bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐314: Daily Evaluation of in Vivo Dose Verification Device
Authors: Knaup C , Stathakis S , Esquivel C , Mavroidis P , Mihailidis D , Papanikolaou N
Published: August 2010

Purpose: To evaluate daily and overall performance of an in vivo dose verification device. Methods and Materials: Five patients were used to evaluate the response of the in‐vivo measurement device DAVID (PTW Freiburg GmbH, Germany) against an ionization chamber array (I'mRT MatriXX, IBA dosimetry, GmbH, Germany). For each patient, two almost equivalent plans were created in Eclipse. The plans were optimized using 5 to 7 static IMRT beams delivered using sliding window and RapidArc. The treatments were delivered using a Varian Clinic 2100 C/D with a 120 leaf Millennium MLC. Each plan was measured by both detectors for six days. The PTW DAVID software automatically compares the reproducibility of each treatment to the reference measurement. Using the OmniPro‐IMRT software, the daily planar dose distribution was compared to the reference, and the gamma index was calculated using 2%‐ 2mm. For each type of IMRT treatment, the PTW DAVID day‐to‐day measurements were recorded and compared. Results: The day‐to‐day differences measured by PTW DAVID were up to 8% and 5% for RapidArc and IMRT sliding window deliveries respectively. The corresponding RapidArc gamma index calculations ranged from 91% to 100% pass rate but no correlation between PTW DAVID and I'm RT MatriXX measurements was observed. Overall, the dose measurements with the I'm RT MatriXX had less variability than with IMRT sliding window, while the opposite was observed for the PTW measurements. Conclusion: An in vivo measurement device for daily QA of the delivered dose was evaluated for its reproducibility. Our results show that the overall good reproducibility of the device is observed when used for RapidArc and IMRT sliding window plans. However, the in vivo daily variations did not correlate to similar dose variations as measured by an ionization chamber array. Research funded by PTW Germany
SU‐GG‐T‐317: Investigation of Dosimetric Characteristics for Lung Tumor Geometries for a 6 MV Photon Beam
Authors: Hsu S , Roberson P , Moran J
Published: August 2010

Purpose: The uncertainty in heterogeneous geometry dosimetry can have a significant impact on lung SBRT. This study investigated dosimetric characteristics of complex heterogeneous geometries for validation of calculation algorithms. Methods and materials: Solid Water and lung‐equivalent phantom blocks were used to construct homogeneous and heterogeneous geometries. The simulated tumor was composed of 3×2×3 cm3 pieces of Solid Water. The lung equivalent media was 12×8×18 cm3 imbedded in Solid Water. The five geometries used were tumor‐in‐lung geometry with tumor and beam axis at 6.5 cm and at 1.5 cm from the interface, lung‐only geometry with beam axis at 6.5 cm and at 1.5 cm from the interface, and homogeneous Solid Water. A field size of 4×4 cm2 was used to cover the simulated tumor. Gafchromic™ EBT film was mounted in the phantom and was scanned in a known orientation and position using a flatbed CCD scanner. Film was used to measure dose parallel and perpendicular to the beam axis. Film perturbation in heterogeneous geometry was investigated to determine an appropriate method for parallel film measurements. The doses for CT‐scanned geometries were calculated using the Dose Planning Method (DPM) Monte Carlo algorithm in the University of Michigan Treatment Planning System (UMPlan) and results were compared with the measurements. Results: The influence of film perturbation in lung material increases with increasing depth. Using a gantry angle (∼2°) for parallel film measurements reduced the film perturbation but still showed a ∼3% discrepancy. The dose distributions along the central axis and off‐axis vary with heterogeneous geometry as expected. DPM was able to track the dosimetric characteristics for the different heterogeneous geometries. Conclusions: Measurements in multiple planes provide more confident information for complex heterogeneous geometries for evaluating the accuracy of calculation algorithms, which increases confidence in lung SBRT dosimetry. This project is supported by NIHP01CA59827.
SU‐GG‐T‐327: Measurement of Small Field Output Factors Using Plastic Scintillation Detectors
Authors: Klein D , Tailor R , Archambault L , Wang L , Therriault‐Proulx F , Beddar S
Published: August 2010

Purpose: To investigate the performance of a small volume, water‐equivalent plastic scintillation detector (PSD) for measuring output factors for small IMRT fields, as compared to 0.01cm3 ionization chamber. Method and Materials: A PSD of 0.5 mm diameter and 2 mm length was irradiated with 6 and 18 MV beams from a Varian Clinac 21EX. The PSD was positioned vertically at a source‐to‐axis distance of 100 cm, at 10 cm depth in a water phantom, and irradiated with fields ranging in size from 0.5 × 0.5 cm2 to 10 × 10 cm2. The field sizes were defined either by the collimator jaws alone, or by a multi‐leaf collimator (MLC) alone. The MLC fields were constructed in two ways: with the closed leaves (i.e., those leaves that were not opened to define the square field) meeting at either the field center‐line, or at a 4 cm offset from center‐line. Scintillation light was recorded using a CCD camera. Measurements were made using a CC01 ionization chamber under conditions identical to those used for the PSD. Results: Output factors measured using the PSD were found to be in good agreement with those measured using the CC01 down to a field size of 2.0 × 2.0 cm2. The PSD measured higher output factors than the CC01 at smaller fields, e.g. by 7.8 % at 0.5 × 0.5 cm2 under 6 MV photons using MLC fields with 4 cm leaf offset. Also, output factors were found to depend on the field defining geometry. Conclusion: This study suggests that PSDs provide a useful alternative to existing dosimetry systems for small fields, as they are less susceptible to volume averaging and perturbation effects than larger, air‐filled ionization chambers. Therefore, PSDs may provide more accurate output factor determination due to their inherent properties. Supported by the NCI 1R01CA120198‐01A2.
SU‐GG‐T‐328: Dosimetric Performance of the Plastic Scintillation Detector for Radiosurgery Quality Assurance
Authors: Gagnon J , Thériault D , Guillot M , Archambault L , Beddar S , Gingras L , Beaulieu L
Published: August 2010

