97% for central axis percent dose difference and mean of the difference, respectively. The preliminary results of the IMRT fields yielded a mean passing rate of 92.5%. Repeated film measurements yielded reproducibility of results to within 2%. Conclusion: This work demonstrated that RCF in conjunction with a commercially available flatbed scanner can be used as a highly accurate and precise absolute dosimeter.
SU‐GG‐T‐286: Performance of An Improved Optical Computed Tomography Polymer Gel Dosimeter System for 3‐D Dose Verification of Static and Dynamic Phantom Deliveries
Authors: Lopatiuk‐Tirpak O , Langen K , Meeks S , Kupelian P , Maryanski M , Zeidan O
Published: July 2008

Purpose: Performance of an improved 3D dosimetry system consisting of polymer gel dosimeters and a next‐generation optical CT scanner (OCT) is characterized. Following improvements are discussed: increased acquisition speed of the scanner; increased useful dosimeter volume achieved by reducing image reconstruction artifacts through background subtraction; efficient calibration procedure using single high‐energy electron field. The characteristics of dose response (range, linearity, saturation) are determined. Feasibility of high‐resolution (<1mm) 3‐D dosimetry is demonstrated under stationary and dynamic delivery conditions. Method and Materials: Large‐volume (2.2 L) polymer gel dosimeters (BANG®‐3) with specially designed TomoBANG™ Virtual Water™ phantom were used to record cumulative, volumetric dose distributions from single‐ and multi‐field deliveries. Calibration procedures using photon (6 MV) and electron (18 MeV) beams were based on using a depth‐dose relationship to correlate delivered dose to the change in optical density of the dosimeter. The accuracy of this approach was verified by delivering a conformal photon plan and comparing the dose distribution to that calculated by the treatment planning system. The feasibility of use for motion (4‐D) dosimetry was demonstrated by comparing dose distributions from static and dynamic deliveries (using a motion phantom) with radiochromic film measurements. Results: Both calibration procedures yielded equivalent results, with the electron‐based method deemed preferable because of its higher efficiency. Dose distributions derived from optical density measurements show good agreement with the treatment planning system and radiographic film measurements. The average distance to agreement between the pairs of isodose curves is below 1 mm for stationary as well as dynamic deliveries. The capability to visualize three‐dimensional dose distributions is also illustrated. Conclusion: In addition to its ability to provide high‐resolution, reliable dosimetric information, the improved gel/OCT dosimetry system allows for 3D visualization and analysis of motion‐induced dosimetric effects, which is unique to this type of dosimeter.
SU‐GG‐T‐287: Deformable Lung Phantom for Verification of 4D Dose Calculations
Authors: Vinogradskiy Y , Starkschall G , Balter P , Followill D
Published: July 2008

Purpose: 4D dose calculations have been developed that explicitly incorporate patient motion and deformation in the calculation of dose. Before these calculations can be used in the clinic it is necessary to evaluate and verify their accuracy. The purpose of this study was to develop a deformable lung phantom for verification of 4D dose calculations. Method and Materials: The Radiological Physics Center (RPC) had previously developed an anthropomorphic thoracic phantom with inhomogeneities and an imageable target. An insert was built for the lung cavity of this phantom to incorporate motion and deformation. The insert was composed of disks of sponges of different compressabilities and a computer controlled motor‐driven piston to compress the sponges. The control system is capable of reproducing measured respiratory motion traces as well as simple sinusoidal motions. A tissue‐equivalent target was placed in one of the sponge disks and planes of films were distributed throughout the target and between sponge disks. Results: The choice of sponge compressibility, thickness, and order has allowed us to shape the lung motion to mimic clinically observed deformations. In addition the design of the deformable phantom offers flexibility of target position and motion, film placement and diaphragm motion pattern. Conclusion: The deformable lung insert for the RPC phantom provides an anthropomorphic phantom that can be used to verify the accuracy of 4D dose calculations. Conflict of Interest: This work is partially supported through a Sponsored Research Agreement with Philips Healthcare.
SU‐GG‐T‐297: Measurements of Proton Dose Distributions Using a Thin Oxide MOSFET Detector
Authors: Kohno R , Hirano E , Kameoka S , Hotta K , Nishio T , Miyagishi T , Hallil A , Kawashima M , Ogino T
Published: July 2008

