bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐247: An Investigation On the Cross‐Calibration Value of the Plane‐Parallel Chamber Against the Cylindrical Chamber for Different High‐Energy Electron Beams with TG‐51 Protocol
Authors: Yu M , Gladstone D
Published: July 2008

Purpose: To investigate the cross‐calibration value of the plane‐parallel chamber for different high‐energy electron beams. Method and Materials: An Exradin parallel‐plate ion chamber (P11) and a calibrated cylindrical ion chamber (PTW N30006) were used for measurements. R50 of electron beams was calculated from Eq.(16) of the AAPM TG‐51 protocol with depth‐ionization curves measured by the cylindrical chamber. Since 2 cm ⩽ R50 ⩽ 20 cm for electron beams 9, 12, 15 and 18 MeV, (K′R50)pp, the electron beam quality conversion factor, was calculated from Eq.(20) of the protocol. Dose per MU at dref using the cylindrical chamber was determined with worksheet B according to the protocol. The parallel‐plate chamber was used with the TG‐51 worksheet C to determine the cross‐calibration value, Kc, of the plane‐parallel chamber against the cylindrical chamber for the above electron beams. Measurements were repeated several times. Results: Kc should be constant for the same parallel‐plate chamber for different energy electron beams since it only depends on chamber's construction, however, our measurements show Kc increases with beam energy. The average cross‐calibration value for 18 MeV is approximately 0.8 % larger than that for 9 MeV. Kc is a function of (K′R50)pp which depends on beam energy via Eq.(20). If Eq.(20) doesn't describe such dependence accurately, Kc will vary with beam energies. When we fit our measurement data with a different set of parameters in Eq.(20), Kc no longer varies with beam energies. Conclusion: Eq.(20) in TG‐51 protocol may not accurately describe the dependence of (K′R50)pp as a function of electron beam energies. Variations of the cross‐calibration value for different beam qualities for the same parallel‐plate chamber are observed. It may be necessary to use different parameters in Eq.(20) to more accurately describe the above dependence.
SU‐GG‐T‐252: An Efficient Technique for Commissioning a Tissue Heterogeneity Correction Algorithm for Treatment Planning for Lung Cancer
Authors: Hall W , Liu T , Stern R , Perks J , Yang C , Mathew M , Purdy J
Published: July 2008

Purpose: To establish a measurement protocol for commissioning lung heterogeneity correction using the Pinnacle3™ convolution/superposition algorithm based treatment planning system (TPS). Method and Materials: A phantom with lung density material was constructed to investigate differences between dose calculated using the Pinnacle3™ TPS and dose delivered using an Elekta™ Synergy™ linear accelerator. The phantom set‐up was 4cm solid water then 4cm lung phantom material (Gammex), then either no bolus, 1cm bolus, or 2cm bolus; then 4cm solid water. Point doses were calculated by placing points of interest (POIs) along the beam central axis at various depths. All dose distributions were calculated using the Pinnacle3™ adaptive convolve (AC) algorithm, for a 10×10cm2 AP beam (6MV and/or 15MV) @ 100 SSD and 200MU prescription. Ion chamber and/or MOSFET measurements were performed between bolus slabs for the bolus containing phantoms and POIs were generated to obtain calculated dose for each dosimeter position. In addition, a MapCheck device was placed under each phantom to measure 2D dose distributions for comparison against the calculated 2D planar dose map. Results: Agreement between calculated and measured dose using ion chamber was within ±1% for 19/23 measurements with a maximum difference of 3.23%. MOSFET measurements showed good agreement, as all measurements were within 3%. Evaluation of planar dose distribution with the MapCheck yielded a pass rate ranging from 93.2% to 98.5% based on 2% difference and 2mm distance to agreement. Conclusion: Pinnacle3™ TPS dose calculations using tissue heterogeneity showed excellent agreement with dose measured in a heterogeneous phantom using 3 different dosimeters. The measurement plan presented represents a simple, efficient, and accurate means for commissioning heterogeneous dose calculation algorithms for clinical use.
SU‐GG‐T‐255: Experimental Verification of Lung Tumor Dose with Radiochromic Film
Authors: Zhang J , Yoo S , Kim S , Yin F
Published: July 2008

Purpose: To assess the absorbed doses in and around lung tumor by performing radiochromic film measurements. Method and Materials: A phantom was made to have a water equivalent central core, a low‐density middle layer and a water equivalent outer layer to simulate tumor in the lung, lung and chest wall, respectively. GafChromic™ EBT films were placed in the anterior and posterior surfaces and middle plane of the tumor. Three plans with a single anterior beam, opposed anterior‐posterior beams and five‐field‐IMRT were delivered separately. A calibration curve was generated to convert the optical density to absorbed dose. The measured doses in the three planes were compared with the calculated doses from Eclipse™ system which uses a Pencil Beam convolution‐superposition Algorithm with modified Batho inhomogeneity correction. The single‐beam, opposed beams were delivered for both low (6 MV) and high (15MV) energy photon. The IMRT plan was delivered with 6 MV. Results: For both single‐ and opposed‐beam plans, the measured dose was very close to the calculated dose in the middle plane (<2 %). In the anterior and posterior surfaces of the tumor, the measured dose was substantially lower than the calculated one. For the single‐beam plan, the difference was 10.0% (6MV) and 13.6% (15MV) for the anterior surface and was 9.8% (6MV) and 6.8% (15MV) for the posterior surface. For opposed beams, the difference was about 8% (6MV) and 11% (15MV) for both surfaces. Similar dose differences were observed for the IMRT plan (∼ 10 %). Conclusion: EBT film measurements demonstrated that the dose calculation algorithm used by a typical treatment planning system is precise (<2%) for the deep locations inside the tumor, but may overestimate the surface dose about 10%.
SU‐GG‐T‐259: Dose Discrepancies in the Buildup Region and Their Impact On IMRT Field Dosimetry
Authors: Hsu S , Moran J , Chen Y , Kulasekere R , Roberson P
Published: July 2008