Purpose: To compare the performance of a plastic scintillation detector (PSD) for quality assurance (QA) in stereotactic radiosurgery to micro‐chamber, gafchromic films, shielded and unshielded diodes. Method and Materials: The PSD consists of a 1 mm diameter by 1 mm long scintillating fiber (BCF‐60, Saint‐Gobain Inc.) coupled to a polymethyl‐methacrylate optical fiber (Eska premier, Mitsubitshi). Output factors and dose profiles for apertures used in radiosurgery ranging from 4 mm to 40 mm in diameter have been measured with the PSD, a micro‐chamber (MC) (A16, Exradin), a shielded diode (SD) (type 60008, PTW), an unshielded diode (UD) (SFD stereotactic, IBA Dosimetry) and gafchromic EBT films. Moreover, a typical stereotactic radiosurgery treatment using 4 non‐coplanar arcs was delivered on a spherical phantom in which UD, MC or PSD was placed. Using the Xknife planning system (Radionics), 15 Gy was prescribed at the isocenter, where each detector was positioned. Results: The output factors measured with the PSD are within a maximum discrepancy of 3.5% compared to UD for the small apertures when normalized to the 35 mm cone. For larger cones the PSD is within 1.3% compared to the SD and the MC. When done with a 35 mm cone, the measured dose for an entire typical stereotactic plan by the PSD is within 1.8% of the measurement performed using the MC and within 1% of the planning system. Volume averaging of the MC can be observed for the 5 mm aperture, where the PSD measurement differs by as much as 9.1%. Conclusion: For stereotactic radiosurgery treatment verification, the PSD gives accurate results compared to the MC once the output factor ratio of both dosimeters are taken into account to correct for the averaging effect of the MC. The PSD shows great potential for stereotactic radiosurgery QA measurements.
SU‐GG‐T‐343: Small Photon Field Dosimetry with Gel
Authors: Ravindran P , Narmatha M
Published: August 2010

Purpose: High spatial resolution of Radiochromic gel dosimeter with optical readout system, its tissue equivalence property and ability to map dose distribution in 3D makes it suitable for dosimetric measurements of small fields used in Radiosurgery. Method & Materials: In this work we have used the FX gel and the optical readout system for measuring the beam profiles and the output factors of Radiosurgery circular collimators and micro MLC (mMLC) collimator for 6MV photon beams. The measurements were repeated with GafChromic film and pin‐point ion chamber. FX gels were also irradiated for two Radiosurgery plans, one with arcs using circular collimators and the other with mMLC and static beams. These were compared with the TPS dose distribution. Results: The field size and the FWHM measured with Gel dosimeter were within 1mm for small fields and maximum of 2 mm for 40 cm diameter filed. The beam profile and the penumbra measured with FX gel matched with those measured with GafChromic films. The output factor measured from the optical attenuation coefficient of FX gel and the factors measured with GafChromic film were within 2%. Conclusion: The dose distribution obtained with FX gel was identical to those obtained from the Treatment Planning System
SU‐GG‐T‐346: Impact of Post‐Irradiation Time on the Accuracy of Dose Measurements for EBT‐2 Model GAFCHROMIC(TM) Film
Authors: Aldelaijan S , Mohammed H , Tomic N , Seuntjens J , DeBlois F , Lning L , Devic S
Published: August 2010

Purpose: A new EBT‐2 GAFCHROMIC™ model radiochromic film has been released recently with addition of a yellow marker dye within sensitive layer that enables correction of non‐uniformity of the active layer of the film. Performance of the EBT‐2 was compared with former EBT model in terms of absorption spectra. Since one of the major drawbacks of the current radiochromic film dosimetry protocols is the post‐irradiation waiting time, we studied the impact of post‐irradiation scanning time on the dose measurements accuracy if shorter times are to be adopted. Method and Materials: Post‐irradiation scanning times employed range from 3 minutes to 5 days and dose range extends from 0 to 6 Gy. Absorption spectra of film samples were measured using a Perkin Elmer Lambda 650 spectrophotometer over the spectral range from 400 nm to 800 nm. Changes in absorption spectra of the samples irradiated to various doses were determined as the net difference between measurement and control film pieces which accounts for changes due to environmental conditions. Results: Both film models experience similar dose change in net absorbance. However, the sensitivity of the latest EBT‐2 model GAFCHROMIC™ film is slightly lower than its predecessor. We show that for two post‐irradiation scanning times of 30 minutes and 24 hours the 1% dose error can be achieved if the scanning time window is less than ±5 minutes and ±2 hours, respectively. Conclusion: By comparing the resultant change in net absorbance between the latest EBT‐2 and previous EBT GAFCHROMIC™ film models we conclude that the addition of the yellow marker dye to the sensitive layer does not affect dosimetric properties of the latest film model. We also describe a procedure by which one can establish an acceptable time window around chosen post‐irradiation scanning time protocol that would provide an acceptable dose error for practical purposes.
SU‐GG‐T‐349: Generalized Formalism and Optimization of the Calibration Sequence of Plastic Scintillation Detectors
Authors: Archambault L , Guillot M , Gingras L , Beaulieu L , Beddar S
Published: August 2010