Purpose: In proton dosimetry, the MOSFET response depends strongly on Linear Energy Transfer (LET). To improve the LET dependence, we focused on the SiO2 thickness of the MOSFET detector. We compared the response of a new MOSFET detector with thinner oxide thickness, TN‐252RD, to the standard MOSFET, TN‐502RD, already investigated. Furthermore, in order to measure proton dose distributions using the MOSFET, a correction method of the MOSFET response to proton beams has been developed. Method and Materials: Dose reproducibility and LET dependence of the MOSFET response were evaluated experimentally using 190 MeV proton beam. A Bragg curve measured by an ionization chamber (IC) was used to evaluate the correction factors (IC/MOSFET) as a function of proton penetrating depth, i.e. residual range. Here, the residual proton range is successfully calculated by the pencil beam dose calculation algorithm at an arbitrary point. Proton dose distributions formed by an L‐shaped bolus were measured by MOSFETs and IC. The MOSFET doses were compensated by those correction factors. Results: The dose reproducibility of the thinner oxide MOSFET, TN‐252RD, was within 2%, almost similar to that of the standard MOSFET. The new MOSFET response at the Bragg peak improved by 10% in comparison to the standard MOSFET. For proton dose distributions measured with the L‐shaped bolus, a deviation between the IC and raw MOSFET measurements was observed. On the other hand, the corrected MOSFET data agree well with the IC results. Conclusion: The MOSFET of thinner SiO2 thickness improved the LET dependence for proton dosimetry, although the accurate depth‐dose estimation is still to be achieved due to the MOSFET LET dependence. However, with a new correction method of the MOSFET response to proton beams, we succeeded in measuring the proton dose distribution using the MOSFET.
SU‐GG‐T‐302: Reduced Energy Dependence Artifact in CR Plate Dosimetry Using Pencil Beam Response Correction and Thoraeus Filters
Authors: Kirov AS , Ilany J , Obcemea C , Donowsky M , LoSasso T
Published: July 2008

Purpose: To improve the accuracy of the Kodak 2000RT Computer Radiography (CR) Beam Dosimetry System by reducing the effect of the energy dependent response of the CR plates. Method and Materials: The energy dependent response of the CR plates is due to the presence of elements with atomic numbers higher than water. This results in larger photoelectric cross section, which leads to dose over‐response for photon energies lower than few hundred keV. The use of lead foils on both sides of the CR plate in the Kodak CR solid water dosimetry cassette strongly suppresses this dependence by reducing the number of low energy photons reaching the plate. To determine the accuracy of these plates when used in a most time efficient way, we scanned them 2 min after exposure. For a very large IMRT field (mean dose 23 cGy) we observe gamma <1 compliance (3 mm distance‐to‐agreement and 3 cGy dose difference) for 73% of the points. To correct for the observed under‐ and over‐determination of the dose in the high and low dose regions, we used Monte Carlo to simulate the response kernel of the CR plates to a pencil beam and then used this kernel in convolution dose calculations. To further reduce the flux of characteristic x‐rays reaching the plates we added more foils forming Thoraeus filters on both sides of the plates. Results: The use of CR response kernel increased the gamma index compliance to 83% for this large IMRT field. The addition of the Thoraeus filters further increased the compliance to 90%, with largest errors seen close to the plate edges. For smaller 15×15 cm IMRT fields, compliance of 96% was observed. Conclusion: With appropriate response correction and low energy photon filtering Kodak's CR plates can be used as an efficient and potentially accurate 2D dosimeter.
SU‐GG‐T‐303: Evaluation of An Implantable MOSFET Dosimeter for Use with Hypo‐Fractionated External Beam Treatments
Authors: Beyer GP , Kry S , Rini C , Baum E , Mann G , Espenhahn E
Published: July 2008