Purpose: The accuracy of the buildup dose for radiotherapy treatment planning suffers from challenges in both measurement and calculation. This study was designed to improve the parameter fit in the contamination term of the convolution/superposition calculation algorithm based on more accurate measurements and to evaluate the impact of residual discrepancies on IMRT fields for normal and oblique incidences. Method and Materials: The percent depth and profile doses were measured in the buildup region in Solid Water using an Attix parallel plate chamber with over‐response correction factors and using Kodak XV film with dynamic calibration curves. The doses were measured at superficial depths ranging from 0.2 to 1.5 cm for 5×5, and 10×10 cm2 fields and 0°, 45°, and 70° incidences. Contamination term parameter fit preference was given to normal incidence. Two segmental head‐and‐neck IMRT fields were recalculated with the new fit parameters as a function of incident angle. Comparison indices (γ and dose‐gradient compensation) were used to quantify the agreement between measurements and calculations for the IMRT fields. Results: After parameter refit, the agreement between measurements and calculations were improved. Differences were still present for some dose profiles. The profile measurements showed larger field widths and sharper penumbras. The local deviations of percent depth dose (PDD) comparisons were within 2% and 5% for normal and oblique incidences, respectively. For IMRT fields, the incident angle did not greatly influence the agreement between calculation and measurement. Comparison indices were lower for shallower depths. Conclusion: Refitting the parameters in the contamination term using more accurate measured buildup doses improved the accuracy of calculations in the buildup region. Contamination term modifications may be required to better account for the influence of incident angle.

Supported by NIHP01CA59827.
SU‐GG‐T‐270: Evaluation of a Novel 3‐D Polymer Gel Dosimetry System for Proton Radiotherapy
Authors: Lopatiuk‐Tirpak O , Langen K , Meeks S , Kupelian P , Maryanski M , Hsi W , Palta J , Zeidan O
Published: July 2008

Purpose: Preliminary evaluation of a novel, large‐volume (2.2 liter), 3D polymer gel dosimeter (BANG®‐3‐PRO) developed for proton radiotherapy dosimetry is presented. The characteristics of dose response and its linear energy transfer (LET) dependence are addressed. The performance of the dosimeter is analyzed under the conditions of pristine and clinically relevant beam configurations. The ability to visualize and analyze proton dose distributions in 3D is demonstrated. Method and Materials: The dosimeters were read out using a characterized optical computed tomography scanner. The calibration was performed by correlating the change in optical density along the beam axis to the depth‐dose curve from a pristine proton beam delivery (28.4 cm range, 4.5 cm circular aperture) obtained using ion‐chamber measurements. To exclude the potential LET dependence, only the plateau region was used. Several pristine‐beam and spread‐out Bragg peak (SOBP) dose distributions were recorded and compared to ion‐chamber measurements, including the assessment of dose response in high‐LET regions. A volumetric dose distribution delivered using a pristine beam and an irregularly shaped aperture was recorded and reconstructed in three dimensions. Results: The dose response of the gel was linear in the studied range (25–150 cGy). Dose profiles from the gel showed good general agreement with ion chamber measurements, with excellent positional reproducibility of the distal and proximal edges along and perpendicular to the beam direction. In the case of SOBP delivery (140 cGy peak dose), no response quenching in the peak region (characteristic of other dosimeter types) was observed. For a pristine beam delivery (170 cGy peak dose), the gel dosimeter demonstrated a ∼10% under‐response at the Bragg peak (BP). Conclusion: Preliminary investigations suggest that this 3D dosimeter is capable of reproducing proton beam dose distributions with high spatial accuracy, with little or no quenching at the BP or SOBP.
SU‐GG‐T‐272: Energy and Irradiation Modality Independence of Calibration Coefficients for Water Equivalent Plastic Scintillation Detectors
Authors: Guillot M , Lacroix F , Thériault F , Beaulieu L , Gingras L
Published: July 2008