Purpose: Effects of Cerenkov light on plastic scintillation detectors (PSD) can be successfully corrected, but an inadequate calibration might result in erroneous measurements. The purpose of this work is to establish a general calibration formalism and to perform a systematic study of calibration sequences to optimize PSD performances. Methods and Materials: A general mathematical formalism has been developed with no a priori assumption on the physical characteristics of the detector system. This formalism was used in simulations of realistic PSD systems (i.e. similar to PSDs published in the literature) for over 1000 different calibration sequences. The accuracy and precision of dose measurements with PSD calibrated with each sequence were then analyzed and compared. We also studied the propagation of errors in the whole measurement chain. Results: Optimized calibration sequence can improve precision by a factor of 3 when dose readings are performed with more than 15 cm of optical fibers irradiated. In addition, a 3‐point calibration sequence prevents systematic error in calibration factors measured in the presence of large (>0.5%) statistical noise. Our study also outlined several “calibration pitfalls” that should be avoided: Calibration points that are too similar will unduly deteriorate measurement precision (e.g. calibration with fields of 10 and 30 cm will increase the measurement standard deviation by 1.5 compared to calibrations with fields of 3 and 30 cm); Small dosimetric errors made at calibration time can result in large systematic measurement error with PSDs. A 1% dose error on one of the calibration measurements can result in a 4% to 10% accuracy error with the PSD when more than 15 cm of clear optical fiber is irradiated. Conclusions: We developed a new PSD calibration formalism. After optimization of the calibration sequence of PSDs we showed improvements in both precision and accuracy of current PSD systems.
SU‐GG‐T‐352: A Tracking Database for Clinical Implementation of Microdosimeters
Authors: Mallalieu L Brewster , Gill G , Gaballa H , Jamshidi A , Kapur A , Potters L
Published: August 2010

Purpose: To develop a database tracking tool to facilitate clinical implementation of a patient microdosimetry system with Optically Stimulated Luminescent Dosimeters. The database allows day to day tracking of treatment dose verification and long term analysis of microdosimeter efficacy and accuracy for all patients treated in a multi‐site department. Methods and Materials: The nanodot microdosimeters (Landauer, Inc., Glenwood, IL) are irradiated for one field per patient, once a week, at three hospital sites. The volume of data produced creates the need for an organized and systematic way of monitoring weekly dose reading results. It also provides a substantial base of information for evaluating the processes involved in the planning, placement, and reading of the nanodots. A nanodots tracking database package was created for this purpose using Microsoft Access. For each nanodot irradiated, parameters regarding the individual dosimeter, patient, staff involved and treatment site are recorded, along with the expected and measured doses. Results: Nanodot data has been collected in a tracking database composed of thousands of readings. This data is followed by the physicist and approved by the physician to verify dose delivery for a given patient's treatment. Examples are given illustrating how the nanodot data can presented graphically to evaluate results over time, and filtered for quality assurance evaluation of patient treatments within a multi‐machine multi‐site department. Conclusion: An organized database is essential for the management of a large scale implementation of patient microdosimetry use. The study of long term trends in microdosimeter data can be used to point to areas in which processes can be optimized and quality of care improved.
SU‐GG‐T‐354: Development and Evaluation of a Low Cost, EBT2 Film‐Based Real‐Time Dosimeter
Authors: Du H , Li S , Zhou S , Deng H , Zhang M , Chen S , Schubert L
Published: August 2010

Purpose: To develop and evaluate a low‐cost GafChromic film‐based real‐time dosimeter for practical radiation dosimetry. Method: The dosimeter employs EBT2 film chips, LEDs at 680 nm and 618 nm, a photodiode, and plastic optical fibers. Red light from the LED, modulated by sinusoidal signal, is guided with a φ=1 mm fiber toward a film chip as small as 2×2 mm2. Transmitted light is carried by another φ=0.5 or 1 mm fiber toward the photodiode. A Java program generates the sinusoidal signal, samples the photodiode signal and computes its amplitude in real time via FFT. Dose response to 6MV photon beams was characterized with a Siemens Primus linac and a Varian Novalis radiosurgery linac. Scp of the Novalis linac was measured down to 0.6×0.6 cm2 using multiple film chips. Results: The dosimeter has 16‐bit resolution with a SNR of ∼80 dB. At 680 nm, optical density responded to dose linearly at ∼10−4/cGy up to 75 Gy. At 618 nm, response was approximately 30 times greater. Response was non‐linear but easily approximated with a cubic polynomial. The accuracy of the dosimeter was estimated to be better than 2% if the film is allowed to settle for one day. The accuracy of the real‐time dosimetry is affected by the settling time the optical density takes to reach asymptotic value. The Scp, measured by the dosimeter was comparable to ion chamber and diode detectors down to sizes of 4.2×4.2 cm2, but was reduced as field size decreased, e.g. by ∼4% at 1.2×1.2 cm2. Conclusions: A real‐time, low‐cost dosimeter was developed based on radiochromic‐film at 618 nm. It can be easily implemented and conveniently used for in‐vivo dosimetry given its demonstrated accuracy. Research is supported by Department of Radiation Oncology, University of Nebraska Medical Center. GafChromic film is provided by the International Specialty Products.
SU‐GG‐T‐358: Indigenously Developed Multipurpose Acrylic Head Phantom for Verification of IMRT Using Film and Gel Dosimetry
Authors: Natanasabapathi Gopishankar , Subbiah Vivekanandhan , Vellaiyan Subramani , Kale SS , Rath GK , Laviraj MA , Kumaran Senthil , Thulkar Sanjay , Bisht Rajkishor , Prabhakar Ramachandran
Published: August 2010

