bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐179: Analysis of Electron Cutout Factor Measurements
Authors: Esquivel C , Rodriguez S , Papanikolaou N
Published: July 2008

Purpose: To compare electron cutout factor (COF) measured with various dosimeters for different cone sizes. Method and Materials: The COFs for various electron fields were measured on a Varian‐2300EX linear accelerator at 6, 9, 12, 16 and 20 MeV. A thimble ion chamber, plane parallel chamber, thermoluminescent dosimeters (TLDs), EDR film, X Omat‐V (XV) film, GafChromic film and Thomson‐Nielsen mosfets were used to measure COFs. Each detector was placed along the beam's central axis at 100 cm from the source. Measurements were made in solid water at dmax for 6×6, 10×10 and 20×∼20 cone sizes with circular cutouts. The thimble chamber was used as the standard to compare COF measurements. The raw measurements made with the two chambers and the diodes were used to determine the COFs. Films were analyzed with RIT software and TLDs were read the next day to determine COFs. Results: Measurements for the 10×10 and 20×20 cone sizes agreed within 3.5% between all the dosimeters and the thimble chamber. Measurements made for the 6×6 cone and cutout were more than 3.5% from those made with the thimble chamber. There was better agreement for COF measurements between the mosfets, TLDs, EDR film and XV film. Conclusion: This study takes a look at the various dosimeters available at the clinic that can be used for determining electron cutout factors. For most cutout sizes, ionization chambers, mosfets, TLDs, EDR, XV and GafChromic film can be used to measure the COFs for all electron energies. For small field sizes, the use of TLDs, mosfets, and film can give more accurate COFs.
SU‐GG‐T‐185: The Effect of Clothing On the Skin Dose During 6 MeV Total Skin Electron (TSE) Irradiation
Authors: Ramer R , Tynan P , Eng T , Papanikolaou CN
Published: July 2008

Purpose: To determine what effect allowing the patient to wear various articles of clothing might have on the dose delivered to the patient during 6 MeV high dose rate total skin electron therapy. Method and Materials: Total skin electron (TSE) radiotherapy delivers a dose of radiation to the total body using electrons to treat the shallow region affected by mycosis fungoides, a cutaneous T‐cell lymphoma, while sparing deeper tissues. A Rando phantom was placed in the treatment position. Thermoluminescent dosimeters (TLDs) were placed at strategic locations on the phantom. The phantom was dressed in various outfits and surface dose measurements were made. These outfits included (1) fully dressed: t‐shirt, sweatpants, underwear and bra, (2) underwear and bra, (3) bra with underwire, (4) hospital gown, and (5) no clothing. Results: When compared with the measurements taken with no clothing, the fully dressed phantom results in a maximum difference of 22.0 cGy at the sternum, and a minimum difference of 0.1 cGy at the center back. Dressed in underwear and bra, the maximum difference from the unclothed measurements was 13.9 cGy at the underwear tag on the waistband and the minimum was 2.7 cGy at the sternum. The underwire bra resulted in a decreased dose of 12.3 cGy. Finally the hospital gown showed only 1 cGy of maximum difference at the stomach center, a difference of 0.2 cGy at the center back, and no difference in measurement under the tied knot. Conclusion: The hospital gown, made of thin cotton, had a minimal impact to the surface dose received. A patient could wear a similar garment during treatment without significant therapeutical impact. All other outfits tested resulted in a greater difference in skin dose, which may make such outfits undesirable for wear during treatment.
SU‐GG‐T‐250: Effect of Titanium Implants On Therapeutic Proton Dose Delivered to Patients
Authors: Ciangaru G , Sahoo N , Arjomandy B , Zhu X , Gillin M
Published: July 2008

Purpose: To assess the accuracy of proton beam dose calculation in Eclipse treatment planning system (TPS) in the presence of Titanium rods in the path of the beam. Method and Materials: All measurements and computations were performed for a phantom consisting of slabs of acrylic with known water equivalent thickness (WET). One of the slabs supported two cylindrical rods of Titanium, similar to the ones used in spine cages, each of 5 cm in length and 0.5 cm in diameter. A treatment plan was then created using the CT scan of the phantom including the rods. EBT films were placed at various depths in the phantom and were irradiated with the proton beam used in the planning. Specifically, a 225 MeV proton beam impinging the phantom surface was modulated to achieve a 16 cm wide spread out Bragg peak (SOBP) inside the phantom. Percentage depth‐dose (PDD) and profiles from Eclipse plan and film dosimetry at different depths in the phantom were compared. Results: The PDD at depths below the Titanium rods from film dosimetry and Eclipse TPS agree within 3% inside the width of the SOBP. A more significant difference (5% to 20%) is seen at depths beyond the distal 90% dose region with high dose gradients, and can be attributed to the uncertainties in the determination of WET of the phantom materials from CT numbers. Lateral profiles, both in the transversal and longitudinal directions along the Titanium rods, from Eclipse and film dosimetry agree reasonably well with each other including in the regions close to the Titanium‐acrylic interface. Conclusion: Eclipse TPS dose calculation in the presence of Titanium implants is found to be moderately accurate, within the experimental uncertainties. The differences are explainable by the limitation of the analytical pencil beam model for proton dose calculations.
SU‐GG‐T‐251: Effect of Range Compensators On Entrance Doses in Proton Therapy
Authors: Zhao Z , Li Z , Hsi W , Slopsema R
Published: July 2008