Purpose: To demonstrate that approximate water equivalence of plastic scintillation detector (PSD) has for consequence the energy independence of the calibration coefficients for both photon and electron beams in the megavoltage energy range. Method and Materials: The PSD consists in a small 1 mm diameter and 2 mm long plastic scintillating fiber made of a polystyrene core (BCF‐12, Saint‐Gobain, inc.). The scintillator was coupled to a 2 meters long non‐scintillating plastic optical fiber and a color CCD camera (Apogee instruments inc.) was used as photodetector. The calibration coefficients of the PSD where extracted for 6 MV, 23 MV photon beams and 9,12,15 and 18 MeV electron beams using a Farmer ionization chamber (Exradin). Complete removal of the Cerenkov radiation produced in the optical fiber was done with a chromatic discrimination technique using the blue and green channel of the CCD camera. All measurements were performed according to the recommendations of the AAPM TG‐51 protocol for clinical dosimetry. Results: The PSD exhibits a maximum deviation of less than 1.7 % (about the mean) of its calibration coefficients over the measured energy range for both irradiation modalities. Conclusion: The energy independence of the calibration coefficients for PSD was demonstrated experimentally for the first time for both photons and electrons. PSDs have the potential to simplify and improve accuracy of dose measurements in clinical situations where photons and electrons are both present in the beam such as electron contamination in photon beams or bremsstrahlung contaminated electron beams.
SU‐GG‐T‐273: Suitability of Synthetic Diamond Films for Radiotherapy Dosimetry
Authors: Betzel GT , Lansley SP , Baluti F , Reinisch L , Lydon JM
Published: July 2008

Purpose: To investigate the suitability of synthetic diamond films as detectors for radiotherapy dosimetry. Method and Materials: A range of commercially available diamond films grown by chemical vapor deposition (CVD) have been studied using spectroscopic, microscopic and electrical characterization techniques, including Raman spectroscopy, optical, secondary electron, and atomic force microscopy. Diamond detectors incorporating Perspex have been fabricated in order to examine various response characteristics, especially transient behavior and priming effects, due to material defects, interface phenomena, etc. Initial dosimetric characterization was performed using a 6 MV photon beam from a Varian 600C linac. Measurements were achieved using a 2570/1 Farmer Dosimeter and a Keithley 6430 SourceMeter. EGSnrc code was used to model simple device structures to assess performance issues that may impede proper measurements e.g. sources of fluence perturbation, absorbed dose distribution, and charge collection efficiency. Results: I–V measurements of polycrystalline diamond films with Ag contacts tested under a ±210 V bias sweep exhibited nonlinear behavior as expected. High leakage currents were a problem for some detectors. Resistivity measurements at 100 V from one manufacturer of films spanned from 1011 to 1012 Ω⋅cm. Dependence on fluence and fluence rates between 50 – 250 MU/min was observed. However, angular dependence appeared to be negligible. Overshoot was also seen during initial exposure, as well as the well‐known priming effect as the signal evolved over time. EGSnrc models have illustrated how electrode thickness alters the dose delivered to the sensitive volume. Conclusion: Experiments will continue to explore the response phenomena of CVD diamond films to evaluate their suitability as detectors for medical applications. The films studied thus far have exhibited typical behavior as seen in previous literature. A thorough characterization of a variety of CVD films and their consequential responses is underway in order to mitigate or ideally eliminate inherent shortcomings.
SU‐GG‐T‐277: Characterization of the OneDosePlus™ MOSFET Detector and Assessment of Its Suitability as An in Vivo Dosimeter
Authors: Ainsley C , Fu L , Ingram M , Novak J , Kassaee A , Both S
Published: July 2008

Purpose: To characterize the OneDosePlus™ MOSFET detector, compare its response to both that of a Thomson‐Nielsen MOSFET detector and that of an ionization chamber, and assess its feasibility for use as an in vivo dosimeter. Method and Materials: Using, alternately, a OneDosePlus™ MOSFET detector, a Thomson‐Nielsen MOSFET detector and an ionization chamber—the gold standard, the output of a 6 MV photon beam from a clinical linear accelerator was measured at the depth of dmax = 1.5 cm in a water‐equivalent plastic phantom (i) as a function of field size and SSD for a fixed 0° gantry angle, and (ii) as a function of gantry angle for a fixed field size of 10×10 cm2 and SSD of 100 cm. Field sizes in the range 5×5–40×40 cm2, SSDs in the range 80–120 cm, and gantry angles in the range 0–80° were studied. For each detector, a similar geometrical setup was employed and the same number of monitor units was delivered. MOSFET measurements were normalized to those obtained correspondingly with an ionization chamber, and then compared. Results: The OneDosePlus™ detector exhibited a response to the delivered radiation dose that was consistent with the corresponding Thomson‐Nielsen MOSFET and ionization chamber measurements for all SSDs and field sizes. As expected, the output increased with increasing field size and decreasing SSD. The OneDosePlus™ measurements demonstrated significant angular dependence with respect to gantry angle beyond ∼40°, deviating by as much as 60% from the ion chamber reading at 80°. Conclusion: The OneDosePlus™ detector is suitable for in vivo skin‐dose determination. It provides a direct, reliable dose measurement for a range of field sizes, SSDs and gantry angles. Though significant angular‐dependent corrections are required beyond ∼40° incidence—owing to the detector's asymmetric construction—it is otherwise a highly convenient alternative to the Thomson‐Nielsen MOSFET.
SU‐GG‐T‐279: Evaluation of a Novel 4D In‐Vivo Dosimeter
Authors: Cherpak A , Hallil A , Ding W , Cygler J
Published: July 2008