Purpose: To validate the newly designed Acrylic Phantom for routine dosimetric purpose in radiotherapy and also simultaneous utilize it to evaluate and compare the calculated dose and measured dose using film and gel dosimetric methods. Method and Materials: In this study, a doughnut shape planning target volume (8.54cm3) and inner organ at risk (0.353cm3) were delineated for an IMRT test planning using the X‐Ray CT image of the phantom. The phantom consists of acrylic slabs which are integrated to form a human head shape with hole in the middle were several dosimetric inserts can be positioned for measurement. An inverse planning with nine coplanar intensity modulated fields was created using Pinnacle TPS. For the film analysis flabed scanner (Microtek 9800XL) and a free download 3cognition Film QA software was used. The 3D‐dose distribution recorded in the MAGAT gel dosimeter was read using a 1.5T MRI scanner (Siemens Avanto). Scanning parameters were CPMG pulse sequence with 8 equidistant echoes, TR = 7210, echo step = 22ms, pixel size = 0.5 × 0.5, slice thickness = 3mm. Using a calibration relationship between absorbed dose and spin‐spin relaxation rate (R2), we converted R2 images to dose images. The dose comparison was accomplished using in‐house MATLAB tage droprem ained ImageJ software. For gel measured dose grid of the TPS was extracted and compared with the measured dose grid of the gel. Results: The results of the film measured dose using Gafchromic EBT2 showed very good agreement between the Pinnacle TPS calculated dose. The preliminary analysis of the gel measurement showed reasonable agreement between the calculated and measured dose. Conclusion: The phantom designed is cost effective and the results are promising but further investigation is required to validate the phantom for dosimetric purpose. Research sponsored by AERB, India through Project no ‐ N‐964.
SU‐GG‐T‐366: Evaluation of Effects of Cable Exposure on the Scanning Data for MV Linear Accelerator
Authors: Kurokawa C , Fox C , Ozawa S , Sugimoto S , Karasawa K , Palta J , Liu C
Published: August 2010

Purpose: To evaluate the effects of cable exposure on the scanning data for MV linear accelerators Method and Materials: Profiles and PDDs from 10 × 10 cm2 to 30 × 30 cm2 field sizes for 6 and 18 MV photons were measured using a 3D automated scanning system (IBA, blue tank and OmniPro 6.5 Accept, Tennessee) and CC04 and CC13 ion chambers (IBA, Tennessee) of volume 0.04 and 0.13 cm3. The measurements were made at ±300V for each detector Results: The cable effect induced a positive change in the measured signal resulting in a reduction in the measured signal for negative polarity measurements and an increase with a positive polarity. The effect is significant for low energy, deep depth and large field size. Although it makes the difference in the GT profiles, the measured field sizes agreed within 0.13 % and 0.07 % for CC04 and CC13 ion chambers. For PDD measurements, the data with a +300V was found to be elevated at the shallow depth (d < dmax) due to the small amount of the cable length in the field. This is consistent with the results of the profile measurements. Although the effect does not change the location of dmax, the measured PDDs of 10×10 and 30×30 cm2 with both voltage, which are normalized at 10cm depth, have 0.3% and 1.2% differences at the dmax, respectively. Conclusion: The effect of cable exposure is significant for low energy, deep depth and large field size measured with the small volume chamber. Ideally, profile and PDD measurements should be acquired as an average of positive and negative bias to remove the effects of cable effects on the measurements.
SU‐GG‐T‐370: Absorption Spectra of a New Radiochromic Film for Various Energies of Therapeutic Photon and Electron Beams
Authors: Hayashi N , Fukuma H , Shibamto Y , Nishii Y , Suzuki Y , Kato H , Teramoto A , Kawano M
Published: August 2010

Purpose: GAFCHROMIC EBT2 (G‐EBT2) films were introduced to replace G‐EBT1 films in 2009. The component and construction of G‐EBT2 films are different from those of G‐EBT1 films. Detailed data of the absorption spectra as a function of absorbed dose are important for understanding the differences in dosimetric sensitivity between those two products. In this study, we investigated the dose response of G‐EBT2 film to therapeutic beams. Method and Materials: The absorption spectra of G‐EBT2 films were measured and compared with those of G‐EBT1 films. The films were inserted in a 30 × 30 × 30 cm3 water equivalent phantom. The radiation energies were 6 and 18 MV, and 6, 9 and 12 MeV for photon and electron beams, respectively. Absorbed spectra of the films were obtained in the wavelength ranging from 350 to 800 nm. The films were analyzed with an ultraviolet photoelectron spectrometer one day after the irradiation. Results: With a marker dye and a yellow colored dye for G‐EBT2 films, the absorption in the wavelength between 350 and 500 nm was high without irradiation. The net frequency of absorption spectra on G‐EBT2 films, however, was similar to that on G‐EBT1 films. For all films, the absorption peaks were observed at the wavelength of 583 and 634 nm (red channel). The sensitivity of dose response increased by approximately 10% compared to G‐EBT1 films. The net absorption below 500 nm (blue channel) was hardly changed. This means that G‐EBT2 films have good sensitivity for photon and electron beams, while maintaining UV sensitivity. These characteristics justify the use of the red/blue channel method. Conclusion: The absorption spectra of G‐EBT2 films were analyzed. While the absorption spectra of the G‐EBT2 films were similar to G‐EBT1 films, the new films exhibited higher radiation sensitivity.
SU‐GG‐T‐371: Measurement of Dose to Pacemakers and Other Implantable Medical Devices
Authors: Pursley J , Oldham M
Published: August 2010

