bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐376: Electron Beam Dosimetry Using a 2D Detector Array:
Authors: Srivastava S , Das I
Published: April 2010

Purpose: To adequately cover a target volume in electron beam radiation therapy, the radiation field size should be increased with respect to the geometric field settings. One could measure the dose distribution in the plane perpendicular to central axis at a depth of prescribed dose and increase the field size or simply change the prescribed isodose line for appropriate target coverage. In this study, lateral electron coverage is studied by using MapCheck a 2D diode array system. Method and Materials: Output factors, measured for 9 and 22 MeV electron beams by using MapCheck and 0.125 cm3 ion chamber on Varian IX (Linac) were compared. Set up geometry was kept the same. Dose calibration was performed for each electron beams and used in final dose measurement for MapCheck. Plans were performed on Eclipse planning system with 2×2 to 15×15cm2 for 9 and 22 MeV. Electron Monte Carlo was used for dose calculation and the plans were delivered on MapCheck and compared by using comparison criteria of 3% dose difference and 3 mm distance‐to‐agreement (DTA) within 10% isodose‐line threshold like IMRT dose verification. Results: Out put factors for 9 and 22 MeV measured for 2×2 to 15×15 cm2 with MapCheck and ion chamber agrees well within ±1.4%. Measured and planned dose comparisons for 9 MeV for 10×10 and 3×3 cm2 small insert shows very good agreement with passing rate of 98.4 and 97.8 % respectively. Conclusions: By measuring the dose profile at a prescribed depth one could easily verify the dose coverage to the target by measuring the lateral distance and also area covered by prescribed isodose line. MapCheck could also be used to measure the output factors for very small and irregular electron fields. Since MapCheck is commonly used in radiation therapy and it provides additional role in electron beam dosimetry.
SU‐FF‐T‐379: Procedure for Dose Rate Determination Via System Response of A Water Calorimeter
Authors: Chen‐Mayer H , Tosh R
Published: April 2010

Purpose: To extract a consistent dose rate from water calorimetry measurements for all irradiation times. Method and Materials: Conventionally, the dose rate is determined at a length of time short compared to the heat conduction time in the system with the correction made by a model calculation for the given length of time (denoted t_open). We advocate a systematic approach by tracing the response of the calorimeter beyond the small t_open (10s of seconds) to 100s of seconds where conduction effect contributes significantly, requiring conduction correction to be valid over the entire time scale. This is made practical by modulating the radiation shutter at various t_open and by frequency domain analysis. The output from a 3D finite element model simulating the experimental protocol in a Co‐60 beam with a 20 mm radius vessel undergoes the same analysis as the experimental data to extract the response to unity radiation dose which can be scaled to any dose rate. Results: We scale the calculated curve to match the data by chi‐square minimization; the scaling factor represents the dose rate for all t_open values. Two sets of data were analyzed; the results for t_open from 60 s to 240 s are compared to Domen's transfer value (t_open=70 s) at standard geometries (SSD=100 cm), yielding a dose rate 1.0% and 0.9% below the transfer values (1 sigma statistical uncertainty 0.44% and 0.17%), respectively. A set of data taken at SSD of 77.4 cm gives a dose rate 3.4% below the transferred value. Ion chamber measurements will be made to resolve this discrepancy. Conclusion: A procedure to derive the dose rate by requiring self‐consistency between the modeled and the measured system response of the calorimeter over large time scales is a step closer to re‐establish a primary standard using a water calorimeter.
SU‐FF‐T‐382: Ball‐Cube II: A New Film‐Based QA Tool to Characterize Stereotactic Dose Delivery
Authors: Purwar A , Main W , Leonovich A , Sorensen T
Published: April 2010

Purpose: To evaluate Ball‐cube II (Accuray Incorporated, Sunnyvale, CA), an improved film‐based QA tool for Stereotactic Radiosurgery (SRS). Method and Materials: Ball‐cube II consists of four plastic sections connected together with clips. It accommodates two pieces of radiochromic EBT film (ISP Corporation, Wayne, NJ) in orthogonal planes. The primary advantage of Ball‐cube II over its predecessor “Ball‐cube I” is that it has four 0.1 inch (2.54 mm) film alignment posts, which when used with matching laser cut film lead to a precise and reproducible fit, thus minimizing errors due to film positioning. The alignment posts also have embedded copper markers to allow automatic registration of the film with the RT dose distribution. These markers are sufficiently visible to be located in the CT image, but are small enough to distinguish them from embedded gold fiducials used for patient alignment and tracking. Ball‐cube II has additional improvements, e.g., nylon nuts in the original design were replaced with clips, which enables easier and faster loading of the film and eliminates the possibility of over tightening the screws, which can cause the film to darken. The cube sits in an anthropomorphic head phantom and can be tracked using either the embedded gold fiducials or bony anatomy. The phantom treatment process is performed just as with a patient: CT image acquisition, treatment planning and phantom irradiation. The films are analyzed to compute spatial targeting error. Ball‐cube II and its predecessor were compared by performing 10 spatial targeting tests for each. Results: The mean targeting error is equivalent (0.40 vs. 0.39 mm), but the standard deviation is 46% smaller for Ball‐cube II (0.07 vs. 0.13 mm). Conclusion: The Ball‐cube II design reduces uncertainty in film analysis with precise film positioning and automatic dose‐to‐film registration.