Purpose: Evaluate the performance of an implantable MOSFET dosimeter designed for hypo‐fractionated protocols. Method and Materials: The DVS® implantable dosimeter has been developed for use with external beam radiotherapy. A new version is currently under development for use with hypo‐fractionated protocols (DVS‐HFT*). This dosimeter is pre‐calibrated for use at body temperature in a dose per fraction range of 340–1200 cGy. The dosimeter response was evaluated in a water phantom at 37°C to validate the accuracy as a function of dose. Since delivery times for high dose fractions can vary, a study of the response as a function of treatment time was performed. In addition, dose rate dependency for high doses was evaluated. Final dosimeter accuracy was validated by delivering simulated hypo‐fractionated treatments on a body phantom at 37 °C. Two dosimeters each (total six) were irradiated inside the phantom with a breast BID plan at 340 cGy per fraction (10 fx), two prostate IMRT plans at 700 cGy per fraction (5 fx) using 6 MV and 18 MV. Results: The dosimeter had minimum dependency on dose fraction size from 340 to 1200 cGy. Phantom testing found a response accuracy of <6% for 50 dosimeters tested with different doses and fractionation schemes. Dose rate dependence measurements in the range of 300 to 600 MU/min showed <0.6% variability. The dosimeter was found to have a slight dose response fall off as a function of fraction number for irradiation times larger than 1 hour. Simulated clinical treatments on six dosimeters resulted in an overall accuracy of −0.65% (σ=1.9%). Conclusion: Hypo‐fractionated treatments have fewer fractions requiring increased treatment accuracy. Phantom testing suggests that the DVS‐HFT could be used to measure the dose per fraction delivered helping optimize hypo‐fractionated treatments.

*Not FDA cleared for use in the US

Research sponsored by Sicel Technologies, Inc.
SU‐GG‐T‐316: Skin Dose Determination for Photon Beam Breast IMRT Patients Using Monte Carlo Simulations and TLD Measurements
Authors: Yang J , Tong S , Li J
Published: July 2008

Purpose: The skin dose is one of the major concerns for breast patient treated with intensity‐modulated radiotherapy (IMRT). According to the ICRU and ICRP recommendations, skin dose was defined as the dose measured at 0.07 mm depth. The skin dose is difficult to be determined because the voxel size in CT scan is much bigger than this depth. In this study, Monte Carlo simulations and TLD measurements were combined to determine the breast IMRT patient skin dose for 6 MV photon beams. Method and Materials: Surface dose on a water phantom calculated using the Monte Carlo method was first validated by comparing the results with the measurements using TLD‐100 chips (3.2 mm × 3.2 mm × 0.38 mm). The doses at a 0.07 mm and 0.89 mm depth on a solid water phantom were then calculated respectively using the Monte Carlo method with two opposite beams for different separations from 5cm to 20cm. A curve of dose‐ratio as function of separations was obtained for 6MV photon beam. The tissue at skin with a 0.89 mm thickness was contoured on the patient phantoms which were converted from patient CT images, Monte Carlo dose calculations for IMRT plans were performed for 4 patients. Skin doses at 0.07mm depth for these 4 patients were then determined by applying the dose ratio between 0.07 mm depth and 0.89 mm depth. Results: The Monte Carlo simulation results for the TLD agreed very well with measurements (4%). The dose ratio between 0.07mm depth and 0.89 mm depth for different separations were obtained. For a given dose of 200 cGy, the deduced skin doses for 4 breast IMRT patients were calculated in different locations on breasts.
SU‐GG‐T‐320: Monte Carlo Generation of Phase Spaces for Dose Computation in TomoTherapy
Authors: Moreau M , Chen Y , Cravens B , Mauer C , Sterpin E , Salvat F , Ruchala K , Olivera G
Published: July 2008