Purpose: Radiotherapy can adversely impair the function of implantable devices such as pacemakers and implantable cardioverter‐defibrillators. Tolerable dose limits are manufacturer dependent, complicating protocol standardization for radiotherapy clinics. This study evaluates the feasibility of using a simple patient dosimeter to estimate the dose received by subcutaneous implanted devices that fall inside of or adjacent to radiation therapy fields. Method and Materials: The OneDose™ is a pre‐calibrated in‐vivo dosimeter typically used use directly in radiation therapy fields. For this study, dosimeters were irradiated on the surface of a Virtual Water™ phantom using 6MV 10 × 10 fields at positions on the central axis to 15 cm off‐axis. Half of the dosimeters were covered with bolus as recommended by the manufacturer. Dosimeter responses were compared to a pinpoint chamber directly underneath at dmax depth. Results: Dosimeters gave good precision in all situations. Dosimeters in the treatment field and under bolus reported dose in good agreement to the ion chamber, while those adjacent to the field slightly under‐reported dose. Further studies were performed using the ion chamber placed on the phantom surface under sections of Virtual Water machined to mimic the bolus shape. These studies confirmed that less dose is deposited at this position than at 1.5 cm depth below. Without bolus, in‐field dosimeters reported low as expected, due to lack of buildup. Those adjacent to the field read high by ∼2cGy, likely due to electron contamination reaching the sensor. More work is planned to better understand this result. Conclusion: OneDose can estimate dose to shallow depths lateral of photon beam fields with a small correction factor. Dosimeters should be covered with bolus to provide buildup for any direct radiation they may encounter. Conflict of Interest: One author is employed by Sicel Technologies.
SU‐GG‐T‐373: The Effect of Vessel Material on Fricke Dosimeter Yield
Authors: Benhabib S , Austerlitz C , Ferreira M C , Sibata CH
Published: August 2010

Purpose: To investigate the effect of solid water vessel in contact with the fricke dosimeter yield in terms of surface to volume ratio and storage time. Material and methods: Fricke solution was made up with the highest pure commercial available chemicals. The optical density was measured using the Varian Cary 400 UV‐Vis spectrophotometer provided with a temperature controller. Three set of ten cylindrical containers (internal radius of 0.5 cm and height 3.5 cm) were made of solid water, polystyrene, and PMMA. Containers have been filled with fricke solution of 4.3, 4.5, and 5 cm−1 surface to volume ration, and sealed with Mylar material. Optical density readings at 304 nm were performed in 120 minutes time interval, 30 minute step. The effects of solid water material on the Fricke dosimeter yield (surface to volume ratio and storage time) were compared against those values obtained for polystyrene and PMMA materials. Results: The deviation of the optical density for a surface to volume ratio ranging from 4.3 to 5 cm−1 was 0.01%, 0.01%, and 0.02% for PMMA, Polystyrene, and solid water, respectively. The maximum deviation among the different materials in the same surface to volume ratio range did not exceed 0.04%. Within the uncertainty of measurement (0.06%) no effect of solid water, polystyrene, and PMMA containers was observed on the Fricke dosimeter yield during the storage time interval of 120 minutes. Conclusion: Solid water plastic can be used to store Fricke solution and can still be comparable to polystyrene and PMMA. It has the advantage to have physical properties very close to water. Keywords: Surface to volume ratio, PMMA, Fricke dosimeter yield, solid water
SU‐GG‐T‐379: GEANT4 Monte‐Carlo Simulation of Dosimetry Parameters of the Passively Scattered Beam Lines at the Proton Therapy Center in Houston
Authors: Ding X , Sahoo N , Song X , Zhu X , Lii M , Gillin M
Published: August 2010

Purpose: To use the Geant4 Monte‐Carlo simulation (MCS) program to generate proton dosimetry data of the passively‐scattered beams of the Hitachi ProBeat machine at the Proton Therapy Center in Houston. Method and Materials: The hardware components of the double scattering beam nozzle of the ProBeat machine and a water phantom were modeled for MCS using GEANT4. The electromagnetic and hadronic models in GEANT4 were used to simulate the interaction among proton and other particles. Transportation histories of 20 million particles were tracked and spatial dose depositions in a large water phantom were recorded. Results from simulation are compared with the beam data measured using an ionization chamber, which includes Pristine Bragg Peaks (PBP), spread out Bragg Peaks (SOBPs), and transverse profiles for proton beams with energy between 100 and 250 MeV to validate the GEANT4 beam model, which was then used to generate clinical dosimetry data. We implemented a phase‐space‐based approach to speed up the simulation. The phase‐space‐based output files contain information about particle's energy, momentum and positions for different combination of incident proton energy and SOBPs. These files were used to calculate the change in the dose output due to the presence of range shifters known as range shifter factor (RSF), which were compared with the measured values. Results: The distal ranges obtained from the GEANT4 agreed within 1 mm both for PBP and SOBP. The profiles agree within 3% or 2 mm in most of the regions. The simulation speed was found to be about 10 times faster in phase‐space‐based approach than particle source model approach. The difference between the simulated and measured values of RSFs was found to be less than 2%. Conclusion: The new phase‐space‐based simulation procedure is found to be a useful approach to generate the needed proton beam dosimetry data for clinical application.
SU‐GG‐T‐389: Mixed Electron and Photon Beams Modulated by the XMLC for Partial Breast Irradiation
Authors: Palma B , Ureba A , Salguero F , Arrans R , Plaza A Leal
Published: August 2010