Research sponsored by Accuray Incorporated.
SU‐FF‐T‐385: Commissioning of a PTW Starcheck 2‐D Ion Chamber Array
Authors: McEwen M , Xu L
Published: April 2010

Purpose: To investigate and commission a PTW Starcheck 2‐D array in megavoltage photon beams and determine to what extent it can be used to replace in‐water ion chamber scans. Method and Materials: The Starcheck was introduced in 2008 and consists of 4 main arrays (0°, 90° and two diagonals) and is designed for both live diagnostics of linac beams and 2‐D QA measurements. Three parameters were investigated — dose linearity, relative precision, energy dependence (kQ) — and the results from the Starcheck mounted in a Virtual Water phantom were compared with scans obtained in a water tank (using both an ion chamber and diode) and with a 1‐D ion chamber array (Victoreen THEBES). Results: The dose linearity was found to be better than ± 0.1 % over the range 0.25 Gy to 8 Gy and short‐term repeatability was at the same level. The measured relative calibration was found to be significantly better than the manufacturer's specification, with a relative precision of 0.3 % (1 sd). The absolute calibration results for 6, 10 and 25 MV photon beams were surprising as the kQ factors for the central chamber did not follow the expected water‐air stopping power ratio, indicating a significant perturbation (perhaps due to the construction of the electrodes). The comparison of scans for a 10×10 cm field showed that the Starcheck in solid phantom agreed with the in‐water ion chamber scan within 0.5% at all points. Conclusion: The Starcheck is an easy‐to‐use device that meets, and generally exceeds, the manufacturer's specifications for relative dosimetry and can be used as a substitute for ion chamber scans where a precision of 0.5% is acceptable.
SU‐FF‐T‐388: Comparison of R2 Estimation Algorithms for Polymer Gel Dosimetry
Authors: Watanabe Y , Kubo H
Published: April 2010

Purpose: To evaluate R2‐estimation algorithms applied for MRI‐based polymer gel dosimetry. Method and Materials: We evaluated weighted least‐squared (WLS), least‐squared (LS), and maximum‐likelihood estimation (MLE) methods. For MLE, we tested both Gaussian and Rician probability distributions (MLE_G and MLE_R). To increase the accuracy of the R2‐estimation, we proposed a variable echo number technique (VAREC), in which the number of echo signals used for the estimation was optimized depending on the expected R2‐value. The VAREC method was used with WLS and MLE_G. These algorithms were used to estimate R2 values of BANG polymer gel, which was irradiated for 12‐different dose levels ranging from 0 to 50 Gy. The R2 measurements were done by using a 32‐echo CPMG pulse sequence as implemented in Siemens Trio 3T MRI scanners. Estimated R2 values, 〈R2〉, were plotted as a function of dose for the four methods, which were compared with a reference. Results: With WLS and MLE_G, 〈R2〉 increased up to an absorbed dose of about 6 to 10 Gy; however, above these doses, 〈R2〉 decreased with increasing dose. This was a manifestation of an algorithmic error of those methods. When the VAREC technique was used with WLS and MLE_G, the erroneous behavior of the 〈R2〉 and dose relationship disappeared. Bothe LS and MLE_R did not show such an error; but, 〈R2〉 values for higher doses were overestimated. The computing times of LS and MLE_R were 10 and 60 times longer than MLE_G, respectively. Conclusion: WLS and MLE_G lead to equally reliable results in a reasonable computing time. Since MLE is known to be more accurate for noisy data than WLS, we strongly recommend the MLE algorithm with the VAREC technique for R2‐estimation to the MRI‐based polymer gel community.
SU‐FF‐T‐389: Prototype Electron Phantom for Radiographic and Radiochromic Film Dosimetry
Authors: Robertson C , Price M , Parker B , Hogstrom K
Published: April 2010

Purpose: To develop a practical electron beam phantom for use with radiographic and radiochromic film to measure relative dose in a plane containing the central axis. Method and Materials: A prototype Solid Water® film phantom was used to measure 2D dose distributions in a principal plane containing the central axis for 6–20 MeV electron beams. Relative depth‐dose and 2‐D dose distributions were measured in the phantom using both Kodak XV radiographic and GafChromic EBT radiochromic films. To evaluate the dosimetric accuracy of the film‐phantom system, film measurements were compared to equivalent measurements acquired using a scanning diode and water phantom. Dose differences were evaluated with respect to regional criteria of acceptability: (1) high dose, low dose‐gradient region (⩽2 % dose), (2) high dose‐gradient region (⩽2 mm DTA), and (3) low dose, low dose‐gradient region (⩽2 % dose). Results: Radiochromic film depth‐dose measurements (9 and 16 MeV) agreed with diode measurements within criteria in all regions. Radiographic film depth‐dose measurements also agreed well in all regions excluding the build‐up region (<10 mm depth) where measurements were approximately 3% low. For both types of film, 2D dose distributions agreed with diode measurements in all regions of the curve with the maximum error (≈3%) near the surface (95% dose contour). Conclusion: The prototype phantom is capable of measuring relative electron dose distributions using radiographic and radiochromic film that are consistent with measurements taken using a scanning diode in a water phantom. Conflict of Interest: Gammex‐RMI, Inc. provided the prototype phantoms for this study.
SU‐FF‐T‐390: In‐Vivo Prostate Brachytherapy Absorbed Dose Measurements
Authors: Gueye P , Velasco C , Keppel C , Murphy B , Sinesi C
Published: April 2010