Purpose: Generate phase spaces using Monte Carlo simulations for TomoTherapy® dose computation, measure accuracy of phase spaces, and present properties of phase spaces. Method and Materials: The PENELOPE 2006 Monte Carlo code system is used to generate phase spaces for the TomoTherapy® treatment device. The PENELOPE geometry package is used to model the head of the treatment device for each one of the three possible field sizes: 1 × 40 cm2, 2.5 × 40 cm2 and 5 × 40 cm2, and a phase‐space plane is positioned downstream from the collimating jaws of the device. Incident electrons are projected onto the linear accelerator's target assembly and Monte Carlo transport is performed through the head of the linear accelerator until particles cross the phase‐space plane. The incident electron energy distribution and the focal spot size are adjusted until the resulting dose profiles computed with a phase space match the TomoTherapy® open‐field gold standard profiles. Fluence maps, dose profiles and off‐axis spectra are presented. Results: A 5.5 MeV monoenergetic incident electron source produced optimal results. Computed dose profiles match the TomoTherapy® open‐field gold standard profiles within 2% / 1mm. The energy spectra, fluence maps and dose profiles exhibit the shape produced by a treatment device with no flattening filter, such as the TomoTherapy® device: minimal beam hardening along the central axis, minimal spectral shape change as a function of off‐axis angle, about 99.5% of the particles in the phase spaces are photons, and cone shape fluence maps and dose profiles in the lateral direction. Conclusion: Phase spaces for the three possible field sizes were generated. Dose computed from these phase spaces match TomoTherapy® gold standard dose profiles within 2% / 1mm and can be used for static or helical delivery dose computations.
SU‐GG‐T‐321: Validation Study of the GEANT 4 Monte‐Carlo Simulation of the Proton Dose Distribution From Passively Scattered Proton Beam Nozzles at the Proton Therapy Center at Houston
Authors: Ding X , Sahoo N , Zhu X , Yang J , Arjomandy B , Lii M , Titt U , Mohan R , Smith A , Gillin M
Published: July 2008

Purpose: To validate the accuracy of the representation of the hardware components and the process of Monte‐Carlo simulation of the dose distributions from the passively scattered proton beam nozzles of the Hitachi ProBeat system at the Proton Therapy Center at Houston (PTCH) using the GEANT4 software package. Method and Materials: GEANT4 is a widely used software package for Monte‐Carlo simulation (MCS) of ionizing radiation transport, which we plan to use extensively, (a) to check the accuracy of both the measured beam data of our proton therapy beams and the patient dose distributions calculated by the treatment planning system, (b) to obtain the dose distribution in many situations where accurate measurement would not be possible and (c) to compare the results with other MCS packages used at our institution. As the first step, we have carried out simulations of the passively scattered nozzles for selected energies by properly including the material and geometry of the first and second scatterer, range modulating wheels, dose and beam profile monitors, and calculated the dose distribution in a water phantom. The depth dose curves for the pristine Bragg peaks (PBP), spread out Bragg peaks (SOBP) and profiles at selected depths are compared with those obtained from ion chamber measurements. Results: The ranges obtained from the GEANT4 MCS agreed within 0.5 mm both for the PBP and SOBP. The overall agreement of the depth dose curves is within 2%. The profiles agree within 3% in most of the regions, with some points differing by as much as 5%. The disagreement for profiles is expected to be reduced by increasing the number simulation histories. Conclusion: We have accurately implemented the passively scattered proton beam nozzle at PTCH for MCS of proton dose distributions using the GEANT4 software package.
SU‐GG‐T‐323: Angular Distribution of Neutron Fluence and Its Effect On Shielding for a Passively‐Scattered Proton Therapy Unit
Authors: Zheng Y , Newhauser W , Klein E , Low D
Published: July 2008