Purpose: To investigate the effect of the range compensator on the entrance dose in proton therapy. Method and Materials: A proton field with a range of 12 cm in water and a modulation width of 9 cm was delivered using our double scattering system (IBA). Measurements of depth dose curves were performed in a water phantom using a PPC05 parallel‐plate ionization chamber. Various thickness (0.5 cm and 2.0 cm) of 30 cm × 30 cm solid water were attached at the end of snout. Different air gaps (7.5 cm, 15.5 cm and 40.5 cm) were achieved by retracting snout to different locations. Variation in relative dose at depth 2.6 mm was evaluated to determine the effect of the range compensator. Results: Without any compensator, no change was observed between measured depth dose curves at different air gaps. For protons passing through 0.5 cm thick solid water at various air gaps, minimal variation was observed at 2.6 mm depth, or at any other position in the PDD. However, with a 2.0 cm solid water compensator, the entrance doses are 82.1%, 83.6% and 84.6% for air gaps of 40.5 cm, 15.5 cm and 7.5 cm respectively. Conclusion: With 2 cm solid water, the entrance dose increases by 2.5% by changing the air gap from 40.5 cm to 7.5 cm. The increase of entrance dose is likely due to the secondary particles produced by primary protons.
SU‐GG‐T‐253: Interface Dosimetry in Heterogeneous Water Phantom Using EBT Film
Authors: Amro H , Roberson P , Smith C , Acosta E , Moran J
Published: July 2008

Purpose: Interface dosimetry measurements in heterogeneous phantoms are subject to additional uncertainties when using solid materials due to the presence of air gaps between the materials and the detector. The use of solid materials also limits the location of dosimeters. In this study, we investigate performing interface dosimetry using EBT film in a liquid water phantom with lung‐equivalent media inside the phantom. Method and Materials: A 6 MV 10×10 cm2 static field was measured at 90 cm SSD with an ionization chamber and EBT film placed in water. Then, interface measurements were made using EBT film or an ion chamber for a 6 MV beam of a 3×3 cm2 static field at 95 cm SSD. The heterogenous phantom consisted of a 4×4×10 cm3 lung equivalent material suspended 5 cm below the water surface in 40×40×40 cm3 water phantom. The phantom was CT scanned and the images were used for dose calculations. EBT film was placed perpendicular to the central axis at multiple depths and in a parallel orientation at central axis and at ±1 cm off axis. The film extended from the water surface to 12.5 cm depth. Data were averaged over multiple measurements. Film data were compared to Monte Carlo simulations and convolution/superposition calculations. Results: EBT film data obtained in both orientations in water agree, to within 2%, with ion chamber data. In the heterogeneous water phantom, a 3% agreement with Monte Carlo simulations is achieved. Large deviations with convolution and superposition calculations was observed. Conclusion: EBT film is a reliable dosimeter in water with and without the presence of heterogeneity when the film is irradiated parallel or perpendicular to central axis. The film can be used in a variety of phantoms to obtain more reliable measurements for commissioning of sophisticated dose calculations algorithms.
SU‐GG‐T‐260: An Experimental and Monte Carlo Study of Output Factors for Small Radiosurgery Beams
Authors: Yang Y , Kim J , Li F , Li X , Li T , Yue N , Heron D , Huq M
Published: July 2008

Purpose: To accurately determine output factors(Sc,p) for small radiosurgery beams through experimental measurements using multiple detectors and Monte Carlo(MC) simulation. Method and Materials: A pinpoint chamber(PTW 31016), two solid state detectors(Scanditronix PFD and SFD) and radiographic film(Kodak EDR2) were used to measure Sc,p for twelve cones(φ5mm∼φ30mm) for a 6MV photon beam on a Varian Trilogy linear accelerator. All measurements were made at a depth of 1.5cm with SAD 100cm. MC models based on BEAM and XVMC codes were developed to simulate the treatment head and detectors to obtain theoretical values of Sc,p and correction factors for the detectors. The experimental data were compared with the MC values. The impacts of detector perturbation and energy dependence on the detector's behavior were also investigated. Results: For the cones larger than 20mm, the differences between the measurements and MC calculations are within 1% for all the detectors except SFD. As the cone size decreases, the differences between the chamber measured and MC calculated values increase significantly. For example, the chamber underestimates Sc,p by almost 23% for the 5mm cone. Sc,p values obtained with films and PFD give most consistent results for all cones. Although the SFD detector has a very high spatial resolution(0.6mm), the data obtained with the SFD detector is found to be less than the theoretical values by about 5%. MC simulations show that the mean photon energy is higher in small beams. The response of SFD detector is larger for low energy beams than that for high energy beams. We contribute 5% underestimation to the energy dependence of SFD detector. Conclusion: It is found that films and PFD diode provide reliable Sc,p results for small beams. MC simulation is a useful tool, which provides benchmark and correction factors for detectors used for determination of output factors in small radiosurgical beams.
SU‐GG‐T‐262: Total Scatter Factors of Small 6 MV Photon Beams Estimated by Simulating the Response of Small Detectors
Authors: Francescon P , Cora S , Cavedon C
Published: July 2008