Purpose: In‐vivo prostate brachytherapy absorbed dosimetry detector using scintillating fibers. Method and Materials: Five pairs of 85.5 ± 0.05 cm long blue shifted scintillating fibers (model BCF‐10) with 1 mm2 cross sectional area were placed in a mixture of gelatin (368.6 ± 0.5 grams) and water (3.78 ± 0.025 liters) to measured the absorbed dose delivered by a 12 Ci 192Ir HDR source. The fibers were held by a 7 × 7 cm2 template grid and optically connected to a 16‐channel multianode photomultiplier tube (Hamamatsu, model H6568). Each pair consisted of one fiber 4 mm shorter than the other one to extract the dose by the subtraction method. A dose atlas was used for radiation delivered to the phantom. The plans followed delivered 5 and 7 Gy to a point located 2.0 centimeters away from the central dwelling positions. A total of 32 data points were acquired in a plan to assess the linearity and reproducibility of the measurements. Results: Reproducibility of the data was found to be within 5% and the overall accuracy of the system estimated to be ±5.5%. The linearity of the data for all 7 measured dose values (ranging from 0.6 to 7 Gy), gives a slope of 312 counts/Gy with a 1.4% relative deviation. Conclusion: This work indicates the possibility of measuring in real‐time the dose effectively delivered to a biological system during prostate brachytherapy treatments. The availability of commercially thin (150 μm) scintillating fibers opens the capability of using such system during clinical treatments (by embedding the fibers within the catheters) with the advantage of performing real‐time adjustment of the dose delivery.
SU‐FF‐T‐391: IC Profiler Study: The End of Water Tank Measurements for Monthly QA?
Authors: Furstoss C , Ayles M , Munger P , Wierzbicki W
Published: April 2010

Purpose: To investigate the possibility of using the IC Profiler (Sun Nuclear Corporation) for monthly QA of linear accelerators. Method and Materials: Photon and electron profiles measured with the IC Profiler, consisting of 251 ionization chambers located along the X, Y and diagonal axes, were compared to the profiles measured in water with an IC10 ionisation chamber. Three different methods were used for energy measurement of the photon and electron beams: 1) different thicknesses of solid water slab on the IC Profiler, 2) an acrylic continuous energy wedge and 3) a step wedge made of solid water. For the last two methods, the measured PDD equivalent curve was corrected for the change in profiles in order to remove the profile effect. All photon measurements were performed at SSD=75 cm with a 30×30 cm2 field size in order to be equivalent to a 40×40 cm2 at the isocenter. The electron measurements were done at SSD=100 cm with a 25×25 cm2 field size. Results: The agreement between symmetry and flatness measurements with the IC Profiler and the water tank was very good: maximum relative difference of −0.7% for 6 MV and 25 MV photon beams and −1.3% for electron beams (from 4 MeV to 22 MeV). Energy measurement using methods 2 and 3 was very fast and seemed to be as sensitive to an energy change as method 1. Method 2 gives a linear interpolation of the relative dose as function of the depth whereas method 3 gives the relative dose averaged over 10 detectors at 4 different depths. Conclusion: The IC Profiler is well adapted to the measurement of photon and electron profiles. Three different methods were tested for energy determination and more measurements are needed to establish the most accurate one.
SU‐FF‐T‐393: Investigation of Varian's Trilogy LINAC to Deliver Fractional MU
Authors: Tong S , Li T , Chu K
Published: April 2010

Purpose: To evaluate whether the Trilogy can correctly deliver the fractional MU specified by IMRT plan. Method: We designed a simple dynamic MLC file with 10 step‐and‐shoot IMRT segments to run on a Trilogy to measure dose‐per‐MU for each individual MLC segment. MLC leaves were open during beam‐On segments. After each segment, some of the MLC leaves were programmed to move back‐and‐forth which forced the Trilogy to hold off the beam for about 5 seconds until next beam‐On segment. During this time the dose delivered in the previous segment was recorded using a Farmer chamber and electrometer. 6 MV photon beam with a 400MU/min was used in this experiment. Results: The Trilogy demonstrated excellent dose linearity (r2 = 0.999997), in terms of total dose vs. total MU for each beam, for both static and dynamic MLC beams. Further data analysis on dose per fractional MU in individual segments revealed that the Trilogy tends to over‐shoot in the first segment before stabilizing. To compensate for the overdosing in the first segment, the last segment of a beam is always underdosing because the Trilogy monitor chamber integration circuit stops the beam when it reaches the programmed MU, regardless of the dose delivered in the individual MLC segments. Dose per MU delivered for the segments in the middle of the beam is relatively stable. Conclusions: Although one can not program the Trilogy from the treatment console to run a fractional MU, the Trilogy has the ability to deliver fractional MU during dynamic MLC treatment such as IMRT. It appears that Trilogy has a tendency of overdosing the first segment and underdosing the last segment. Relative error can be 20 to 30% when the segment MU is less than 3.
SU‐FF‐T‐394: Monte Carlo Computation of the Effective Dose in Scanning Beam Proton Therapy
Authors: Mirkovic D , Titt U , Sawakuchi G , Zhang X , Li Y , Mohan R
Published: April 2010

