bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐272: Measurement of Leakage Radiation From Fixed Electron Applicators On Siemens Primus Accelerators
Authors: Yeboah C , Karotki A , Hunt D , Holly R
Published: April 2010

Purpose: To characterize leakage radiation from fixed electron applicators on Siemens Primus accelerators and to ascertain whether placement of bolus and/or lead rubber outside the field on a patient's skin reduces dose to the skin from leakage radiation. Method and Materials: First, measurement of leakage from electron applicators (10×10–25×25 cm2) on a Primus unit was performed as a function of vertical position along and lateral distance from applicator body using XV films and cylindrical ion chamber with 1 cm build‐up cap. Second, using a parallel‐plate chamber inserted at the surface of a “solid water” phantom, the leakage at 4 cm from applicator body was measured after “solid water” slabs of thicknesses 0–3 cm were placed between detector surface and applicator sidewall. Finally, to determine the thickness of lead rubber needed for shielding leakage radiation on a patient's skin, transmission of leakage from 20×20 cm2 applicator through various thicknesses (0–6 mm) of lead rubber was measured. Results: Maximum leakage observed at 2 cm from applicator body at the front and right sides of applicator were 16% and 14%, respectively; these maxima were recorded for 18 MeV electron beams and applicator sizes of ⩾20×20 cm2. At lateral distances of ⩾4cm and ⩾10cm from applicator body, the maximum leakage observed was <10% and ⩽5%, respectively, for each energy and applicator size combination. In addition, introduction of water‐equivalent material of thicknesses 1–3 cm or sheets of lead rubber of thicknesses 1.5–6 mm between the detector‐phantom surface and applicator sidewall for 6–18 MeV electron beams, respectively, attenuated the leakage radiation by ⩾75%. Conclusions: The highest average leakage observed at a distance of 2 cm from applicator body was 15%; this value exceeds the IEC 60601‐2‐1 recommended limit of 10%. Dose to skin from leakage can be minimized by shielding with bolus or lead rubber.
SU‐FF‐T‐273: Energy Dependence of TLD‐100 Chips and Microcubes for the Xoft Axxent® 50 KV Miniature X‐Ray Source and Cs‐137 Relative to Co‐60
Authors: Pike T , Raffi J , Davis S , DeWerd L
Published: April 2010

Purpose: To use experimental methods to measure the overall energy dependence of LiF:Mg,Ti thermoluminescent dosimeters (TLDs) for the Xoft Axxent® miniature x‐ray source (mean energy ∼27 keV) and 137Cs (662 keV) relative to 60Co (1250 keV), to calculate the absorbed‐dose energy dependence with the Monte Carlo (MC) code MCNP5, and to determine the intrinsic energy dependence of the TLDs by dividing the measured overall energy dependence by the absorbed‐dose energy dependence. Method and Materials: TLDs were irradiated to a known air kerma with all three sources. The air kerma rate of the miniature x‐ray source was measured using an Attix free‐air chamber. Each irradiation was performed for TLDs of two different sizes, microcubes (1 mm3) and chips (3×3×1 mm3). MC simulations were performed for each setup to calculate dose to the TLDs including photon scatter from the holders. MC methods were also used to calculate the air kerma. Results: The overall measured energy response was greater than the MC calculated absorbed‐dose energy dependence for both sources and both chip sizes. The difference between the measured and calculated energy dependence for the miniature x‐ray source was greater than for 137Cs, which means that the intrinsic energy dependence of LiF:Mg,Ti TLD chips and microcubes relative to 60Co is more pronounced at lower energies. Conclusion: This experiment provides further evidence that TLDs have intrinsic energy dependence in addition to the absorbed‐dose energy dependence that can be calculated using Monte Carlo methods. MC corrections alone are not able to calculate the overall energy dependence of TLDs. Therefore, appropriate energy dependence corrections must be measured and applied to TLD measurements when calibration irradiations are performed at a different energy.