Purpose: To estimate total scatter factors (sc,p) of small 6 MV photon beams from a Siemens Primus linear accelerator. Method and Materials: A method of estimation of sc,p of small beams previously applied to circular fields of the Cyberknife radiosurgery system has now been applied to square fields of 0.3 cm and 0.5 cm field size (reference field 10 cm). Experimental measurements were made with a PTW31014 PinPoint chamber and with 3 diodes (PTW60012, PTW60008 and SunNuclear EdgeDetector™). The method consists of Monte Carlo simulation (BEAMnrc) of the active volume of the detectors and of the treatment head in the hypothesis of 6 different values of FWHM of the electron beam incident on the target (1.0 to 2.5 mm). Possible outcomes are then checked for consistency with experimental results to determine correction factors to be applied to experimental sc,p. Results: Corrected sc,p values of the 2 fields were 0.085 and 0.243 with the PinPoint chamber compared to 0.049 and 0.234 resulting from uncorrected measurement. Corrected sc,p values of the 2 fields were on average 0.075 and 0.256 with the diodes compared to 0.082 and 0.272. The FWHM of the electron beam was consistently estimated between 1.8 and 2.0 mm for all the detectors. Conclusion: This strategy has the advantage of avoiding any “a priori” beam tuning of the electron beam width in the Monte Carlo model because it takes into account the whole range of possible values and uses the actual shape and chemical composition of the detectors in the simulation. Results are consistent with underestimation of sc,p from the ion chamber, partly due to volume effect, and overestimation from the diodes due to non‐water equivalence of the silicon substrate (all) and of the metal shield (PTW60008 and SunNuclear). The method will be extended to include more field sizes.
SU‐GG‐T‐269: Experimental Determination of Detector Volume Effect Using a Gaussian Function On Output Factor Measurements in the 5 Mm Small Field
Authors: Lah J , Yoon S , Suh T
Published: July 2008

Purpose: We investigated the effect of detector volume for output factor measurement in the 5 mm collimator. Comparisons of measured and calculated output factors from a Gaussian fitting approaches the real doses to remove detector volume effect were performed. Method and Materials: A 6 MV photon beam from a third‐generation CyberKnife was used in this study. We have used a Gaussian function to correct for the spatial response of finite‐sized detectors and to obtain the real beam profiles from measurement. The beam profiles were measured with a PTW 31006 pinpoint ionization chamber and a PTW 60008 diode. The calculated output factors were obtained from a Gaussian fitting for the beam profile with the same detector, which were compared to measured value with radiochromic film. Results: The 5 mm collimator output factor measured with the ionization chamber was 0.615±0.009, the diode was 0.702±0.015 and radiochromic film was 0.695±0.022. Ionization chamber show significant differences of more than 12.4% between measured and calculated output factor by Gaussian function for the 5 mm collimator. The main reason for an underestimation of the output factor is the increase of lateral electron disequilibrium with an increase of the ionization chamber measuring volume. The agreement between output factors measured and calculated value with the diode was within 1.5%. A diode was found to be suitable for output factor measurements of small beam because of its high spatial resolution. Also, we found good agreement between the measured output factor obtained with radiochromic EBT film and the calculation values of another detector. Conclusion: The volume effect of the detector could lead to inaccurate in small field dosimetry. The results of this study indicated that the choice of a suitable detector is an important problem for dose measurement in high dose gradient region such as the 5 mm collimator.
SU‐GG‐T‐274: Dosimetric Characteristics of a Commercially Available Optical Stimulated Luminescence Dosimeter (OSLD)
Authors: Lim S , LoSasso T
Published: July 2008

Purpose: To assess the dosimetric characteristics of the Landauer Microstar optical stimulated luminescence dosimetry (OSLD) system, the energy, dose rate, and angular dependencies, linearity, temporal and reading fade, and intra‐batch variation were systematically tested. Method and Materials: All detector chips were irradiated a custom‐milled 25×25 cm polystyrene phantom. Calibration of the system was done for 6 MV, 10×10 cm2 field size, 100 cm SAD, and dmax. Relative energy response was determined for 6 and 15 MV xrays and 6, 9, 12, 16, and 20 MeV electrons. Groups of multiple chips received up to 20 Gy to assess the linearity and reproducibility of the system. The angular dependency was studied by varying the angle of incidence. Reading fade dependency was quantified by repeatedly reading out an exposed chip over a one hour period. Temporal fade was quantified by readouts up to 14 days post irradiation. Results: OSLD response is linear to within 5% over the dose range 0 to 20 Gy. OSLD response decreased by 5% at 90° incidence. Energy dependence showed the largest variation, 5%, was between 6 MV xrays and 12MeV electrons. Measurement uncertainty is 2.5% (1σ, n=70). Reading fade is found to be 0.1%/rdg. Temporal fade is 5%, 2%, and 1% for 1cGy, 5 cGy, and ⩾50 cGy, respectively, between 15 minutes and 2 hours after irradiation, and 15%, 7%, and 3% at 14 days. Conclusion: OLSD exhibits some nonlinearity, as well as angular, energy, and intra‐batch variation under reference conditions. Refinements in calibration software and tighter quality control by the manufacturer should improve the accuracy and chip to chip variation. Temporal and reading fade are not clinically significant.
SU‐GG‐T‐275: Investigation Into the Use of a MOSFET Dosimeter as An Implantable Fiducial Marker
Authors: Kry S , Price M , Wang Z , Mourtada F , Salehpour M
Published: July 2008