Purpose: Currently most clinical proton centers use constant, generic relative biological effectiveness (RBE) value of 1.1. However, the RBE for protons depends on the dose per fraction, the tissue type, the biological endpoint and the local energy spectrum characterized by the linear energy transfer (LET). In this work we describe the tools for automatic Monte Carlo (MC) calculation of the LET in clinical scanning beam proton plans and investigate the effects of variable LET on the effective dose. Method and Materials: A clinical prostate scanning beam proton treatment plan was used to automatically generate the input for the MC code MCNPX. The input contained a detailed model of the scanning beam nozzle and delivery system with spot positions and intensities extracted from the actual patient treatment plan. The input also contained a voxelized, CT based patient model or a water phantom. A rectangular mesh tally was used to score the energy deposition and the fluence in the patient or in the water phantom. These values were used to compute the absorbed dose, the LET, the RBE as a function of the LET, and the RBE‐weighted effective dose in the patient or in the water phantom. We have adopted a simple functional dependence of the RBE on LET from literature. Results: Three dimensional distributions of the absorbed dose, the LET, and the effective dose were computed both in the water phantom and in the patient. The results show a steady LET and RBE increase along the beam path with a number of spikes resulting from the end‐of‐range effects of the individual energy layers. Conclusion: Monte Carlo model can be used for computation of the RBE‐weighted effective dose distributions in clinical scanning beam proton plans. This model can be used to investigate whether the effects of variable RBE can be clinically significant.
SU‐FF‐T‐397: Positioning Recommendations for Parallel‐Plate Ionization Chambers in Clinical Electron Beams According to Different Dosimetry Protocols ‐ a Monte Carlo Study for Three Different Chamber Types
Authors: Zink K , Altintas S , Poppe B , Wulff J
Published: April 2010

Purpose: Current dosimetry protocols recommend the use of plane‐parallel ionization chambers for the dosimetry of clinical electron beams. Regarding the allowance for the non‐water equivalence of the entrance window and therefore the exact positioning of the parallel‐plate chambers, the recommendations given in these protocols are not unique, resulting in positioning deviations in the range of tenth of millimeter. Method and Materials: The dose and the spectral fluences inside the active volume of three parallel‐plate chamber types (Roos‐, Advanced‐ Markus‐ and NACP‐chamber) were calculated in a water phantom in the reference depth zRef and the half‐value depth R50 for three electron energies (6, 11 and 21 MeV) using the EGSnrc Monte Carlo code. From these results the perturbation correction pwall was calculated as pwall = Dcav/Ddet where Ddet is the dose inside the active volume of the chamber and Dcav the dose inside the air cavity with walls entirely made of water. To inspect the consequences of the different positioning recommendations given in the dosimetry protocols (AAPM TG‐51, IAEA TRS‐398, DIN 6800‐2), the chamber's reference point was shifted by an amount Δz to fulfill the recommendations given in the different protocols. Results: Although the recommended shifts Δz are in the range of tenth of millimeters only, the spectral fluence inside the active volume of the chambers changed strongly with Δz. At the depth zref these spectral fluence changes had no impact on pwall. For all chambers its numerical value is larger than 1, the recommended value given in all protocols. Contrary, at the depth R50 the different positioning recommendations resulted in deviations of pwall up to 15%. Conclusion: The different positioning recommendations have no impact on the results of dose measurements at the clinical relevant depth zref. The resulting deviations in pwall at the depth R50may be of academic interest.
SU‐FF‐T‐400: Monte Carlo Simulations of Compact Plasma Accelerators for Proton Radiotherapy
Authors: Constantin M , Keall P , Perl J , Poehlmann F , Rieker G , Cappelli M
Published: April 2010

Purpose: One common characteristic of the compact plasma proton accelerators is a broad energy spectrum which can not be used directly for radiotherapy. The purpose of this study is to analyze the effect of the geometrical parameters of the treatment head components, their materials, and the magnetic field configuration, on the proton and neutron dose distributions to identify an optimal design for the plasma proton accelerator's treatment head and provide important feedback to ongoing plasma proton experiments. Method and Materials: Calculations of proton dose distributions and the neutron dose equivalent were performed using Monte Carlo simulations by irradiating a water phantom with protons in the energy range of 175 to 250 MeV. The simulations were performed using the Geant4 (version 4.9.2) toolkit. The input spectra were experimental energy spectra scaled to energies relevant for radiotherapy, having a broad Gaussian distribution. Results: We have designed a collimating system, magnetic field region, energy selector, and shielding components for a plasma accelerated proton therapy system. The collimating and energy selector systems are needed in order to achieve proton dose distributions superior to the conventional X‐ray radiotherapy distributions. It is shown that a great number of unwanted electrons and photons is stopped by a sequence of tungsten absorbing disks placed inside the collimating and energy selector systems. Also, a sequence of three boronated polyethylene layers is shown to substantially capture the secondary neutrons. Dose deposited by neutrons in the water phantom is found to be 6 orders of magnitude lower than the dose deposited by protons, thus giving negligible contribution to the integral dose. Conclusion: We have studied the design of the plasma proton accelerator's treatment head. It is shown that the proposed system is capable of delivering clinically relevant proton dose distributions. The implemented shielding components ensure extremely low neutron dose distributions.
SU‐FF‐T‐402: Source Modeling for CyberKnife Monte Carlo Dose Calculation Using the IRIS Variable Aperture Collimator
Authors: Fan J , Li J , Wang L , Jin L , Xu Q , Chen L , Price R , Ma C
Published: April 2010