Purpose: To present the latest quality assurance and characterization tests of the RADPOS 4‐D in vivo dosimetry system. Method and Materials: The dosimetric evaluation of this system included the measurement of in‐air dose profiles in , 6 MV, and 18 MV beams, and the investigation of the dependence of detector response on beam angle and field size. The stability and accuracy of the positioning component of the RADPOS detector was studied as well as the effect of metals and other commonly used materials on the RADPOS signal. Results: The dose profiles measured with the RADPOS detector and the diode agreed in within 0.41%, 0.53%, and 2.69% for the , 6 MV, and 18 MV beams, respectively. The angular response of the RADPOS probe over 360° was isotropic within 1.6% (1SD). Over a period of seventy minutes, the position of the RADPOS was read every 30 s and found stable within 0.37 mm. The system can also measure the displacement of a RADPOS detector with an accuracy of (0.45 ± 0.07) mm and (0.75 ± 0.07) mm for step sizes up to 50 mm and 200 mm respectively. The only materials that caused significant interference with the RADPOS signal were aluminum, brass, steel and lead. However, once the separation between the detector and the sample was >100 mm, the interference was minimal (the average deviation was less than 1.00 mm for all samples and sizes). Conclusion: Results of the preliminary tests indicate that the device can be used for in‐vivo dosimetry in and high‐energy beams from linear accelerators. Future work will involve technical improvements to the device, experiments in a 4D phantom and finally patient in‐vivo dosimetry.

Acknowledgement: This project has been supported by a grant from HTX‐OCE‐IRAP and Best Medical.
SU‐GG‐T‐280: Dose Characteristics and Applications of Optically Stimulated Luminescent Dosimeters in Proton, Electron and Photon Radiotherapy
Authors: Reft C
Published: July 2008

Purpose: To investigate dosimetric properties of optically stimulated luminescent detectors (OSLDs) and their application to proton, electron and photon radiotherapy relating to their energy dependence, linearity, reproducibility and use in small field measurements. Method and Materials: The OSLDs used are 0.7 cm diameter, 0.02 cm thick plastic disks containing in a light‐tight plastic holder available as InLight/OSL Dot detectors (Landauer, Inc). For the small photon field measurements the sensitive diameter of the detector was reduced to 0.05 cm by making a pin‐hole in the holder. The thermoluminescent detectors were 0.3×0.3 cm2, 0.038 cm thick chips. A micro‐MOSFET dosimeter was used for the small field measurements. A Varian Cl 2100 provided 6 and 18 MV photons and 6–20 MeV electrons. A Philips RT‐250 provided photons from 80–250 kVp. Three proton facilities in the United States provided the proton irradiations. Results: Irradiation of detectors followed by optical bleaching for dose increments of 0.5 Gy from 0.5–4.0 Gy up to 30 Gy showed less than 1% variation in their relative sensitivities. Dose response for the megavoltage photons and electrons was independent of energy while for protons it was 3% less than for 6 MV photons. The response to kilovoltage photons increased inversely with energy from 30–300% relative to 6 MV photons. Supralinearity was observed at doses above 1.5 Gy. Measured output factors, Spc, for 6 MV at 5 cm depth for 0.5×0.5–3.0×3.0 cm2 fields agreed with the other detector measurements. Conclusion: These results demonstrate their potential for applications in megavoltage proton, photon and electron radiotherapy. Their favorable properties include relative energy independence, dose linearity below 1.5 Gy, dose independent relative sensitivity and use in small field dosimetry. Their response variation for kilovoltage photons is due to their higher Z relative to water.
SU‐GG‐T‐284: Modelling the Effect of Optical Pathlength On Light Scattering Artefacts in Optical CT for Polymer Gel Dosimetry
Authors: Bosi S , Brown S , Sarabipour S , De Deene Y , Baldock C
Published: July 2008

Purpose: Dosimetric polymer gels (e.g. PAGAT [1]) undergo a change in opacity on exposure due to polymerisation and cross‐linking. Cone beam optical CT is a simple, inexpensive method to read exposed gels. To ensure accuracy of dosimetry using optical CT, it is necessary to account for the effect of light scattering in a dosimeter as a function of optical pathlength. In this paper we report the results of modelling of experiments using a simulated gel phantom. Method and Materials: As described earlier [2], opacity change in a dosimetric gel was simulated by adding known concentrations of Dettol™ to gelatin. A phantom composed of a cylindrical PETE vessel (9.5 cm diameter, 13 cm high) containing two regions of different turbidity (optical density OD) was produced; an outer, low turbidity region surrounding a funnel‐shaped, high turbidity region (simulating exposure). Different horizontal slices of the funnel phantom represent regions of uniform, high OD but with differing diameters (pathlengths). The phantom was scanned with a Vista Optical CT Scanner (Modus Medical Devices Inc.). Horizontal profile slices were obtained from the reconstruction (resolution; 0.25 mm per voxel), each containing circular slices of the funnel of differing diameter. Results and Discussion: Larger diameter slices of the funnel exhibited dishing artefacts — the central region of the slice showed an apparent reduction in OD, similar to the dishing artefact in X‐ray CT. Lower diameter slices were found to exhibit the opposite — “doming” artefacts where the central region showed an apparent increase in OD. These effects are modelled by taking into account the back‐reflection of scattered light.
SU‐GG‐T‐289: Out‐Of‐Field Dose Equivalents Delivered by Passively Scattered Therapeutic Proton Beams for Clinically Relevant Field Configurations
Authors: Wroe A , Clasie B , Kooy H , Flanz J , Schulte R , Rosenfeld A
Published: July 2008