Purpose: To evaluate the potential for an implantable dosimeter to be also used as an internal fiducial maker. Additionally, to evaluate the synergy of the dosimeter/fiducial capabilities. Method and Materials: Two implantable MOSFET detectors (DVS®, Sicel Technologies, Inc.) were placed inside an acrylic pelvic phantom for which a wedged‐field photon plan and an eight‐field prostate treatment plan were developed. For each plan, conditions were simulated so that detectors were in their correct positions or slightly displaced to represent patient setup error and/or organ motion. Doses measured by the two detectors after irradiation were compared to those calculated by the treatment planning software. Additionally, using localization software (MarkerVision, Varian Medical Systems) and kilovoltage images of each setup, the displacement of the detectors from their correct locations was calculated and compared to the induced physical displacement. Results: For all alignments and detector positions, measured and calculated doses showed an average disagreement of 2.7%. The detectors were easily visualized in AP and lateral radiographs, and the induced detector displacements were typically recognized by the localization software within 0.1 cm, and were recognized within 0.16 cm at worst. The relationship between the geometric displacement and measured dose was dependent on the positioning of the dosimeter and dose gradients. If the detector was placed in a uniform dose region then geometric displacements did not induce any dosimetric discrepancies. Alternately, if the detector was placed in a strong dose gradient then even high geometric accuracy resulted in dosimetric discrepancies. Conclusion: The implantable detector functioned well as both an internal dosimeter and as an internal fiducial marker and thus may be useful as a clinical tool to localize the target volume and verify dose delivery in vivo. Conflict of Interest: Research was supported by a grant from Varian Medical Systems.
SU‐GG‐T‐276: Comparison of the Epson Perfection V700 Photo Flatbed and the Vidar VXR‐16 Dosimetry PRO AdvantageTM Film Scanners for Use with Radiochromic Film
Authors: Matney J , Parker B , Neck D , Rosen I
Published: July 2008

Purpose: The purpose of this study was to compare the performance and utility of a commercially available flatbed scanner with that of a widely‐used medical film scanner for use with radiochromic film. Method and Materials: This project compares the Vidar VXR Dosimetry Pro and the Epson Perfection V700 Photo flatbed scanner. Using radiochromic EBT film irradiated to a known range of doses 0–512 cGy using eight 3×3 cm fields on a single film, both scanners were characterized for scan repeatability, orientation effects, uniformity, and scanner noise. RIT V5.0 was used to determine pixel value (PV) of Vidar scans, while ImageJ software was used for the Epson scans. Results: For the range of doses measured, average pixel values of a central 1.5×1.5 cm region of interest (ROI) were reproducible within 0.1% standard deviation with Vidar and approximately 0.2% with Epson. Epson mean PV differed by up to 18% for all dosed regions from landscape to portrait orientation, while only up to 5% for Vidar. From 0 to 512 cGy, identical films showed a 10% wider PV range for Vidar than Epson. Scanner noise was minimal; Epson showed a maximum SD of 1.00% from the mean of the central ROI, Vidar showed 1.14%. In both systems, cross profiles for an unirradiated film showed deviations in PV no more than 2% of the mean. Conclusion: No clear significant accuracy advantage was noted in either system, provided the leading and trailing 1 cm film edges were neglected with the Vidar system. Although the Epson scanner is a relatively inexpensive method for analyzing radiochromic film, the lack of commercially available software for it could be a major disadvantage. Hence, an institution already having a Vidar system may find its use with radiochromic film attractive.
SU‐GG‐T‐278: Dosimetric Characteristics of An Amorphous Silicon Electronic Portal Imaging Device for Dynamic Intensity Modulated Radiation Therapy
Authors: Krongyuth K , Krongyuth K
Published: July 2008

Purpose: To investigate the dosimetric characteristic of an amorphous silicon electronic portal imaging devices (EPIDs) for dosimetric verification of dynamic intensity modulated radiation therapy (IMRT). Method and Materials: The dosimetric characteristic of amorphous silicon EPID (aS500, Varian Medical Systems) studies were dose response, field size response, effect of dead time in frame acquisition, response with leaf speed, response of dose‐rate fluctuation, memory effect and long term detector reproducibility. Relative dosimetric measurements using EPID such as open and wedged static fields, IMRT test pattern field and actual fluence from clinical IMRT fields were included. Results: The results showed that the dosimetric characteristics from EPID were linear with dose and dose rate, and response to MLC leaf speed up to 3.0 cm/s. The reading from EPID and ionization chamber measurements at the field size response were up to 4%. The dead time in frame acquisition was found to be a source of error of up to 20 % in the EPID measured signals. Response with dose‐rate fluctuation was within 3.5%. Memory effect was not significant. The long term detector reproducibility was found to be within 2% for dynamic field delivery. The result of relative dosimetry of EPID for IMRT fields was similar to the film measurements; however they were different when compared to CadPlan fluence measurements. Conclusion: The EPID has potential for use as a dosimetric instrument for IMRT verification.
SU‐GG‐T‐282: Investigation of Optical Artifacts in a Complex Light‐Scattering Gel Phantom for Optical CT Polymer Gel Dosimetry
Authors: Bosi S , Naseri P , Baldock C
Published: July 2008