Purpose: The goal of this project is to model the Accuray CyberKnife Linear Accelerator (“Linac”) using the Iris Variable Aperture Collimator for Monte Carlo treatment planning. We developed and tested a straightforward source modeling and commissioning process for clinical Monte Carlo calculations. Method and Materials: The Iris Collimator consists of a dual bank of six‐sided collimators that dilate and contract in unison. The two banks are rotated 30 degrees with respect to each other in order to provide dodecahedral shaped collimation. The Iris collimator will have 12 configurations that correspond to 12 field sizes that vary from approximately 5 to 60 mm (projected to 800 mm). Our commissioning process of fitting treatment planning system data includes fitting the source energy spectrum, source space distribution, matching penumbra fluence blurring and the collimator scattering. The source distribution was de‐convoluted from the fluence penumbra distribution. The source energy spectrum is determined from the measured central axis PDD in water at 80 cm SSD using a 60 mm cone. Scattering radiation from the Iris collimator is modeled as a ring source at the bottom of the lower bank with a Gaussian angular distribution. Results: We tested our methodology on dose distributions between calculations using the source model, calculations using the Monte Carlo simulated phase space and measurement. The agreements in percentage depth dose curves and dose profiles between the two were generally within 1%/1mm for all the collimators from 5 mm to 60mm. Conclusion: In this work we investigated the method of source modeling for the Cyberknife Linac system with the Iris collimator. Dose calculation using the source model demonstrates the efficiency and the simplicity of the new model, which lays the groundwork on a simple source model from a set of measurement data that can be easily obtained clinically.
SU‐FF‐T‐405: Dose to Water Versus Dose to Medium in Proton Beam Therapy
Authors: Paganetti H
Published: April 2010

Purpose: Dose in radiation therapy is traditionally reported as dose‐to‐water, Dw. Monte Carlo (MC) dose calculations report dose‐to‐medium, Dm. A methodology is needed to convert Dm into Dw for comparing MC and planning system results. This work addressed the following questions for proton therapy: What formalism allows the conversion from Dm to Dw?

1. Can we convert MC calculated Dm into Dw retroactively or do we have to convert during particle tracking?

2. What is the difference between Dm and Dw when analyzing patient data in terms of dose distributions, dose volume histograms and absolute doses?

3. Is there a difference in the predicted beam range between Dm and Dw?

Method: We did develop a formalism to convert Dm into Dw for proton beam dose calculations. Three different methods are introduced, an approximate method based on relative stopping power which allows retroactive conversion, a method considering energy dependent relative stopping powers, and a method incorporating nuclear interaction events as well. A total of 33 patient fields were analyzed. MC calculated dose distributions, dose volume histograms and absolute doses to assess the clinical significance of differences between Dm and Dw were compared. Results: We found that the difference between the three conversion methods was within 1% in most cases when analyzing mean doses to contoured structures. Further, we found that the difference between Dm and Dw can be up to 10% for high CT numbers. The difference is clinically insignificant for soft tissues. For proton beams stopping in bony anatomy, the predicted beam range can differ by 2–3 mm when comparing Dm and Dw. Conclusion: For comparison between MC and analytical dose distributions in proton therapy, a dose conversion is required. However, retroactive conversion seems to be sufficiently accurate in most cases, except for the end of range.
SU‐FF‐T‐406: Toward a More Accurate Dose Calculation Technique Using a Semiautomatic Organ Contouring in Monte Carlo Post‐Implant Assessment of Breast LDR Brachytherapy
Authors: Afsharpour H , Pignol J , Keller B , Carrier J , Reniers B , Verhaegen F , Beaulieu L
Published: April 2010

Purpose: We assess the impact of the uncertainty of glandular/adipose tissue proportion on dose distribution inside the breast for LDR brachytherapy. Furthermore, we propose a semiautomatic tissue segmentation method to be used in Monte Carlo (MC) calculations for breast LDR patients. Methods & materials: Post‐implant CT exams of five breast brachytherapy patients are imported into our Geant4 MC platform. The CTV and the PTV were contoured by the oncologist; the external contours of the breast, skin, lungs and the ribs were made manually by a physicist. In a first step, the breast is assigned with different proportions of glandular/adipose tissue (ρ = 100/0, 75/25, 50/50, 25/75, 0/100) in order to study the impact of this variation on clinical dosimetries. In a second step, a density‐based semiautomatic segmentation of breast tissue is used to identify glandular from adipose regions inside the breast and to create a more representative breast anatomy with appropriate chemical compositions. A TG43‐like simulation (SMC: Superposition MC) is also performed for each patient. Does‐Volume‐Histograms are used to visualize the effect of dose reduction. Results: In all patients, the glandular/adipose proportion, affects the dose distribution across the breast. The higher the adipose proportion in the breast tissue, the larger is the dose reduction across the organ. The D90 clinical parameter is reduced by up to 22% when the breast is entirely made of adipose. The semiautomatic contouring enabled a patient‐dependent segmentation of the glandular and adipose regions leading to more accurate dose calculations for each patient. Conclusion: Using realistic chemical compositions in MC simulations is achievable for MC calculations. Unlike external‐beam radiotherapy, the low‐energy emission of 103Pd is strongly affected by the heterogeneities adipose proportion in breast. A patient‐dependant glandular/adipose segmentation in breast is important for accurate dose determinations in MC, especially for breast LDR brachytherapy.
SU‐FF‐T‐410: Simulation of Large X‐Ray Fields Using Independently Measured Source and Geometry Details
Authors: Sawkey D , O'Shea T , Taranenko V , Faddegon B
Published: April 2010