Purpose: Neutron production is of principal concern in shielding of proton therapy units. Conventionally, neutron calculations are based on the analytical methods, which do not take the actual beam shaping components and the nozzle shielding into account. The goal of this study was to calculate neutron spectral fluence produced at a passive scattering proton treatment unit with detailed Monte Carlo modeling and to estimate the corresponding neutron dose equivalent and shielding requirements at various angles relative to the beam direction. Method and Materials: We modeled all key components in the nozzle including a range modulator, scatter, range shifter and aperture for a passive scattering proton therapy unit with MCNPX. We simulated the spectral neutron fluences at different angles and distances around the nozzle. The neutron spectral fluences were then converted to neutron dose equivalent using conversion factors from International Committee of Radiological Protection (ICRP) Publication 74. Based on the neutron dose equivalent and spectral fluence, the shielding requirements for walls facing the beam and parallel to the beam were assessed. Results: The spectral neutron fluence varied significantly with angle relative to the beam. High energy neutrons were forward peaked and decreased with increasing angle. Low energy neutrons did not change much with angle. Neutron dose equivalent in treatment room did not change significantly with angle, but more shielding was required for the wall facing the beam than other walls because the high energy neutrons were greatest at zero degrees. Conclusion: The neutron spectral fluence from proton therapy units changes significantly with angle relative to the beam direction. Compared to the analytical methods, Monte Carlo simulated results provided a more accurate estimation of the neutron angular distribution for shielding design.
SU‐GG‐T‐326: A Fast Monte Carlo Code for Proton Transport in Radiation Therapy Based On Pre‐Calculated Tracks From MCNPX
Authors: Jabbari K , Seuntjens J
Published: July 2008

Purpose: This work extends Pre‐calculated Monte Carlo (PMC), successfully developed for electrons to protons. PMC uses pre‐calculated tracks for each material in the simulation and applies these to heterogeneous geometries such as CT‐data. Method and Materials: The tracks of 10000 primary protons are generated in the middle of a large homogenous material for various energies using MCNPX. The proton range was 20, 40,…100, 110, …200 MeV with ECUT=200 keV. The ptrac routine of MCNPX writes the various events of each particle in a particular format. An in‐house Fortran code was developed to read and extract the position, direction, energy and deposited energy of a particle in each step from the ptrac file. Unlike electron and photons, protons produce many different secondary particles such as neutrons, deuterons, tritons, alphas, secondary protons, etc and they are handled in three categories: 1‐Secondary protons: treated like a primary protons and transported using a track picked up from pre‐calculated tracks; 2‐ Neutrons: The energy of the neutron are deposited far from the initial point and neglected. 3‐ All other secondaries: Since other secondaries have a very short range their energy is deposited locally. Results and Discussion: The size of the pre‐calculated data for each material is about 100 Mb. The performance of the code is evaluated in various homogeneous and in‐homogeneous phantoms. In comparison of the code with MCNPX as the reference the difference is generally between 2–5% and it runs 200 times faster than MCNPX. For electron transport the code runs 40–60 times faster than EGSnrc.
SU‐GG‐T‐328: Human Anatomy‐Based Monte Carlo Dose Calculation for External Protons
Authors: Liu Y , Shi C , Xu X , Papanikolaou N
Published: July 2008

Purpose: In this work, we aim to build an accurate dose calculation algorithm based on human anatomy‐based model using Monte Carlo simulation for providing basic and benchmarking data for therapeutic protons. Method and Materials: Both phantom‐based and human anatomy‐based models were used. The human anatomy model was developed from VHP® at National Library in Medicine. The human anatomy‐based model was built with 4 mm × 4 mm × 4mm voxel resolution with total over 6 million voxels for describing the whole body. Each voxel was assigned physical properties, including density and isotopic composition. MCNPX was used to simulate the transport and energy deposit to each voxel. An in‐house dosimety software package, Human Anatomy‐based Monte Carlo Dose (HAMD) was developed to analysis the huge dose dataset based on Monte Carlo simulation and the three dimensional dose matrix was super positioned to the CT image correspondingly. Results: The Monte Carlo simulation provided very close agreement to the two widely used proton range‐energy tables with average depth peak difference less than 0.70% and −0.37% to ICRU Report 49 and Janni DNDT respectively from 40 MeV to 250 MeV energy range. HAMD performed well in proton treatment dose calculation. HAMD offers very friendly and familiar interface for physicians to conveniently review a treatment plan. The isodose lines in transverse, sagittal and coronal views provided very conformal coverage to the contours in lung. Conclusion: The simulated proton range‐energy table has been accurately benchmarked compared to measurements. The in‐house developed dose algorithm HAMD performs very well in dose calculation both in phantom‐based and human anatomy‐based heterogeneity. The HAMD needs further validation by using additional human anatomy‐based models and specified beam source configuration. The long‐term and board objective is to provide an extreme accurate dose calculation based on human model for benchmarking clinic treatment planning systems.
SU‐GG‐T‐331: A New Monte Carlo Code for Gamma Knife Simulations
Authors: Yang J , Tong S , Fan J
Published: July 2008