Introduction: A test phantom of complex shape was developed to simulate artefacts in dosimetric polymer gels (e.g. PAGAT [1]) when read using cone beam optical CT. Here we report the results of modelling of experimental data using this phantom. Method and Materials: Using a method reported earlier [2], opacity change in an exposed dosimetric gel was simulated by adding known concentrations of Dettol™ to gelatin. Different doses were simulated with gelatin of different opacity (optical density, OD). The phantom was contained is a cylindrical PETE vessel (9.5 cm diameter, 13 cm high). It consisted of an outer mantle of pure gelatin (“zero dose”) surrounding a central, vertical hollow finger. The hollow finger was constructed of 10 transverse layers 15 mm thick of increasing OD, the uppermost layer having the lowest OD. The cavity through the finger centre was filled with a pith of pure “zero dose” gelatin. Inclusion of this central pith simulates a treatment where a high dose region surrounds a low dose region. The phantom was scanned with a Vista Optical CT Scanner (Modus Medical Devices Inc.). Horizontal profiles of OD versus position through each of the 10 layers were obtained from the reconstruction (resolution; 0.25 mm per voxel). Results and Discussion: Consistent with earlier work [1], higher OD slices exhibited dishing artefacts — inner regions within the slice exhibiting an apparent reduction in OD, similar to dishing artefacts in X‐ray CT. More interestingly, the central “zero dose” pith exhibited spurious increases in OD. These effects are modelled by taking into account the back‐reflection of scattered light.
SU‐GG‐T‐283: Dosimetry for Hybrid 1.5 T MRI Radiotherapy System: Impact of the Magnetic Field On a NE2571 Ionisation Chamber
Authors: Raaijmakers AJE , Raaymakers BW , van Dijk I , Kok JGM , Lagendijk JJW
Published: July 2008

In collaboration with Elekta and Philips Research Hamburg we areconstructing a prototype radiotherapy accelerator with integrated 1.5 TMRI functionality for on‐line soft tissue based treatment guidance. In such a system radiation dosimetry has to be performed in the presence of a magnetic field. This paper investigates the feasibility of using a Farmer NE2571chamber (NE Technology Limited, Berkshire RG7 5PR, England) for dosimetry. A laboratory magnet was installed next to an accelerator. The NE2571 chamber was placed in a build‐up cap at the center of themagnet and the relative response was measured as function of ahomogeneous magnetic field ranging from 0–1.2 T. The measurements were done for two orientations of the chamber, 1)perpendicular and 2)parallel to the radiation beam, while perpendicular to the magneticfield. These set‐ups were also simulated. A second set of simulations is performed to determine the impact of the magnetic field ranging from 0–1.5T on the NE2571 under calibration conditions, i.e. in a sleeve in a NE2528/3A phantom. In this set‐up a third orientation is simulated too: 3)perpendicular to the incident beam while parallel to the magnetic field. Both measurements and simulations show that the response for orientation 1 increases until 1 T by approximately 10% and then start to decrease. For orientation 2 the response is opposite, it decreases until 1 T by approximately 10% and then starts to increase again. For orientation 3 only simulations are available and the response is constant as function of the magnetic field. The behaviour of the NE2571 for absolute dosimetry as function of the magnetic field can be simulated using Geant4 simulations and is in correspondence with measurements. The exact response is determined by the orientation of the chamber with respect to the magnetic field and the radiation beam.
SU‐GG‐T‐292: The Response of LiF:Mg,Ti Thermoluminescent Dosimeters to Low‐Energy Photons
Authors: Rasmussen B , Davis S , Micka J , DeWerd L
Published: July 2008

Purpose: To characterize the component of the LiF:Mg,Ti TLD response to the low‐energy photons of and LDR brachytherapy sources and x‐ray beam qualities of M40 and M80, that cannot be predicted by cavity theory or Monte Carlo methods. To provide a methodology for determining accurate energy correction factors for experiments performed in a variety of scatter conditions and to provide an example of how to apply the results of this work will also be presented. Method and Materials: TLD‐100 chips were exposed to and LDR brachytherapy sources using the known geometry of the University of Wisconsin Variable Aperture Free Air Chamber. Dose calculations were based on primary determinations of air‐kerma strength and Monte Carlo simulations of the full irradiation and source geometry. For comparison purposes and to examine the effects of dosimeter size and scatter conditions, a series of x‐ray experiments were performed to compare the response of chips (3×3×0.89mm) to microcubes (1×1×1mm) and free in air irradiations to irradiations in a PMMA holder. Results: The results of the x‐ray experiments agreed well with the work of Nunn et al. (Med. Phys. 2006) and confirmed that the “solid‐state” component of the energy response was largely independent of irradiation geometry. The results of the experiments exhibited good reproducibility for the single source, but the measurements seem to exhibit source‐to‐source variability. More experiments are needed for both isotopes to determine the nature of these observations. Conclusion: For all radiation qualities used in these experiments it was found that Monte Carlo simulations appear to underestimate the dose response of LiF:Mg,Ti when compared to measured response. As a result, the previously published values for the dose rate constants of LDR brachytherapy sources are likely overestimates.
SU‐GG‐T‐304: Comparison of Three Commercial MOSFET Dosimeters for Use in Total Body Irradiation
Authors: Ingram M , Novak J , Ainsley C , FU L , Both S , Kassaee A
Published: July 2008