Purpose: Measure and simulate dose distributions from a 6 and 18 MV x‐ray linac with open jaws, including asymmetry, with independent measurements of the source and geometry details. Materials and Methods: Energy of the incident electron beam was measured by removing the target and flattening filter and measuring the percent‐depth ionization curve of the raw beam. Spot size was measured with an alternating layer spot camera. Treatment head parameters were determined from disassembly of the treatment head, or deduced from the penumbra position. Flattening filter density was determined by weighing it. Distance between target and flattening filter determined from position of the penumbra in the diagonal profiles. Simulations were with EGSnrc. Measurements and simulations were done both with and without the flattening filters. Results: Measured and simulated dose distributions inside the treatment field agreed to 1%/1mm. The exception was the buildup region with no flattening filters, where measurements were greater than simulation by 3–5%, corresponding to a 1 mm shift. This suggests the Roos chamber over‐responded to low energy electrons. Conclusions: Close agreement betweem measurement and simulation was obtained, with simulation parameters determined independently. It was not necessary to remove the flattening filters to obtain the simulation parameters.

Support from NIH R01 CA104777‐01A2 and Siemens OCS.
SU‐FF‐T‐414: Monte‐Carlo BEAM Modeling of Off‐Axis Headscatter for Megavoltage Photons with MLC
Authors: Chang C , Zhu T
Published: April 2010

Purpose: The study examines the off‐axis headscatter for megavoltage photons with a Multi‐Leaf Collimator (MLC). Methods and Material: The effect of off‐axis headscatter becomes most pronounced in low dose regions IMRT fields. We define the headscatter off‐axis ratio (HOA) as the ratio of the kerma from headscatter photons at off‐axis positions to that from direct primary photons on the central axis. Monte‐Carlo (MC) simulations of HOA are performed for an accelerator with an MLC replacing the lower jaws. Headscatter contributions from components of the LINAC, e.g. flattening filter, MLC, primary and secondary collimators, are separated for lateral mean energy distribution, HOA, energy fluence distribution and spectrum. Results: Lateral distribution of the mean energy differs for individual components. Mean energy distribution within the beam collimation is dominated by the primary beam and decreases radially from 1.8∼1.6MV. The mean energy distributions of primary collimator and flattening filter remain relatively flat within the treatment field at 1.5MV and 1.2MV, respectively. MC simulation also shows that dose outside of treatment field is dominated by the headscatter from the flattening filter, the primary collimator, and the secondary collimators. Primary collimator and flattening filter's contributions to HOA on‐axis are estimated to be 1.8% and 3.7%, respectively, for a 20×20 cm2 field. Total headscatter contribution in this case is ∼6.1%, which agrees with experimental results to within 1%. Contributions to HOA from the Y‐Jaw and the MLC are consistently smaller than 0.3%. The asymmetric nature of HOA, due to the vertical displacement of the secondary X & Y collimators, is also examined. Conclusion: Contributions from individual components in the treatment head to off‐axis headscatter are characterized. It's found that the asymmetric distribution of HOA due to contributions from MLC and Y‐jaw may need to be incorporated into future IMRT treatment planning systems.
SU‐FF‐T‐417: Effect of Transverse Magnetic Fields On MV Photon Dose Distributions in Heterogeneous Media
Authors: Gauthier M , Bouchard H , Després P
Published: April 2010

Purpose: To study the effect of transverse magnetic fields on dose deposition profiles in regions of mass density gradients. This work characterizes this effect with a simple Monte Carlo model of a lung tumor irradiated with a 6 MV photon beam. Method and Materials: The Geant4 Monte Carlo toolkit was used to simulate the irradiation of a simple phantom representing a lung tumor. A 4 cm spherical tumor made of soft tissue (1.06 g/cm3) was positioned at the center of a 14×14×14 cm3 box made of lung tissue (0.3 g/cm3). A uniform magnetic field was applied throughout the phantom perpendicularly to the beam axis. Simulations were conducted with field intensities ranging from 0 to 5 tesla. The phantom was irradiated with a 6 MV photon beam created with the BEAMnrc code. Results: Significant dose modulation was observed at the lung‐tumor interface. Regions of dose enhancement and symmetrically opposed regions of dose reduction were observed at the vicinity of the tumor boundary. These regions vary in shape, intensity and location with field strength in a non trivial way. As a rule of thumb, these crescent‐like regions tend to become sharper and shift rotationally with field strength. Conclusion: Magnetic fields were shown to affect significantly dose deposition profiles in heterogeneous media. Further investigations are required to understand the pattern of dose modulation as a function of magnetic field intensity, heterogeneity size and heterogeneity surface orientation relative to beam axis. Devising approaches to potentially exploit or cancel out these effects is also of interest.
SU‐FF‐T‐424: Benchmark Investigation of MCNP5, EGSnrc, and PENELOPE Monte Carlo Codes for Calculating the Emitted Photon Spectrum From An Electronic Brachytherapy Source
Authors: Davis S , DeWerd L
Published: April 2010