Purpose: Microdosimetric measurements were performed at the Francis H. Burr Proton Therapy Center at Massachusetts General Hospital to assess the dose equivalent external to passively delivered proton fields for various clinical treatment scenarios. Method and Materials: Treatment fields evaluated included a prostate cancer field, a cranial field for a medulloblastoma patient, a spinal field for a medulloblastoma patient, an ocular melanoma field, and a field for an intracranial stereotactic treatment. Measurements were completed using patient specific bolus and aperture combinations with clinically relevant treatment settings. This work was conducted using a silicon‐on‐insulator microdosimeter that was placed on the surface of and at various depths within a homogeneous plexiglass phantom to assess the dose equivalent and average quality factor as a function of both lateral distance from the treatment field edge and distance past the distal edge of the spread‐out Bragg Peak (SOBP). Results: Dose equivalent values ranged from 3.5–0.12 mSv/Gy (2.5–60 cm lateral displacement) for a typical prostate cancer field, 4.3–0.23 mSv/Gy (2.5–40 cm lateral displacement) for the cranial medulloblastoma field, 1.0–0.23 mSv/Gy (5–20 cm lateral displacement) for the spinal medulloblastoma field, and 0.2–0.03 mSv/Gy (2.5–10 cm lateral displacement) for the ocular melanoma field. Measurements of external field dose equivalent for the stereotactic field case exhibited differences as high as 50% depending on the modality of beam collimation. Average quality factors derived from this work were typically in the region of 2–7, with the value largely dependant on the position within the phantom in relation to the primary beam. Conclusion: This work has allowed for accurate comparison of the external field dose equivalents for various passively scattered proton treatment fields and provides valuable, clinically relevant data.
SU‐GG‐T‐290: A Fast Optical CT Scanner for Gel Dosimetry
Authors: Xu Y , Wuu C , Maryanski M
Published: July 2008

Purpose: To evaluate the performance of a fast optical CT scanner and discuss methods for improving scanning speed in 3D gel dosimetry. Method and Materials: The performance of a fast optical laser CT scanner for 3D gel dosimetry (OCTOPUS‐10X, MGS Research, Madison, CT) is evaluated. This scanner uses three stepper motors to guide a single laser beam to move in three dimensions relative to the gel to be scanned. Fresnel lenses were used to generate parallel‐ or fan‐beam geometry inside the scanning tank. Non‐uniform averaging was applied to the parallel‐beam data to compensate for the non‐uniform linear speed of the laser beam refracted by the first Fresenel lens. Image reconstruction was done using filtered back‐projection functions “iradon” and “ifanbeam” built in Matlab. The geometric accuracy of the scanner in two fast‐scanning modes was tested using test objects. Dosimetric accuracy of the scanner in conjunction with BANG polymer gel dosimeter was tested by comparing dose distributions from the gel dosimeters with those from Eclipse treatment planning system and film dosimetry for several static and dynamic treatment plans. Results: The scanner can acquire a single slice image of 100 pixels ×100 pixels from an irradiated gel in 30 seconds. This is about 20 times faster than the previous model of optical CT scanner based on translational‐rotational motion. The reconstructed images of test objects show the geometric accuracy of the scanner is at sub‐pixel level. The reconstructed dose distributions from the two different scanning modes agree well between themselves and are also in good agreement with the planning system and film dosimetry. Conclusion: Optical CT scanner based on a single laser beam and rotational motion of 3 stepper motors can reproduce complete 3D dose distributions from regular external beam treatment plans in a matter of half an hour.
SU‐GG‐T‐294: A New Cone‐Beam CT Daily QA Phantom for Stereotactic Radiosurgery
Authors: Zhao Z , Duvvuri S , Flampouri S , Horne D , Huh S
Published: July 2008

Purpose: In Stereotactic Radiosurgery (SRS), sub‐milimeter accuracy for patient setup is necessary. We developed a phantom for SRS using CBCT for patient alignment. Method and Materials: The phantom is a semi‐spherical shaped acrylic with the dimension of 11 cm diameter and 12 cm height. It consists of four sections: hollow cone‐shaped top, house‐made large water level, metal target supporter with 15 mm diameter stainless steel ball target and hollow cone offset between radiation and mechanical axis of the CT canner. The bottom of the phantom is attached to a frame that can be placed on treatment table of the linear accelerator. The phantom was scanned by using diagnose CT and exported to the ADAC Pinnacle planning computer. The phantom is then aligned by using lasers and CBCT images are taken. The image correlation was carried by comparing external contours of the phantom itself and two contours of the chamber in the phantom. Results: The geometrical accuracy of the stereotactic radiosurgery system using the CBCT as secondary double check was investigated over 8 different gantry angles and table rotations, and their average uncertainty is approximately 0.5 mm in three major orientations with maximum 0.8mm errors. Four physicists were asked to do image fusion, intra‐user variations were less than 0.25mm. The dosimetric uncertainties of 75% isodose volumes were also evaluated. Between planning computer and film irradiation, the uncertainties were less than 1 mm in three major axis. Conclusion: The CT and CBCT images can be correlated with 0.5mm steps. The daily QA phantom was very effective to investigate any systematic errors including CT images, planning procedure, geometrical uncertainty of CBCT, and gantry flex of the linear accelerator. The unique geometrical features of the phantom provide minimum intra‐user variations with 0.5mm uncertainty, which is comparable to the setup uncertainty of the laser.
SU‐GG‐T‐295: Determination of the Dose Equivalent Near Proton Pencil Beams
Authors: Clasie B , Wroe A , Kooy H , Paganetti H , Flanz J , Schulte R , Rosenfeld A
Published: July 2008