Purpose: To determine the efficacy of three MOSFET dosimeters to perform in vivo measurements of both entrance and exit dose in total body irradiation. Method and Materials: Three commercially available OSFET dosimeters including the Thomson Nielsen Mobile Mosfet (Best Medical) OneDose and OneDose Plus (both from Sicel) were tested under TBI conditions. Measurements were made with two phantoms: a 30cm × 30cm × 21cm solid water phantom and an anthropomorphic phantom. The midlines of the phantoms were placed at 492 cm from the source, the standard TBI condition in our clinic. TBI procedures in our clinic involve irradiation of the patient decubitis with two opposed 6 MV photon beam. To ensure equilibrium, 1.5cm of bolus was used for the Thomson Nielsen MOSFET as well as OneDose. The OneDose Plus system possesses an internal buildup cap. For the solid water phantom, measurements were made at five positions beginning with the central axis. Measurement positions on the anthropomorphic phantom included the head, neck, chest, and umbillicus. Entrance and exit doses were recorded and normalized to an ion chamber reading. Results: All three MOSFET detector measurements were within 2% of the ion chamber measurements when measuring entrance dose on the solid water phantom. When measuring exit dose, OneDose Plus recorded a 40% over response while the other detectors were within tolerance. Entrance and exit dose measurements using the anthropomorphic phantom saw more variation with results deviating from the ion chamber reading as much as 5%. Conclusion: The MOSFET dosimeters provide a convenient method for in vivo dosimetry for total body irradiation. Entrance and exit doses under ideal conditions were accurate, but degraded slightly when placed on an anthropomorphic phantom. OneDose Plus showed an over response when measuring exit dose.
SU‐GG‐T‐306: Evaluation of Two‐Dimensional Diode Array Dosimeter for An Application of Exit Dosimetry
Authors: Chung H , Li J , Liu C , Samant S
Published: July 2008

Purpose: To characterize and evaluate the performance of MapCHECK™ two‐dimensional diode array dosimeter for accurate measurement of exit fluence at an extended source‐to‐surface distance (SSD) for the application of in vivo dosimetry based on dose reconstruction. Method and Materials: Dose profile measurements of open field for varying field sizes (3×3cm2, 5×5cm2, 7×7cm2 and 10×10cm2), air gaps (30cm, 40cm, 50cm and 60cm) and thicknesses of primary beam attenuator (10cm and 20cm of solid water) using MapCHECK™, CC04 ionization chamber and GafChromic® EBT radiochromic film were carried out. For clinical measurements, the 2D dosimetry measurements for five IMRT boost fields were carried out at an air gap of 50 cm with MapCHECK™, and compared with GafChromic® EBT radiochromic film, which served as the gold standard. Results: With increasing air gap, the difference between MapCHECK™ system and CC04 increased slightly for both primary beam attenuator thicknesses. For air gaps of 0cm, 30cm, 40cm, 50cm and 60cm, the overall agreement between MapCHECK™ and CC04 was ±1.25%, ±1%, ±2%, ±2.5% and ±4, respectively. The overall agreement averaged over all air gaps between MapCHECK™ system and CC04 was within ±2% while agreement between GafChromic® EBT radiochromic film and CC04 was ±3.5%. For the clinical IMRT fields and using the gamma test, the comparison between MapCHECK™ and GafChromic EBT radiochromic film yielded a mean passing rate of 91.9%, with a minimum and maximum passing rate of 87.6% and 97.2% respectively. Conclusion: MapCHECK™ yielded to have an accurate dose distribution (profile and 2D dose distribution) at an extended SSD (> 130cm). It can be considered a suitable candidate for exit dosimetry.
SU‐GG‐T‐327: Simulation of Neutron Equivalent Doses in Proton Therapy as a Function of Patient's Age for Different Tumor Sites
Authors: Athar B , Jarlskog C Zacharatou , Paganetti H
Published: July 2008

Purpose: There is a growing concern of increased risk of treatment complications among radiation therapy patients as the age of patients diagnosed with cancer is decreasing, due to early cancer detection. Pediatric and young adult patients receive higher doses at organs and tissues distal to the PTV and stand a higher risk of developing secondary cancers during the span of their longer post treatment lives. It has become imperative to study the effects of secondary radiation doses at various organs distal to the PTV. Method and Materials: Age and gender specific whole‐body phantoms have been implement in Geant4 in order to determine the doses from the secondary radiation in patients undergoing proton treatment. We have used different fields planned for different treatment sites (head & neck, abdominal) to determine the secondary organ doses. This work is an extension of the previous work done at the Massachusetts General Hospital and is based on six voxelized phantoms: an adult, a 9 month old male, a 4‐year old female, an 8‐year old female, an 11‐year old male, and a 14‐year old male. Results: We found large variations on the amount of dose to various organs distal to the PTV. The equivalent neutron doses depend on age and treatment site. Conclusion: We found the neutron equivalent dose at various organs and tissues decreases with phantom age and depends on the field size. Furthermore, our study suggests that organ specific neutron doses in proton therapy depend on the treatment site.
SU‐GG‐T‐329: Investigation of Alpha Particle Beam Therapy as a Lower Neutron Dose Alternative to Proton Beams
Authors: Hecht A , Zhang M , Perez‐Andujar A , Mackie T , DeLuca P
Published: July 2008