Purpose: Gamma Knife is widely used in stereotactic radiosurgery (SRS) to treat brain tumors. In this study, a new Monte Carlo code was developed to simulate Gamma Knife system, which can be used to calculate Gamma Knife collimator factors, to verify Gamma Knife monthly and annual QA results and to evaluate treatment plan from Leksell GammaPlan system. Method and Materials: An EGSnrc based user code DOSIMETER was used for this study. The Cobalt‐60 source and collimators were simulated based on their material, size and location in the system. Because of the equal geometries and materials of the sources and collimators, one source system is generated for all 201 sources in corresponded angle. For each collimator (4, 8, 14 or 18 mm), a phase space file was produced at the final helmet collimator surface. A CT image of a round QA phantom with 160 mm in diameter was introduced in the simulation system. Source linearity and profiles at isocenter were calculated using Monte Carlo simulations and compared with pin chamber and film measurement results. The relative collimator factors for 4, 8 and 14 mm were calculated and compared with ultra‐thin TLD and OSL measurements. Finally, a plan from Leksell GammaPlan system was evaluated by the Monte Carlo simulations. Results: A Monte Carlo code was implemented for Gamma knife simulation. It showed good dose linearity. The profiles for each collimator were consistent with film measurements. The relative collimator factors from Monte Carlo simulations are 0.970, 0.938 and 0.873, which are within 3% against TLD and OSL results. The DVHs from Monte Carlo are in good agreement with GammaPlan system. The treatment plan dose difference at isocenter between Monte Carlo and the planning system is less than 1%.
SU‐GG‐T‐336: A Track Repeating Algorithm for Fast Proton Dose Calculations
Authors: Yepes P , Newhauser W , Randeniya S , Taddei P
Published: July 2008

Purpose: Monte Carlo tracking programs are utilized for accurate dose calculations in proton radiation therapy. They are superior to commonly used analytical dose calculations in accuracy, however, they are highly demanding computer resources. The aim of this work is to characterize fast algorithm for accurate dose calculations based on a track repeating Monte Carlo approach. Method and Materials: A few hundred thousand proton trajectories in water, generated with GEANT4, were stored in a database in discrete steps. The trajectories were utilized to calculate dose in various materials by rescaling the step length and angles between consecutive steps. The extrapolation is applied to a variety of materials, including bone and metals, which are important for patient with prostheses. Results: The track repeating approach reduced the CPU time required for a complete dose calculation in a patient by more than two orders of magnitude. The track repeating approach reproduced the results from the traditional Monte Carlo approaches within 4% in dose and within 1 mm in distance to agreement. Conclusion: A fast track repeating algorithm based on GEANT4 provided a substantial reduction in CPU time with very little loss in accuracy. The approach is not limited to GEANT4 and the developed tool can be utilized with other Monte Carlo programs.
SU‐GG‐T‐344: Modifications of the MCNP Code to Facilitate Monte Carlo Dose Calculations with a CT Geometry
Authors: Jiang H
Published: July 2008