Purpose: To present a modulated radiation therapy technique as a more accurate alternative to current approaches for the planning of partial breast irradiation (PBI). This technique is based on the use of modulated electron beams alone or in combination with intensity modulated photon radiation therapy (MERT+IMRT), delivered with the same collimation device (xMLC). Method and Materials: A Monte Carlo treatment planning system based on BEAMnrc/DOSXYZnrc code, named CARMEN, was developed by our group. A Linear Programming (LP) optimization procedure calculates the weight for each beamlet generated from phase space data below the jaws. An algorithm to reduce beamlets and voxels was developed to make possible the LP formulation and to solve the problem in operative times. A specific sequencer capable to consider electron and photon beams was also developed by taking into account previous full MC simulations of the xMLC. Four PBI cases were planned with CARMEN following the recommendations from the RTOG‐0413/ NSABP‐B39 trial protocol for 3D‐CRT PBI. Two cases were planned by MERT alone and the other two by MERT+IMRT as a function of PTV position in depth. Results: More than 90% of PTV volume received 90% of prescribed dose with a maximum dose ≤ 110%. A reduced dose to ipsilateral breast, lung and heart, was also achieved, especially in the low‐dose range. The addition of IMRT to MERT improved PTV coverage in depth diminishing overdosed superficial areas. MERT and MERT+IMRT solutions improved PTV homogeneity compared with the other techniques evaluated and in most of the cases reduced dose to normal tissues. Conclusions: An improved PBI plan was obtained with MERT+IMRT solution compared with 3D‐CRT and brachytherapy techniques. The accuracy of this technique by means of the xMLC opens the chance to implement the accelerated partial breast irradiation in the clinical practice.
SU‐GG‐T‐390: Dose to Proximal Bronchial Tree in Lung SBRT Treatments: Comparison of Pencil Beam and Monte Carlo Dose Distributions
Authors: Altunbas C , Miften M , Stuhr K , Gaspar L , Kavanagh B
Published: August 2010

Purpose: Increased dose to proximal bronchial tree (PBT) has been associated with higher incidence of toxicities in prior lung SBRT trials. In this study, the effect of pencil beam (PB) dose calculations on the PBT dose for targets within the PBT zone was evaluated using Monte Carlo (MC). Methods: Lung SBRT plans with PTVs within the PBT zone were generated for 10 patients using a commercial treatment planning software equipped with PB and MC algorithms. PBT within 2 cm of the PTV region was contoured on CT datasets for all patients. Treatment plans were generated using two different dose calculation methods: i) Pencil beam algorithm without inhomogeneity correction (PB‐IHC), and ii) PB with inhomogeneity correction (PB+IHC). Both plans for each patient had identical beam geometry, but different MLC shapes were used to achieve similar conformai dose coverage of PTV. All plans were normalized to have 98% of PTV receive 60Gy in 3 fx. Dose distribution for each plan was recalculated with MC by keeping the monitor units the same in respective plans. Dosimetric comparisons were performed between PB‐IHC, PB+IHC, and MC based dose distributions, where MC dose distributions were assumed to be the gold standard. Results: The MC mean values for PTVD95, PTVDmax, PBTDmean, and PBTDmax outside the PTV region were 67.7Gy, 83.1Gy, 38.5Gy, and 64.5Gy respectively. These values were higher than PB‐IHC by 9%±4.5%, 17%±9%, 19%±6%, and 16%±5.8%, and were lower than PB+IHC by 13%±4.7%, 9%±1.5%, 5%±1.8%, and 8%±2.4%, respectively. Conclusion: Compared to MC, PB+IHC overestimates the dose to both PTV and PBT, and PB‐IHC underestimates the dose to both PTV and PBT. Unrecognized dose hotspots in this region might influence the risk of toxicity to proximal airways. This issue should be considered when analyzing clinical toxicity data calculated using PB‐based algorithms without heterogeneity corrections.
SU‐GG‐T‐392: Surface Dose Investigation for Open and Wedge Fields: Measurements vs. Monte Carlo
Authors: Kim J , Hill R , Kuncic Z
Published: August 2010

Purpose: To accurately predict the surface doses of radiation fields commonly used in breast cancer radiotherapy using Monte Carlo simulations. Method and Materials: Monte Carlo simulations were carried out using the EGSnrc/BEAMnrc Monte Carlo code on a computing cluster. A Varian 21iXs linear accelerator was modeled using BEAMnrc. Characteristic beam data for the 6MV x‐ray beam on this linear accelerator was measured using an ionisation chamber for different beam sizes (5×5 to 15×15 cm2) and various static field arrangements. Monte Carlo calculated doses were verified by comparing dose distributions with measured data including depth doses and profiles of the open fields. The verified Monte Carlo data were then used to model various beam setups and the corresponding surface doses. Surface doses were also calculated in the Eclipse treatment planning system using the Pencil Beam Convolution algorithm. Results: Measured and Monte Carlo calculated depth doses agreed to within 2% for all field sizes and shapes, except within the first 2 mm depth from the surface where differences of up to 30% are found with measured data being consistently higher. The dose profiles agree to within 2.5%, however, relatively large systematic differences (up to 6%) were found at the edge of the fields. These differences tend to increase with field size. Largest difference in surface dose is found to be more than 5% after the over‐response chamber reading correction. Treatment planning system calculated surface doses were systematically over predicted compared to our Monte Carlo data for all field sizes and shapes by more than 10%. Conclusion: Our results confirm there are significant uncertainties in predicted and measured skin doses for breast cancer radiotherapy as compared to Monte Carlo calculations. This suggests that clinical dose measurements and/or treatment planning system alone may be inadequate for assessing the possible risk of adverse skin reactions.
SU‐GG‐T‐394: Proton SOBP Reconstruction for Low‐Density Media and Divergent Beams
Authors: Tafo A Guemnie , Fourkal E , Veltchev I , Ma C
Published: August 2010