Purpose: To perform a benchmark investigation of three general‐purpose Monte Carlo codes used to calculate the emitted photon spectrum from the Xoft Axxent® electronic brachytherapy source. Method and Materials: Three different Monte Carlo codes, MCNP5, EGSnrc, and PENELOPE were benchmarked for parameters relevant to simulation of the Axxent® source. The tested parameters were electron backscatter from a high atomic number slab, electron impact ionization and bremsstrahlung production in tungsten, and yttrium fluorescent photon production. A full model of the Axxent® source was constructed in each code, and the differences in the emitted photon spectra were explained based on the results of the simple benchmarks. Results: The three electron transport algorithms in MCNP5 produced up to 48% differences in electron backscatter, but the best agreement with EGSnrc, PENELOPE, and experimental data was with the ITS‐style energy indexing. The choice of electron transport cutoff energy in PENELOPE had a large effect on the number of backscattered electrons, but less of an effect on the total backscattered energy. Bremsstrahlung production in tungsten was in good agreement for all three codes, but large differences in the production of tungsten L‐shell fluorescent photons were observed. The yttrium K.. fluorescent photon production in MCNP5 was 35% low compared to PENELOPE when executed with full photon/electron transport, but it was within 3% of PENELOPE when executed using photon transport only. The simulation results for the full model of the Axxent® source were consistent with the results from the individual benchmarks, with significant differences in the tungsten L‐shell fluorescent photons and yttrium fluorescent photons. Conclusion: The three Monte Carlo codes produced differences in the calculated photon spectrum for the Axxent® source, and these differences will have an effect on the corrections applied for air‐kerma strength measurements. Conflict of Interest: This study was funded in part by Xoft, Inc.
SU‐FF‐T‐427: Monte Carlo Dose Distributions From Photon Beams Obtained with the Bethe‐Heitler and NIST Bremsstrahlung Cross‐Sections
Authors: Fragoso M , Faddegon B , Chetty I
Published: April 2010

Purpose: Reports on Monte Carlo‐based clinical dose calculations have identified a paucity of information on the influence of cross‐sections on such calculations. A recent investigation on BEAMnrc‐based linac phase space (PHSP) calculations showed systematic differences (up to 3%) between fluence distributions computed using the Bethe‐Heitler (BH) and NIST bremsstrahlung cross‐sections. To understand the impact of these differences, we performed a study involving dose calculations in simulated water and heterogeneous phantoms. Method and Materials: DOSXYZnrc was used to calculate the dose distributions from a 6 MV (Siemens, Primus) photon beam. BEAMnrc was used to generate PHSP files, using the BH and NIST bremsstrahlung cross‐sections, for field sizes ranging from 5×5 to 40×40 cm2. Dose was computed in phantoms composed of water and heterogeneous media, including air, lung, bone and aluminum slabs, following the ICCR benchmark. The voxel dimensions were set to 5×5×2 mm3 (x, y, depth dimensions). Results: MC‐calculated dose distributions using the NIST and BH bremsstrahlung cross‐sections agreed within 2% for all geometric configurations and field sizes considered, including at interfaces in the ICCR benchmark phantom. Larger differences (up to 4%) were noted for the 40×40 cm2 field size, due to statistical fluctuations resulting primarily from the higher latent variance in the PHSP file for this large field size. Conclusions: Despite relatively large, systematic differences observed in the fluence distributions generated with the NIST and BH bremsstrahlung cross‐sections, calculated dose distributions in water and heterogeneous phantoms agreed within 1–2% on average. On the other hand, the insensitivity of dose distributions in phantom illustrates the importance of studying fluence distributions in MC‐based dose calculations, particularly when generating beam models from PHSP. Though we have shown that the differences in the fluence distributions have no impact in dose calculations, further investigation is needed on the cause of this discrepancy.

Acknowledgement: NIH‐R01CA106770, NIH‐R01CA104777
SU‐FF‐T‐428: Deformable Computational Breast Phantoms for Monte Carlo Based Calibrations of Detector Systems Used for Assessing Internal Radioactivity Burden in the Lungs
Authors: Mille M , Hegenbart L , Na Y , Zhang J , Xu X
Published: April 2010

Purpose: To demonstrate the feasibility of deformable patient modeling for the virtual calibration of detectors used to measure inhaled radioactivity in a female patient's lungs for internal dose assessment. Materials and Methods: We have developed the ability to deform a mesh‐based phantom that consists of 140 highly detailed organs or tissues. The phantom can be adjusted to match a desired patient. A software was developed to deform the breasts of this phantom to create new models representing female patients with different breast cup sizes (ranging from AA to G) and breast glandularities. The geometries of these phantoms and a Phoswhich detector system were defined in a Monte Carlo code for virtual in‐vivo lung counting simulations involving various photon emitting radionuclides. The counting efficiencies for each of the virtual patients were calculated and compared. Results: The counting efficiency was found to decrease with increasing breast size and mass because of greater attenuation in the patient. For low energy emitting radionuclides such as Am‐241, roughly a 50% drop in counting efficiency was observed for the model with G cup size as compared to the smaller breasted AA model. Higher breast glandularities resulted in lower counting efficiencies; however, this effect was small. For the E breast cup size model, the counting efficiency for low energy emitters decreased by roughly 2% as the glandularity increased from 7% to 40%. Conclusions: In order to obtain accurate internal dosimetry estimates for female patients, the in‐vivo measurements of the activity in the lungs should account for breast size. The effect of breast glandularity can be ignored because it is negligibly small compared to other sources of experimental uncertainty.
SU‐FF‐T‐429: Electron Monte Carlo Dose Calculation in Eclipse for Siemens Linear Accelerators
Authors: Fix MK , Cygler JE , Vandervoort E , Karle H , Tertel J , Frei D , Volken W , Born EJ , Manser P
Published: April 2010