Conflict of Interest: This research was partially sponsored by Xoft, Inc.
SU‐FF‐T‐277: Measuring High Energy Neutrons at a 230 MeV Proton Therapy Facility
Authors: Nazaryan V , Keppel C
Published: April 2010

Purpose: Examine neutron field spectrum and angular distribution characteristic to medium‐energy (230 MeV) proton accelerator facilities. Measure effectively the high energy neutron component of the radiation field. Method and Materials: Primary shielding walls in modern proton therapy facilities are typically made of an over 7′ thick of ordinary (2.3 g/cm3) concrete. Our analysis demonstrate that behind such a thick wall in forward direction with respect to the proton beam neutrons with energies greater than 8 MeV still contribute considerably to the total dose. High energy neutron contribution in lateral direction is potentially even larger. Traditionally neutron rem‐meters are designed to have their response function match well an appropriate (ICRP 1990, or NCRP‐38) fluence‐to‐dose conversion function over an energy range extending from thermal (0.025 ev) to 10 MeV. We have performed over thirty measurements of the neutron dose equivalent at various locations in and around a state‐of‐the‐art 230 MeV proton therapy facility using the Wide Energy Neutron Detection Instrument (WENDI) that has a useful energy response in the energy range from thermal to 5 GeV. Results: We have obtained neutron attenuation lengths in forward and lateral directions from our measurements most appropriate for use at 230 MeV proton therapy facilities (PTFs) and compared our results with some previously published values. We have also obtained a new parameterization for neutron attenuation in the maze suitable for modern PTFs. Conclusion: In surveying and area monitoring modern PTFs the neutron detector of choice must be capable of detecting with sufficient efficiency the high energy component of the neutron field to avoid large dose underestimation in an environment where greater than 8 MeV neutrons may have a significant contribution. New maze attenuation parameterization will provide for adequate maze design in these facilities, where previously used attenuation models provided at best for a marginal maze design.
SU‐FF‐T‐282: Development of a Multi Electrode Well Ionization Chamber to Be Used in Nuclear Medicine
Authors: Takeda F , Santos M , Vieria J , Calvo W , Netto T Ghilardi , Nicolucci P
Published: April 2010

Purpose: Development and perform preliminary test of a multi electrode well ionization chamber to be used in Nuclear Medicine. Method and Materials: The well ionization chamber wall was constructed in stainless stell (ANSI 304). The well has 72mm‐diameter and 238mm‐height with a wall thickness of 0.5mm. The external dimensions are 168mm‐diameter and 300mm‐height. The sensitive volume is filled with high pure argon (0.3 Mpa). The electrodes are made with brass and have a multi‐layer paralel configuration allowing low voltage saturation. Sources of 57Co (∼123MBq), 133Ba (∼8MBq) e 137Cs (∼7MBq) were used to study the variation of response with pressure of argon. 99Tc (∼1GBq) and 131I (1GBq) sources were used to study response saturation with polarizing voltage (10 to 500V). Sensitivity and linearity of response with activity was studied for 99Tc (60MBq to 7GBq) and 131I (220MBq to 4GB). Results: The tests performed with 57Co, 133Ba and 137Cs sources showed that a voltage of 100V already saturates the chamber response. Similar result was also found when 99Tc and 131I sources were used. The chamber response is linear (r = 0.999) as a function of activity for the sources and range of activities used. The response repetitivity was better than 99.6% for the activities measured for the 99Tc and better than 95.2% for the 131I source. The chamber response in A/MBq was found to be doubled for 131I when compared to the 99Tc source. Also the relative sensitivity for the 131I source was found to be 1.56% the relative sensitivity for the 99Tc source. Conclusion: The preliminary tests performed show that the response of the developed well ionization chamber is adequate to allow its use in Nuclear Medicine. Aditional tests have still to be performed and Monte Carlo simulation will be used to study construction parameters to optimize its response.
SU‐FF‐T‐287: Characterization of In‐Air Anisotropy of Brachytherapy Sources Using Clinical Ionization Chambers
Authors: Mitch M , Tong L , Seltzer S
Published: April 2010