Purpose: Secondary neutron production is a serious issue in ion‐beam radiotherapy. As ion therapy becomes more popular, this issue must be addressed. In this study, we investigated whether alpha‐particle beam therapy would have an advantage over proton beam therapy in lowering neutron flux. Method and Materials: Simulations were performed using the Geant4.9.1.p01 Monte Carlo toolkit for alpha‐particle and proton beams. Fluxes and spectra were modeled for neutrons produced in a 40cm cubic water phantom and in a 50mm thick Lucite plastic range shifter for several beam energies. Results: Alpha particles have four times the stopping power than protons for similar speeds. For equal depth penetration, they must carry four times the energy. Per beam particle, alpha particles produce more secondary neutrons than do protons, but only 1/4 the beam flux is required. Monte Carlo calculations show that, for comparable beam dose, alpha‐particle beams produce lower neutron fluxes than proton beams at lower treatment energies (∼100MeV protons, ∼400MeV alpha‐particles). This is observed both in water and in Lucite. For higher energy beams, neutron spectra and fluxes are comparable between alpha particles and protons. For all cases, the proton‐induced neutron flux falls off at lower neutron energy than does the alpha‐induced neutron flux. For higher energy beams, with otherwise similar neutron spectra, this differential drop‐off means a higher integrated neutron energy using alpha particles. For the lower energy primary beams, this is more than compensated for by the lower total integrated neutron energy. Conclusion: Calculations suggest that for lower energy treatment beams, alpha particles produce fewer secondary neutrons with a lower integrated energy than do proton beams. This is significant for depths of ⩽80mm in a water phantom, which is good for juvenile patients where a reduced neutron dose is paramount. There is less benefit seen using higher energy alpha‐particle beams.
SU‐GG‐T‐332: Monte Carlo Collimator Transport Methods for Photon Treatment Planning
Authors: Fix MK , Schmidhalter D , Frei D , Volken W , Manser P
Published: July 2008

Purpose: Monte Carlo (MC) based investigation of the impact of different radiation transport methods in collimators on beam characteristics, dose distributions and efficiency. Method and Materials: Within the Swiss Monte Carlo Plan (SMCP) — a GUI‐based framework for photon MC treatment planning — different MC methods are available for the radiation transport through the collimators (secondary jaws and MLC): EGSnrc, VMC++ and Pin (an in‐house developed MC code). Additional MC methods were implemented in order to provide different complexity levels for the MC simulation: Considering the collimators as totally absorbing, considering attenuation only, considering first order or all Compton scatter. Furthermore, either a simple or an exact geometry can be selected for the absorbing or attenuation method. Phase spaces above and dose distributions in a water phantom are analyzed for artificial and clinical treatment fields using 6 and 15 MV beams. Results: For all MC methods, differences in the radial mean energy and radial energy fluence are within 1% inside the geometric field. Below the collimators the energy fluence is underestimated for non‐full MC transport methods ranging from 5% for all Compton to 100% for absorbing. Gamma analysis using EGSnrc calculated doses as reference shows that the percentage of voxels fulfilling a 1%/1mm‐criterion decreases from 99% when using VMC++ to 80% (non‐IMRT) and 40% (IMRT) when the absorbing method in a simple geometry is used. However, compared with EGSnrc calculations, the gain in efficiency is a factor of 10 for VMC++ and 90 for the absorbing method. Conclusion: The results of this investigation suggest that it might be suitable to use a simple transport technique in the initial treatment planning process and use more accurate transport methods towards the end of the planning process accepting longer calculation time. Conflict of Interest: This work was supported in part by Varian Medical Systems.
SU‐GG‐T‐334: Improvement of Off‐Axis Energy Sampling in XVMC Beam Model for Varian 2100C/D
Authors: Kim JO , Huq MS
Published: July 2008

Purpose: To improve the off‐axis energy sampling in the Virtual Photon Energy Fluence (VEF) model of XVMC using an energy‐dependent scaling factor for Varian 2100C/D. Method and Materials: When the VEF model of XVMC was commissioned for Varian 2100C/D, the calculated outputs significantly disagreed with measured outputs. Moreover, the off‐axis output becomes lower than measured output as the off‐axis distance increases. Because the XVMC does not reflect the monitor backscatter effect, this study employed the monitor backscatter factor to correct the central axis output. The XVMC samples photon energy from the central axis spectrum and then converts to off‐axis photon energy by a scaling factor which is the ratio of mean off‐axis photon energy to the mean central axis photon energy. Since the spectrum changes with the off‐axis distance, the scaling factor must vary with not only off‐axis distance but also with energy. This study developed an energy‐dependent scaling factor using a fifth order polynomial equation and fitted coefficients to off‐axis spectrum generated by a full scope Monte Carlo accelerator head model using BEAMnrc. The coefficients are functions of photon energy to account for the off‐axis spectrum changes. Results: The calculated outputs agree with the measurement after the monitor backscatter correction is applied. It ensures that the central axis spectrum commissioned is reliable and accurate. The energy‐dependent scaling factor enabled the XVMC to sample photon energy correctly at any off‐axis distance and to reproduce the measured profiles within an acceptance criterion for all field sizes. Conclusion: Although the XVMC photon energy sampling is fast and efficient with the VEF model, the inaccuracy of output has delayed the implementation of XVMC for Varian 2100C/D. It is expected that the improved beam model will enhance the clinical implementation of the XVMC for Varian 2100C/D.
SU‐GG‐T‐337: A Method of Effective Attenuation That Preserves Detailed MLC Characteristics and Replicates Spectral Effects of Deliberate Transport
Authors: Speiser M , DeMarco J
Published: July 2008