Purpose: To utilize a silicon‐on‐insulator (SOI) microdosimetry technique to determine the out‐of field dose equivalents and average quality factors in proximity to an actively scanned and an analogous passively scattered and shaped prostate cancer proton field. Method and Materials: From measured microdosimetry spectra, we determined the dose equivalent laterally (up to 10 cm from the central axis) and distally (up to 8 cm from the Bragg peak) of a single proton pencil beam. Microdosimetry measurements were also performed for a passively delivered prostate cancer proton field, and out‐of field dose equivalents and quality factors were determined. Using GEANT4.8.0, we modeled the microdosimeter response to the single pencil beam and simulated an actively scanned proton treatment field comparable to the passively scanned field with a range of 29 cm in water. Results: GEANT4 simulations showed that the dose equivalent lateral to the pencil beam was dominated by contributions from scattered primary protons while neutrons were the major contribution to the dose equivalent downstream of the Bragg peak. This explains that the average quality factor was 2 lateral to the pencil beam and 5 downstream of the Bragg peak. The dose equivalent values in the lateral regions of the actively scanned prostate cancer field were within +10% to −30% at analogous positions in the passively scattered field. Distal to the Bragg peak, the quality factor was 7 for the passively scattered field compared to 5 for the actively scanned field, which demonstrates that the delivery method affects the secondary particle spectra in this region. Conclusion: Preliminary results suggest that there are differences in the dose equivalents and quality factors near the treatment field from actively scanned and passively scattered and shaped proton beams. These data highlight interesting directions for further measurements and simulations.
SU‐GG‐T‐296: A Method for Removing An Artifact in Time‐Dependent Dose Rate Data Obtained with An Ionization Chamber
Authors: Watanabe Y
Published: July 2008

Purpose: To develop an algorithm to remove the RC‐like artifact with time‐dependent dose‐rate measurements using an ionization chamber and electrometer system. Method and Materials: A 0.07‐cc Farmer‐type ionization chamber, Capintec Model PR05‐P, was used for dose measurements. The electrometer was Standard Imaging MAX‐4000, which was connected to a laptop computer. Electrometer control software, a special version of the MAXcomm program, was used to sample the electric current readings every 100 msec. We formulated a rate equation for the number of electrons in the cavity by hypothesizing that the RC‐like artifact is caused by the finite time needed for electrons to recombine with ions in the cavity of the ionization chamber. The rate equation includes terms for the ionization, the loss of electrons due to two‐body recombination, and the loss of electrons due to the electrons flowing out from the chamber cavity to the electrometer. The unknown parameters in the equation were obtained by iteratively adjusting the values until we achieved the best fit of the corrected ionization current data with the ideal time‐dependent ionization current curve. For the parameter estimation, we used the data of one 100MU irradiation of a linac photon beam. Next, the correction algorithm with the estimated parameter values was applied to the time‐dependent dose‐rate data obtained for a test case of Gamma Knife irradiation. Results: By iteratively adjusting the values of the two parameters in the rate equation, we could recover the ideal rectangular shaped beam pulse of the linac photon beam. Using these parameters, the temporal variation of the ionization current for the Gamma Knife experiment was successfully corrected by removing the RC‐like artifact. Conclusion: The proposed method is capable to eliminate the artifact in the time‐dependent dose‐rate measurements.
SU‐GG‐T‐301: Evaluation of An Ion Chamber Array for Volume Modulated Radiation Therapy and Helical Tomotherapy
Authors: Neeley M , Ramsey C , Charara Y
Published: July 2008

Purpose: The primary objective of this study was to determine if the MatriXX could be used as a substitute for radiographic film in rotational IMRT delivery techniques. Method and Materials: Twenty random patients treated with helical tomotherapy were selected for this comparative analysis. The tomotherapy and VMAT QA plans were created in the respective planning systems by copying the patient's plan to the CT images of the MatriXX and re‐computing the dose. Horizontal profiles, vertical profiles, Gamma pass/fail analysis, and Gamma histograms were calculated for each test case. For the Gamma calculations, the threshold parameters were set to a 3% dose difference and a 3‐mm distance‐to‐agreement. The analysis region of interest was defined as the area of the film receiving over 10% of the prescribed dose. Results: A total of twenty helical tomotherapy and volume modulated arc test cases have been measured and evaluated. From the gamma analysis, the percent of pixels exceeding the threshold with a 3% dose and 3‐mm distance‐to‐agreement criteria was 10.8 ± 6.9% for the film verses 23.4 ± 14.7% for the MatriXX. The MatriXX appears to function well in high‐dose low‐gradient regions and low‐dose low‐gradient regions. However, the MatriXX has difficulty in regions with steep dose gradients. This is likely due to 1.) Volume averaging across the 4.5 (Ø) × 5 (h) mm chambers, and 2.) the coarse 7.62 mm center‐to‐center spacing of the chambers. Conclusion: Based on the analysis of the MatriXX data, half of the patients in this study would not have passed our institution's patient‐specific IMRT QA testing. In contrast, only one patient would not have passed for measurements taken with film. As with all array devices, the MatriXX is limited in its spatial resolution. It is difficult to adequatly verify plans with steep dose gradients using the MatriXX.
SU‐GG‐T‐310: Detour Factors for Low‐Energy Electrons in Water
Authors: Wiklund K , Lind BK
Published: July 2008