Purpose: To determine the feasibility of using well, spherical cavity, and cylindrical cavity ionization chambers to characterize the in‐air anisotropy of low‐energy photon‐emitting brachytherapy sources. Method and Materials: Anisotropy in the plane of the long axis of I‐125 and Pd‐103 brachytherapy sources was characterized by calculating air‐kerma rate as a function of rotation angle from emergent spectra measured with a high‐purity germanium spectrometer. This measurement is a critical component of the seed calibration program at the National Institute of Standards and Technology (NIST), used to identify anomalous sources and explain variations in well‐chamber response relative to the U.S. air‐kerma strength standard due to anisotropy variations. To investigate whether such a measurement was possible using instrumentation commonly found in therapy clinics, the responses of three different types of ionization chambers were measured as the source was rotated by 360 degrees in small increments. Initially, ionization chamber current was measured as a function of distance from the source to determine the optimum position for the anisotropy measurements, a compromise between signal strength and angular resolution. The well‐ionization chamber was placed on its side with the centering jig removed. Results: All three ionization chambers were able to reproduce the characteristic shape and features of the in‐air anisotropy for a given model source as measured by the spectrometer, albeit with slightly lower angular resolution. Using an ionization chamber results in significant time savings, as a detailed source anisotropy characterization can be completed in 1.5 days as opposed to 2.5 days with the spectrometer. Conclusion: Measurement of the in‐air anisotropy of brachytherapy sources can be accomplished using ionization chambers of the types used in clinics, including well‐ionization chambers. This technique could serve as an additional quality assurance check on seeds prior to their being used for dosimetry measurements critical for accurate treatment planning.
SU‐FF‐T‐290: A Practical Method of Composition Ratio QA for 103Pd and 125I Hybrid Seeds: Derivation and Monte Carlo Verification
Authors: Zhang G , Saini A , Biagioli M , Forster K
Published: April 2010

Purpose: In prostate cancer treatment, Brachytherapy using radioactive seeds of 125I or 103Pd is a common practice. Studies using the concept of BED have shown that it would be beneficial to treat prostate cancer with mixture of radioactive seeds of isotopes with different half‐lives. Commercially, the technology to manufacture multi‐isotope seeds has been developed by IsoAid LLC (IsoAid, Port Richey, FL). To be used clinically, a method of QA regarding the composition ratios of the hybrid sources must be developed in addition to other physical quantities QA. Method and Materials: The difference in energy spectra between 125I and 103Pd determines that the attenuation of the two isotope components is different when the γ‐rays passing through the same material. Thus for hybrid sources, the attenuation magnitude through a certain thickness of a certain material depends on the composition ratio. Simple methods were derived based on attenuation measurements using a cylindrical chamber or well chamber. Monte Carlo simulations were performed to verify the methods. Doses at 1 cm depth in water were calculated with and without attenuation materials for pure 125I and 103Pd sources and hybrid sources of different composition ratios. Different measurement settings using a well chamber or cylindrical chamber were simulated. The energy spectra of the radioisotopes that were used in TG‐43 were used in the Monte Carlo calculations. Results: Monte Carlo simulations verified that the method based on attenuation measurement is accurate and practical. Corrections for measurement depth variation and measurement time are discussed. If the measurement depth is 0.5 mm off, the error of the composition ratio can be up to 1.6%. The advantages/disadvantages of different measurement techniques using a cylindrical chamber or well chamber are presented. Conclusions: composition ratios of hybrid seeds can be determined by attenuation measurement method presented in this work.
SU‐FF‐T‐292: An Efficient Method for Image Quality Analysis of the On‐Board Imager®
Authors: Atkins R , Jones A
Published: April 2010