Purpose: To show a realistic analytical model for energy spectrum calculation leading to a flat depth dose distribution for low density media and divergent beams. We put the accent on the divergence of the beam in order to make this model applicable to new proton accelerator technologies such as dielectric wall or laser induced proton beams where the gantry size would be smaller than the conventional gantry size. Although the model is suitable for any density and divergence of the beam, we focus our effort on low density media, in order to investigate lung cancer treatments using such novel systems. Methods and Materials: Monte Carlo simulations have been performed using the code Fluka. We first used a uniform phantom to validate the model and then double layer geometry (7 cm tissue ICRP slab followed by 20 cm lung ICRP slab). The lung density has set to 0.2 g/cc. The beam size has been set to 10×10 cm2, while the divergence of the beam ranged from 80 cm SSD to 1000 cm. Results: We showed that for any divergence of the beam, density of the media or proton range, the model gives a flat SOBP in a very fast way. Calculations showed that in double layer geometry, the in‐water calculated proton energy spectrum does not give a flat dose distribution anymore, especially for high divergent beam. We showed the limitations of the model over the field size, down to 5×5 cm2 the lack of equilibrium starts to affect the depth dose distribution. Conclusion: The analytical model we developed permits to reconstructed a flat physical dose distribution in a very fast way for any beam parameters and low density material. It can easily be implemented in a treatment planning system to get the appropriate proton energy spectrum for uniform tumor coverage.
SU‐GG‐T‐395: Monitor Unit Calculation for Electron Arc Therapy Using Monte Carlo Simulations
Authors: Chow J , Jiang R
Published: August 2010

Purpose: This study reported a monitor unit (MU) calculation method for electron arc therapy. This MU calculation was carried out using Monte Carlo simulation and verified by measurement. Methods and Materials: The 9 MeV phase space electron beam (2.6 × 21 cm2) attached to the block tray holder of a Varian 21 EX linac was generated using the EGSnrc‐based BEAMnrc code, and was verified by dosimetric measurements using ionization chamber and radiographic film. The relative output factor (ROF), source‐to‐surface (SSD) offset and dwell factor (DF), required in the MU calculation, were determined using measurement and Monte Carlo simulation. Solid water rectangular and cylindrical phantom made of polystyrene were used in the measurements. Results: Deviations of ROF, SSD offset and DF between measured and Monte Carlo results were 1.2%, 0.18% and 1.5% respectively. Our results show that the DF decreased with an increase of the treatment arc angle (α), and such decrease of DF was more significant in the α range of 0 – 60 deg than 60 – 120 deg. Comparing the DF determined using the rectangular and cylindrical phantom through measurement and Monte Carlo simulation, we found that the DF determined by the rectangular agreed well with that by the cylindrical phantom within ±1.2%. It shows that a simple setup of solid water rectangular phantom was sufficient to replace the cylindrical phantom to determine the DF associated with the electron arc. Conclusions: Monte Carlo simulation was proved to be an alternative way to determine the MU for electron arc therapy. Since Monte Carlo simulation can generate pre‐calculated database of ROF, SSD offset and DF for the MU calculation with a reduction of human effort and linac beam‐on time, it is suggested to integrate Monte Carlo simulation into the commissioning of electron arc therapy.
SU‐GG‐T‐396: Secondary Neutron Production by a Scanning Proton Beam
Authors: Moskvin V , Das I , Cheng C , Zhao L , Johnstone P
Published: August 2010

Purpose: A scanning proton beam is a highly desired option in new proton facilities. However, the patient specific brass aperture used to shape the beam is one of the major sources of secondary neutrons, which contribute to whole body dose. Thus, minimizing neutron yield from the aperture is a critical criteria for optimization of scanning parameters. In this study we investigated this neutron yield, and the concurrent generation of the neutrons inside the patient, in a uniform scanning proton beam using Monte Carlo simulation. Materials and Methods: The general purpose Monte Carlo code FLUKA was used to simulate the interaction of a 204.8 MeV uniform scanning beam with brass apertures between 2 and 10 cm in diameter using a 10 cm shout. A water phantom 5 cm from the aperture surface was used to simulate a homogeneous absorbing medium. The neutron fluence, spectrum and dose equivalent in the phantom was acquired for various scanning patterns. Results: Neutrons generated in the brass aperture affect the ambient dose equivalent H*(10) near the surface of the phantom. However, neutrons generated inside the phantom have a dominant contribution to the ambient dose equivalent H*(10) close to isocenter. Their contribution ranges from 50% to 90% to the total neutron H*(10), corresponding to a dose equivalent of 2.5 to 5.5 mSv/Gy for the different apertures used in this study. The H*(10) values 30 cm from the beam axes are one order of magnitude less than at the isocenter. Conclusions: The neutrons produced in the aperture are the major contributor to secondary dose only in the superficial region. However, neutrons generated inside the patient are the most important factor contributing to the total body dose in proton therapy.
SU‐GG‐T‐400: A Monte Carlo Based Proton Therapy Dose Distribution Verification System
Authors: Beltran C , Jia Y , Slopsema R , Yeung D , Li Z
Published: August 2010