Purpose: The aim of this study is to calculate the average penetration depth, zav, and detour factors of 10–10 000 eV electrons in water using an in‐house Monte Carlo track structure code. These low electrons are known to be the most toxic in sense of radiation damage and the calculation of their true depth of penetration and energy depositions is required in areas such as radiation therapy, to be able to calculate the correlation between ion tracks and the potential cell damage. Method and Materials: The CSDA‐range largely overestimates the range of electrons of energies between 10–10000 eV because of the high amount of elastic scattering. Up to 50% of the interactions can be elastic for energies below 100 eV. This phenomena will shorten the longitudinal distribution, average penetration depth zav, but @@ elongate the transverse distribution . Several definition of the detour factor can be found in the literature. In this work is the detour factor defined as in ICRU (1993): , were ro is the CSDA‐range. The zav is computed according to: , were N is the number of histories and zabs is the depth of absorption of every primary electron. ro is calculated by integrating the stopping power, which is extracted from the MC track structure code. Results: The calculated zav is shorter for low energies compared to Wilson et al(2004) and slightly higher for energies around 10 keV. zav is substantially shorter than the calculated CSDA‐range for energies below 1 keV, which results in a small Detour factor. The sigmoid shape of the Detour curve decompose at very low energies around 10 eV due to a drastically reduction of the ionization cross section and a still increasing elastic cross section. Conclusion: The results show that the CSDA‐range should be used with caution at these low energies.
SU‐GG‐T‐318: Validation of PENELOPE Code for X‐Ray Photons Transport with Energy Between 20 and 150 KeV in Thin Geometry
Authors: Pianoschi T , Nicolucci P , Góes E , Pelá C
Published: July 2008

Purpose: PENEPOLE is a general Monte Carlos package for the simulation of coupled electron‐photon transport. In this study, the simulation input parameters that control the transport algorithm have been optimized so that PENELOPE could properly describe X‐rays interactions with thin materials in the energy range used in diagnostic radiology. Method and Materials: The PENELOPE code was validated through the simulation of the linear attenuation coefficient (μ) considering thin geometries and energies between 20 and 150 keV. The parameters associated with: threshold energy for local absorption (EABS); angular distribution (C1) and loss energy (C2) of charged particles and energy loss in collision (WCC) and radiation emission (WCR). The validation was performed by comparison of the simulated values and those presented in literature. Results: The differences between the values of μ obtained in this study and those presented in the literature were less than 1% for all the combinations of input parameters and in the whole range of energy used. It was found that the combination of input parameters has more influence on the results in the energy of 150 keV. For the range of energy considered in this study, the values of simulated μ were superior to those presented in the literature, except for the energy of 80 keV, for the acrylic, and 20 keV, for the aluminum. Conclusion: The value that should to be selected for each input parameter depends on the thickness and atomic number of the material and energy of the X‐ray photons. Considering that variations of input parameters did not significantly changed the values of μ, as compared to those provided by literature, we concluded that the code PENELOPE correctly simulates the transport of particles in thin geometries and low X‐rays energies, validating its use on studies of construction characteristics of ionization chambers used in diagnostic radiology.
SU‐GG‐T‐322: Determination of the Optimum Statistical Uncertainty and Voxel Size to Perform Monte Carlo Based Dose Calculation in the Context of Radiotherapy
Authors: Isambert A , Morrow S , Husson F , Salvat F , Lefkopoulos D
Published: July 2008

Purpose: To evaluate the influence of statistical uncertainty and voxel size on target dose volume histograms (DVH) calculated with a Monte Carlo based treatment planning system (TPS). To determine the maximum uncertainty level and voxel size acceptable in the context of external beam radiotherapy. Method and Materials: First, the effect of increasing voxel sizes was investigated by studying the display of material and mass density matrices related to patient CT images. Then, the dose calculations were performed using the TPS ISOgray™ (Dosisoft) based on a fast Monte Carlo simulation code called PENFAST. The dose deposition due to electron beams was calculated for one breast plan. Intramammary node DVH were calculated first for increasing statistical uncertainty (1σ) levels of 0.7%, 1%, 1.3%, 2%, 3% and 6%, and secondly by increasing voxel sizes from 2×4×2 mm3 to 8×8×8 mm3. The same study was conducted for one medulloblastoma radiotherapy plan. Spinal cord DVH were calculated for uncertainty levels (1σ) of 1.3%, 1.7%, 2%, 3%, 5%, 12% and voxel sizes from 2.1×3×2.1 mm3 to 5.7×6×5.7 mm3. Results: The consequence of increasing voxel sizes is the blurring of material and mass density matrices by averaging values contained in neighbouring voxels which must be avoided at the interface between heterogeneities. The study of the DVH for both breast and medulloblastoma plans showed that they were not modified for statistical uncertainty levels (1σ) below 2% and voxel sizes below 4×4×4 mm3. Conclusion: This work indicates that, in clinical context, to save computation time while keeping the reliability of the target DVH for clinical use, Monte Carlo based dose calculation can be performed with uncertainty level (1σ) below 2% and voxel size which must not be greater than 4×4×4 mm3.