Purpose: To develop a fast, accurate method of incorporating spectral effects of photon transport through a Multileaf Collimator while maintaining detailed representation of MLC geometry in a virtual source model. Method and Materials: A virtual source model for a 6MV Novalis LINAC with micro‐Multileaf Collimator (mMLC) has been developed for use with the MCNPX Monte Carlo engine. This source model uses a comprehensive ray‐tracing algorithm to determine pathlengths through a detailed representation of mMLC geometry in order to clearly represent leaf edge and rounded leaf tip effects on the virtual source grid. To ensure sufficient resolution, the virtual source grid elements are only 0.5 × 0.5mm2 when projected to the isocenter plane. In addition to representing open‐field energy spectra and effective mMLC attenuation of open beam fluence, a sub‐source has been developed to simulate the energy spectra of mMLC leakage. For a given mMLC configuration, pathlength and effective attenuation maps are determined at the start of a simulation. For each particle simulated, transport through the mMLC is determined by comparing one random number generation to an effective attenuation value. Subsequent sampling from the leakage sub‐source is triggered by comparing the sampled photon's pathlength through the current mMLC configuration against the pathlength at that location if the mMLC were completely closed. Results: The virtual source model accurately represents leaf edge and rounded leaf tip effects as demonstrated by comparison with coronal film measurements. Simultaneously, the spectral effects of photon transport through the mMLC are effectively reproduced without any deliberate or abridged transport. Simulation results demonstrate these principles and are shown to agree with axial and coronal film measurements. Conclusion: Explicit or pseudo‐explicit transport through MLC leaves may be obviated while preserving mMLC representation by incorporating a more sophisticated ray‐tracing algorithm that triggers the sampling of a leakage sub‐source.
SU‐GG‐T‐338: Recent Improvements in the Geant4 Monte Carlo Simulation Toolkit for Medical Physics Applications
Authors: Perl J , Faddegon B , Paganetti H
Published: July 2008

Purpose: Rapid growth of the medical user base for the Geant4 Monte Carlo Simulation Toolkit over the past few years has led to major improvements in the code addressing specific needs of the medical physics community. Problems in applying the toolkit to the energy domain of medical physics have been indicated by users (e.g. by the paper by Poon et al. [1]) and subsequently addressed by the code developers. Consequently, many recent papers show greatly improved results for simulations [2,3]. The purpose of this presentation is to update the medical physics community about key improvements in the Geant4 toolkit for radiotherapy physics applications. Method and Materials: A recent Technical Note published by the SLAC (Stanford Linear Accelerator) Geant4 team has provided the first and most useful summary of code improvements in the energy regions of interest to radiation therapy. We have analyzed this information and assessed the impact on simulations in electron and photon therapy. Further, we have expanded the information to issues affecting proton and ion therapy as well as imaging applications. Results: The original multiple scattering model in Geant4 was based on the Lewis formalism. Results from Poon et al. and others showed that the approach was insufficiently precise. In response to these findings, modifications were made to the model based on ideas from Penelope and EGSnrc including: limit the particle step size near geometry boundaries, use correlations between lateral displacement and scattering angle and add options to use single or plural scattering near boundaries (instead of multiple scattering). Single Scattering processes were added which can be used to entirely replace multiple scattering if desired. Models for energy loss fluctuations were substantially revised. Conclusion: This presentation summarizes significant recent improvements in Geant4, which may lead to an even more widespread acceptance of this toolkit in radiation therapy.
SU‐GG‐T‐342: Dosimetric Characteristic of 137‐Cs Sources Used in Afterloading Selectron System by Monte Carlo Method
Authors: Hosseinabad M Tavakoli , Hoseinpour F
Published: July 2008

Purpose: In brachytherapy, for effective treatment planning it is necessary to know the accurate source dosimetric information such as air kerma strength, exposure rate constant, dose rate constant and redial dose distribution. Usual method to determine these factors is TLD dosimetry but now a day more accurate method is the Monte Carlo simulation method, which have widely used in determining of Brachytherapy sources dosimetric factors. The aim of this research is determining of dose distribution of sources configuration used in interacavitary LDR Selectron brachytherapy by Monte Carlo simulation method. Method and Materials: Simulation has been performed for stainless‐steel encapsulated source in 2.5 mm in diameter. Six different combinations of sources were simulated and each combination was put into different applicator. MCNP4C (Monte Carlo N‐Particle version 4C) was used for Monte Carlo simulation. Cylindrical layer tally volumes were used to score dose (using the MCNP *f4 tally) radially outward from sources. MCNP tally f6 was used to score kerma in air. Cutoff energy for electrons and photon were taken at 10 KeV to reduce variation. The number of history was taken 1 million for calculating air kerma strength and exposure rate constant and 0.1 million for calculating dose rate constant and redial dose distribution. Results and conclusion: Maximum air kerma strength is obtained for the combination with fourteen active sources and found to be 919.3 μGym2h−1 while its minimum value was 133.9 μGym2h−1 for the case with two active sources. Maximum and minimum dose rate constant of 3.3 cGyh−1mCi−1 and 2.05 cGyh−1mCi−1 obtained for the combination with two and eight or fourteen active sources, respectively.

Key Words: , Brachytherapy, Redial dose; Dose rate constant, Monte Carlo simulation.