Purpose: Create a Monte Carlo based proton therapy dose distribution verification system which can be used as a clinical aid in determining the adequacy of proton treatment plans. Method and Materials: The GEANT4 Monte Carlo toolkit is used for all simulations. The absolute dose value per proton of the Bragg peak and its location were benchmark against NIST and other published data to ensure the accuracy of the physics models used in the simulations. For the patient specific dose distribution verification, the full nozzle is not simulated, only the beam spectrum, patient specific snout components, and the patient's CT. The beam spectrum, which for the IBA double scattering machine, depends on the time dependent proton current and range modulator, is calculated based on a spreadsheet which uses water equivalent thicknesses for the nozzle components. This allows for relative fast calculations compared with full nozzle simulation. The speed is needed if this system is to be used in a clinical setting. Results: The beam spectrum has been verified for multiple ranges and SOBP widths for the different options. The MC distal and proximal 90% of the SOBP matches the TPS to within 3% or 3mm for all tested cases. The MC penumbra for different aperture sizes match the TPS at 20%, 50% and 80% within 3% and 3mm, however the 10% value of the penumbra for the MC is wider than the TPS, which has been noted as a limitation of the TPS in literature. Comparisons to measurements have not been made to date and adjustments to model parameters will be made to match measurements. Compensator simulations have not been performed to date. Conclusion: This Monte Carlo dose verification system is fast and accurate; therefore it holds promise as a clinically useful tool.
SU‐GG‐T‐405: Quantitative Analysis of Dose Enhancement Contributed by Different Photon Interactions in X‐Ray Irradiation with the Presence of HZ Materials with Geant4 Monte Carlo Simulation
Authors: Huang K , Yu Y
Published: August 2010

Purpose: To study the overall dose enhancements and those contributed by different processes quantitatively in the presence of HZ materials and X‐ray with different energies with Monte Carlo method. Method and Materials: A virtual phantom is constructed with three layers, with the first and the third layers made of water, sandwiching the second layer composed of water and HZ materials, including Gold (Au), Platinum (Pt), Gadolinium (Gd), or Iodine (I). The phantom is irradiated by X‐ray with mono and spectrum energies. The dose deposited in the phantom along photon incidence direction is recorded with a space resolution of 0.1mm. The percent depth dose (PDD) curves for these sources are generated with dmax calculated. The overall dose enhancement ratios and those contributed by different processes, such as Pair Production, Compton Scattering, and Photoelectric Effect, are analyzed quantitatively. Results: The dose enhancement ratios (rD–E) for different HZ materials with the same molar concentration increase with the atomic numbers. For example, with concentration at 0.1882 mmol/ml and 0.25MeV X‐ray energy, the overall rD–Es are 152.69%, 150.57%, 127.64%, and 114.32% for Au, Pt, Gd, and I, respectively. Higher energy source gives lower rD–E. For example, a 5 MeV source gives a rD–E of 105.2% for Au. These can be explained by analyzing the dose contribution from each process and the rD–E for that process. Conclusion: A new method is implemented in MC simulation for analyzing percentage dose contribution from different processes to the total dose. This helps to select the right X‐ray energy and HZ atom as well as the concentration in HZ‐enhanced Radiation therapy. Lower energy source gives higher rD–E because more doses are contributed by Photoelectric effect, which gives very high rD–E. At the same molar concentration, materials with larger atomic number give higher rD–E.
SU‐GG‐T‐407: Modeling a New Varian Linac Using a CAD to Geant4 Geometry Implementation: Dose and IAEA‐Compliant Phase Space Calculations
Authors: Constantin M , Perl J , Constantin D , LoSasso T , Salop A , Narula A , Svatos M , Keall P
Published: August 2010

Purpose: To (1) use accurate CAD linac treatment head geometry for Geant4 Monte Carlo calculations to minimize the errors due to approximations of the linac treatment head components used in previous studies, (2) investigate the CAD to Geant4 linking procedure through 6MV dose calculations, and (3) provide IAEA‐compliant patient‐independent phase space files. Method and Materials: Geant4 v4.9.2.p01 was employed to simulate the treatment head geometry of the newest Varian linac for a 6MV beam. The geometry components are tessellated solids included in Geant4 as GDML (Generalized Dynamic Markup Language) files obtained via STEP (STandard for the Exchange of Product) export from Pro/Engineering, followed by STEP import in Fastrad, a STEP‐GDML converter. An IAEA‐compliant phase space writer was implemented between the shielding collimator and the upper jaws on a cylindrical geometry due to the compact treatment head and the divergent arc of the jaws. The simulation was run in parallel on the SLAC cluster. The dose calculations were based on a combined total of 26 billion histories. The voxel size for the 60×60×40cm3 water phantom was 4×4×4 mm3. Results: For the percent depth dose profiles, the agreement between experiment and Monte Carlo is within 2% for 4×4, 10×10, 20×20, 30×30 and 40×40 cm2 field sizes. For the lateral dose profiles, the agreement is within 3%. The fractional uncertainty of the average dose for the voxels with dose larger than 0.5 of the maximum dose decreased from ∼4% to ∼2% for field sizes increasing from 4×4 to 40×40 cm2. Conclusions: We have developed and validated the Geant4 simulated IAEA‐compliant phase space of the new Varian linac for the 6MV beam obtained using a high precision geometry implementation from CAD. These files will be made publicly available and could be used for further dose calculations. Supported in part by Varian Medical Systems