This work was performed in the framework of the Project MAESTRO (IP CE503564) granted by the European Commission.
SU‐GG‐T‐325: Improved Organ Sparing by Electronic Brachytherapy: A Monte Carlo Study
Authors: Dingley J , Taranenko V
Published: July 2008

Purpose: To compare the absorbed dose delivered per photon to the lungs, heart wall, and breast by irradiation of a breast tumor via a novel electronic x‐ray brachytherapy source to that delivered by a standard seed. Method and Materials: X‐ray spectra for 20–50 keV monoenergetic electrons, in 5 keV increments, were generated using the GEANT4 Monte Carlo code and realistic apparatus geometry. These spectra were then used as simulated point sources located 2 cm beneath the surface of a breast of a female voxelized phantom, generating 108 photons. The emission spectra of was used to simulate a point source in the same location for comparison. Results: An average decrease by a factor of 1.75 in dose to breast was revealed for when compared to . Dose to the lungs showed a decrease by factors of 2 × 103 and 11 at 20 and 50 keV electron energy, respectively, while the heart wall exhibited a decrease varying by factors of 1.5 × 104 to 17.6 for the same energies. Conclusion: The lower dose (factor of 1.75 on average)delivered to localized tissue by a x‐ray brachytherapy source is more than compensated by a substantial decrease of dose (up to a factor of 1.5 × 104) delivered to vital organs adjacent to the treatment area. Due to the softer specta a novel electronic x‐ray brachytherapy source spares better the healthy tissues.
SU‐GG‐T‐343: Comparison of MCNP and GEANT4 Monte Carlo Codes On Photo‐Neutron Generation in High Energy X‐Ray Beams
Authors: Jiang H
Published: July 2008

Purpose: Photo‐neutrons generated in high‐energy X‐ray beams are of increasing interest. Secondary neutrons introduce concerns such as neutron shielding and additional dose to patients. The Monte Carlo method is an appropriate choice for studying photo‐neutron production and transport. However, unlike interactions of photons with orbital electrons, photonuclear reactions suffer from the incompleteness and inconsistence of the cross section data. The purpose of this investigation is to compare two major Monte Carlo codes MCNP and GEANT4 in implementing the photonuclear process. Method and Materials: Cross section data (total photonuclear cross section, neutron production yield, and neutron energy/angular distribution data) were extracted from the respective external cross section files (MCNP) or from the code (GEANT4) directly for the major materials/elements in linear accelerator and human body. The Varian linear accelerator with multi‐leaf collimator (MLC) was modeled identically with MCNP and GEANT4. Neutron generation in major components in the linear accelerator was simulated and compared. Results: MCNP and GEANT4 implemented the photonuclear process in very different manners. Significant differences in cross section data between MCNP and GEANT4 were discovered. Several institutions provided the cross section data used in MCNP, and the cross section data for the same isotope from different providers may also present large discrepancies. The results of photo‐neutron production in linear accelerator components simulated with the two Monte Carlo codes can be quite different. Although neutrons were not transported in our simulations, appreciable differences in neutron fluence or neutron dose can be expected. Conclusion: MCNP and GEANT4 are two major Monte Carlo codes, capable of simulating photon‐neutron generation and transport. The discrepancies between the two codes indicate the lack of comprehensive understanding in physics of the photonuclear process. The investigators who worked with either code should be aware of the potential uncertainties in their results regarding to secondary photo‐neutrons.
SU‐GG‐T‐350: X‐Ray Beam Model Benchmarks
Authors: Sawkey D , Schreiber E , Faddegon B
Published: July 2008

Purpose: Determine fluence, differential in energy and angle, of Siemens Oncor 6/18 MV x‐ray beams, a necessary step towards an accurate, easily‐commissionable beam model for Monte Carlo treatment planning. Method and Materials: Dose to water is measured under various conditions, including: various field sizes including the largest; flattening filter present and removed; energy, steering tuned from nominal values to outside manufacturer acceptance criteria; target position moved along beam axis; filters. Energy is determined from depth dose curve measured with no target, flattening filter, or primary scattering foil (in electron mode, using x‐ray beam parameters). Spot size is determined using an alternating layer spot size camera. Spot position, beam angle, primary collimator and flattening filter positions are determined geometrically from profile scans vs. distance. Positions of treatment head elements, density of flattener, verified by direct measurement. Monte Carlo simulations with EGSnrc, taking asymmetry into account with beam angle and position, modified BEAMnrc code allowing lateral shifts of component modules, and modified ‘flatfilt’ component module allowing shift of flattening filter relative to primary collimator. Results: Comparison of electron PDD to Monte Carlo yields the beam energy to within positional accuracy and systematic uncertainty of Roos detector. Precision of geometrical measurements is sufficiently high to constrain inputs. Simulations agree to measurement within 1%/1 mm. Fluence is determined accurately over range of clinically acceptable beams. Conclusion: We determine, with reliable uncertainty estimates, input parameters for Monte Carlo simulation of Siemens 6/18 MV x‐ray beams. Close agreement is obtained to measured data over range of beam tuning. The method is applicable to other types of linacs and provides fluence benchmarks of clinical x‐ray beams for development of an accurate, easily‐commissioned beam model.

Support from NIH R01 CA104777‐01A2.