Purpose: MCNP is a powerful Monte Carlo code, which is capable of modeling three‐dimensional complex geometries and in the meanwhile provides high computational efficiency. However, as a general‐purpose code, not all the issues in the specific application of Monte Carlo dose calculation in a patient geometry were fully addressed. The purpose of this study is to develop a modified version of the MCNP code to facilitate Monte Carlo dose calculations involving a large number of CT voxels and geometry change. Method and Materials: A new type of BOX object is added to MCNP for being the container of a CT volume. Unlike the universe/fill structure in MCNP, this CT box is only filled with a single ordinary MCNP BOX object of the dimensions of a CT voxel. The location, dimensions, and material of the CT voxel box are allowed to change. Therefore, this voxel box is able to represent any CT voxel in the CT volume. Such a scheme has the advantage of saving computer memory. The material for CT voxels can be a Hounsfield unit (HU). The HU will be converted to a MCNP material on the fly. With the density effect ignored, the density of a material is allowed to change. Hence, each HU is associated with a unique mass density without defining a material for each HU. The geometry can also be changed in a simulation to enable simulations involving motions such as MLC leaf movement. Results and Conclusion: A modified MCNP code is currently under development and debugging. The new code will one hand takes advantage of the great features in MCNP, and on the other hand facilitate and speed up the procedure of CT based Monte Carlo simulations. It will especially simplify motion‐involved simulations such as those for IMRT.
SU‐GG‐T‐345: Verification of a Monte Carlo‐Based Source Model for a Varian 10 MV Photon Beam
Authors: Davidson S , Cui J , Deasy J , Ibbott G , Followill D
Published: July 2008

Purpose: To apply a measurement‐driven source model using the Monte Carlo Dose Planning Method (DPM) dose calculation engine to a Varian 10 MV photon beam. Method and Materials: A measurement‐driven model using the DPM dose calculation algorithm is being extended from a Varian 6 MV photon beam to include Varian 10 MV, Elekta 6 MV and 10 MV, and Siemens 6 MV and 10 MV photon beams. The present work details the model commissioning for the Varian 10 MV photon beam. The multi‐source model consists of a primary photon point source, an extra‐focal exponential disk source, and an electron contamination uniform disk source. The model accounts for fluence and off‐axis energy effects due to the flattening filter. The photon energy spectra for the primary and extra‐focal sources are modeled by the statistical fatigue‐failure function combined with a Fermi‐cutoff function. The energy spectrum of the electron contamination source is modeled as an exponential distribution. Model parameters are determined by an optimization process that minimizes the differences between measurement and calculation. The set of standard measurements used for optimizing consists of the percent depth dose (PDD) and dose profiles in water for 10×10 cm2 and 40×40 cm2 field sizes. Results: Comparisons between calculation and measurement of the PDD and dose profiles for the 10×10 cm2 field size show agreement within ±2%/2 mm except for the off‐axis low dose regions where calculations underestimate the dose by up to 3% of dmax. Conclusion: This work demonstrates that the model, previously shown to be accurate for the Varian 6 MV beam, can be successfully extended to the Varian 10 MV photon beam. Work is ongoing to further refine and validate the model to include Elekta and Siemens linear accelerators. Conflict of Interest: Work supported by PHS CA010953, CA081647, and R01 CA85181 awarded by NCI, DHHS.
SU‐GG‐T‐347: Methods to Calculate Organ Doses Due to Scattered Radiation From External Beam Radiation Treatment Using Various Patient Phantoms and Monte Carlo Simulations
Authors: Bednarz B , Xu X
Published: July 2008

Purpose: It has been long known that patients treated with radiation carry a risk of developing second cancers in their lifetimes as a result of their treatments. In addition to hosting a symposium on “Secondary Cancer Risks for Emerging Radiation Treatments,” the AAPM has recently formed Task Group 158 that will provide recommendations on the dosimetry protocols for quantifying scattered radiation dose outside the treatment volume. Measurements are useful in providing a relative comparison of different treatment modalities or beam characteristics. On the other hand, more precise organ dose equivalent values need to be calculated from Monte Carlo simulations and anatomically realistic computational phantoms. This study outlines a computational platform that integrates anatomically‐refined virtual patient models with a detailed model of a medical accelerator head to calculate scattered doses to patients treated with radiation. Method and Materials: A detailed model of a 6‐MV Varian Clinac 2100C was developed in MCNPX. Several virtual patient models have been integrated with the accelerator head model in MCNPX. Various conventional and IMRT treatment plans have been considered. Results: Average organ doses are significantly different than point‐wise measurement values. The differences can be attributed to various sources of scattered radiation that deposits dose in the organ. Large dose gradients will be present throughout organs at short and intermediate distances from the tumor volume, while organs at longer distances will have a more uniform dose distribution. Conclusion: A computational platform to calculate organ dose from scattered radiation during radiation treatments has been demonstrated. Results indicate that average organ/tissue doses are significantly different than point‐wise measurements. Organ doses calculated in this manner will likely provide more accurate organ dose information for epidemiologic studies and for assessing the risk during the treatment planning.
SU‐GG‐T‐349: Large‐Field Sensitivity Analysis of a Non‐Cylindrically Symmetric Monte Carlo Beam Model of a Varian Electron Accelerator
Authors: Schreiber E , Sawkey D , Faddegon B
Published: July 2008