Purpose: Initially, the implementation of the macro Monte Carlo (MC) method into Eclipse (Varian Medical Systems), named eMC, was only carried out for Varian accelerators. This leads to limitations in accuracy if eMC is applied to Siemens machines. In this work eMC has been adjusted in order to allow accurate dose calculations of electron beams for Siemens accelerators. Method and Materials: Several changes have been implemented into the eMC algorithm. First, the beam model has been modified by introducing a 2D fluence instead of a 1D radial fluence distribution for the primary electrons and by including all scrapers of the applicator. Second, the resolution of mono‐energetic depth dose curves used during beam configuration has been increased to determine the initial electron energy spectrum. Furthermore, the size of the sphere within the macro MC transport has been reduced when the incident energy of the electron is below certain thresholds. Calculated and measured dose distributions are compared for Siemens machines using electron energies of 6, 7, 9, 13, 17, 20 and 21 MeV and applicators ranging from 10×10 to 25×25 cm2. Results: Calculated and measured absolute depth dose curves agree within 1% or 1 mm for all energy and applicator combinations investigated. Calculated and measured absolute dose profiles at depths of dmax, R50 and in the photon tail generally agree to within 2%. Some dose values close to the field edge show slightly larger differences. Conclusion: In this work several improvements have been implemented into the eMC algorithm. Due to these improvements the eMC algorithm was able to successfully configure electron beams from Siemens machines. The results of the dose comparison suggest that eMC is suitable to predict dose distributions also for Siemens linear accelerators. Conflict of Interest: This work was supported by Varian Medical Systems.
SU‐FF‐T‐432: Measurement Driven, Electron Beam Modeling and Commissioning for Monte Carlo Treatment Planning with Improved Accuracy
Authors: Thebaut J , Deblois F , Alexander A , Seuntjens J
Published: April 2010

Purpose: Numerous Monte Carlo (MC) models of therapeutic electron beams are presented in the literature. However, beam models built solely with manufacturer specifications of the accelerator do not systematically provide acceptable agreements with measurements. Clinically, accurate beam models are crucial to MC treatment planning as electron dose calculations found in commercial treatment planning system (TPS) are generally inaccurate. Therefore there is a strong motivation to use highly accurate MC simulations as standard information for commissioning commercial TPS. The current research project consists of developing an improved accurate electron beam model based on detailed information of the linac head and to incorporate it into an in‐house TPS. Method and Materials: A model for the dual scattering‐foil linac CL‐21/EX (Varian) has been built with the EGSnrc/BEAMnrc‐MC codes for all clinical electron energies available. Measured PDD and transverse dose profiles at various depths of clinical interest were used to tune‐up the model. Our commissioning procedure also includes measurements made with fully opened jaws without applicators. This procedure yields a more insightful understanding and accurate description of the source geometry. Measuring tools consisted of an IBA‐Blue phantom water tank, diodes, parallel plate and cylindrical ionization chambers. Calculations were performed on a 24‐cores MC cluster. Results: Our model has been validated for regular, irregular fields and custom cutouts against measurements and is accurate within 2.9%. The model has been incorporated into an in‐house planning system. A previously developed EGSnrc code was used to transport the particles of the phasespace through the custom cutouts allowing further simulation into CT patient. Conclusion: Excellent agreement was achieved between measurements and MC simulations. Our accurate electron beam model incorporated into our MC TPS is a valuable tool for evaluation of treatment planning systems.
SU‐FF‐T‐441: Application of the Simplified Monte Carlo Algorithm to a Clinical Case for Proton Treatment Planning
Authors: Hotta K , Kohno R , Takada Y , Himukai T , Hara Y , Akasaka H , Kimura T , Tansho R , Nihei T , Nishio T , Ogino T
Published: April 2010

Purpose: We have developed a simplified Monte Carlo calculation (SMC) algorithm of dose distribution in proton therapy to improve the accuracy of dose calculations yet with a reasonable calculation time. We verified the new SMC by measurements, applied it to the clinical case, and compared the results with those by the conventional pencil beam algorithm (PBA). Method and Materials: The SMC takes into account the edge‐scattering effects in an aperture and a range‐compensating bolus ignored in the conventional pencil beam algorithm (PBA). In the SMC, since the dose in the human body is calculated using the measured dose distribution in water, the calculation time can be reduced compared with the full Monte Carlo method. To verify the accuracy of the SMC, the dose distribution formed by 235‐MeV protons traversing a step‐shaped bolus were measured with the PTW 2D‐ARRAY detector. We applied the verified SMC to the clinical case and compared with the results obtained by the PBA. Results: While the new SMC reproduced well the measured dose distribution in water of protons traversing the step‐shaped phantom, the PBA could not reproduce the hot spots due to the surface scattering effects of the aperture and the bolus. In the clinical case, while the high‐dose parts at the entrance region and complex dose distributions have been found in the SMC, such dose structures have not been seen in the PBA. The calculation time of the SMC was 20 minutes for simulated 4 mega particles, the calculated voxels of 0.7 mega, the rms statistical error of 4.5%. Conclusion: We verified that the SMC method reproduced well the measured dose distributions with a reasonable calculation time. We applied it to the clinical case and found that a complex dose distribution predicted by the full Monte Carlo method also appears.