Purpose: To facilitate the development and commissioning of radiotherapy electron beam models by studying the dosimetric response of a Varian Trilogy electron accelerator to variations of the geometric and electron beam parameters of the treatment head, including lateral offsets of linac components and lateral and angular offsets of the incident electron beam. Method and Materials: We have performed a sensitivity study examining the effect of varying beam and geometric parameters of a Monte Carlo model of a Varian Trilogy accelerator. The accelerator was modeled using a modified BEAMnrc package that included asymmetric accelerator and beam models. The dose output was modeled using DOSXYZnrc. Electron beams were modeled for 40×40 cm2 fields with applicators removed. Profiles at dmax and in the bremsstrahlung region were calculated for 4, 12, and 20 MeV configurations. Results: The dose distributions were evaluated for flatness, symmetry, and OAR. Scattering foil thickness had a negligible effect on dmax profiles, but increased the bremsstrahlung production rate. A 3 mm shift in the interfoil distance had a 5% effect on the OAR at 12 cm, but only 2% at 5 cm radius. Profiles in the bremsstrahlung region help differentiate between similar dmax behavior for changes in incident electron angle and lateral secondary scattering foil position. A 4 mm variation in the electron spot size had a negligible effect on the 4 MeV dose distribution, but varied the flatness of the 12 and 20 MeV beams by 2% and 5%, respectively. Conclusion: Lateral and angular offsets of beam and accelerator components have strong effects on dose distributions, and need to be included in any high‐accuracy electron beam model. Large fields without applicators and bremsstrahlung profiles can give additional insight into how parameters should be adjusted when commissioning a Monte Carlo beam model.

Support from NIH R01 CA104777‐01A2.
SU‐GG‐T‐352: Dosimetric Validation of a Commercial Monte‐Carlo Based IMRT Planning
Authors: Grofsmid D , Dirkx M , Woudstra E , Marijnissen H , Heijmen B
Published: July 2008

Purpose: Recently, the inverse planning planning system Monaco (CMS Inc) version 1.0.0 was installed in our department. The Monaco system incorporates a two‐stage IMRT optimization procedure and a version of the XVMC Monte Carlo dose calculation algorithm. In the first stage the fluence distribution of the IMRT beams are optimized using hard (‘biological’) constraints for the organs at risk. In the second stage, the beams are segmented and their weights optimized while still satisfy the hard constraints. In this study we investigated the dosimetric accuracy of the XVMC dose engine implemented in MONACO. Method and Materials: For an ELEKTA(r) linac (6, 10 and 18 MV) depth dose curves, dose profiles and output factors were measured in a waterphantom using source‐to‐surface distances (SSD) between 80 and 100 cm. Symmetric fields ranging from 2×2 to 30×30 cm and off‐axis fields of 2×2 3×3 cm and 2×10 cm were used. In addition, GAFCHROMIC(r) film measurements were performed in anthropomorphic phantoms to validate the dose accuracy in inhomogeneous media. Furthermore, the dosimetric accuracy of IMRT fields for prostate and head‐and‐neck cancer treatments was evaluated. All fields were simulated in Monaco using a grid size of 2 mm and variances of 0.5 and 1%. Results: The agreement between measured and calculated dose distributions was generally within 2%. Only in the build‐up region larger dose differences were observed, especially for the highest photon energy. No impact of the SSD on the dose accuracy was observed. Calculation times for a 10×10 cm field and a 0.5 or 1% variance were 75 and 23 minutes, respectively. Conclusion: A very good agreement was observed between Monaco Monte Carlo dose calculations and measurements, allowing clinical introduction.