Purpose: To develop a quality assurance procedure to test image quality of a kilovoltage On‐board Imager system (On‐Board Imager®, Varian Medical Systems, Inc., Palo Alto, CA). Method and Materials: An imaging phantom (TOR 18FG, Leeds Test Objects, Ltd.) was used to analyze kV imaging quality. The phantom includes 21 line pairs (0.5 – 5.0 lp/mm) and 18 low contrast discs. An image of the phantom was taken weekly using 70kV, 40mA, and 100ms. The images were analyzed using the ImageJ software package (U.S. National Institute of Health, Bethesda, MD). A macro detects the circular edge of the phantom and performs a least squares circle fit to detect any translational shift in the phantom. ROIs are automatically selected and mean and standard deviation are measured. An interface to our in‐house quality assurance database was developed. The database calls the macro, receives the measurement results and calculates noise, contrast scale and MTF of the image. Low contrast is resolved by taking the difference between the mean of the low contrast discs and areas adjacent to them. Visibility is determined by a minimum difference. The database stores analyzed data and graphs results over time. Results: Images were acquired between October 2008 and February 2009. Images were exported from the R&V system and were typically analyzed in under a minute. The software was tested by introducing additional specific noise to the images using ImageJ. Using the software, we were able to track MTF, contrast scale, and noise over time and confirm the imaging quality of kV imager. Conclusions: This work demonstrates an efficient way to perform weekly quality assurance for on‐board kV imagers.
SU‐FF‐T‐293: Quantitative Determination of the Alignment of Accelerator Light and Radiation Fields Using Optical Methods
Authors: Vicic M , Ohrt J , LaNeave S , Tailor R , Prado K
Published: April 2010

Purpose: The evaluation of the coincidence of light vs. radiation fields is an important and mandatory component of linear accelerator QA. Currently, the fields' alignment is often only coarsely estimated visually. We describe a new methodology that enables actual quantification of the differences between the light and radiation fields of accelerators. Method and Materials: A fluorescent screen was positioned at a 100 cm SSD reference plane of a Varian 21EX accelerator. The screen was imaged during irradiation with a consumer grade 8.0 Mega pixel camera mounted on a tripod next to the accelerator's couch. A computer generated grid with 1 cm spacing printed on a high‐resolution printer was used for system calibration. Three series of images were taken: (i) a 10×10 light field projected on the screen, (ii) the corresponding radiation field as defined by the screen's fluorescence and (iii) the calibration grid placed on top of the screen. The images were registered with the ideal grid using a projective transformation algorithm. Results: The half height positions of the penumbrae for the light and radiation fields respectively were obtained by analyzing 1D profiles of the images. With the current setup, it is possible to determine fields' mismatch at the order of 0.05 mm. Conclusion: Traditional methods for checking of the field alignment such as pin‐pricking radiological film are somewhat subjective as they greatly depend on the operator's skills. We have developed a novel method that enables accurate quantitative determination of the fields' overlap. The method utilizes inexpensive off‐the‐shelf components and produces results an order of magnitude more accurate. We believe that use of this quantitative and precise optical methodology could be expanded into other areas of accelerator mechanical QA.
SU‐FF‐T‐297: Total Electron Therapy Calibration with TG‐51
Authors: MacKenzie J , Johnson M
Published: April 2010

Purpose: To determine if slight modifications and assumptions can be made to allow for a TG‐51 based absolute dose calibration for a Total Skin Electron (TSE) Therapy program. We show the results of a TG‐51 based absolute dose calibration for TSE Therapy which are compared against three other dose calculation methods. Method and Materials: A TG‐51 calibrated Markus parallel‐plate chamber was used for our absolute dose calculation. This chamber was also used to determine the best gantry angles to produce dose uniformity and to determine the percent depth dose for the six dual field electron beams. Full rotation uniformity was analyzed with thermo luminescent dosimeters (TLDs') and clinical rotation of an anthropomorphic phantom to six different positions (Stanford technique.) Results: The absolute dose calibration of this TSE program was completed with slight modifications of a standard TG‐51 calculation which included extrapolation of the K′R50 value to 1.05 +/− 0.01 and extended target to skin distance (TSD) of 361cm. Verification measurements were done with TLDs, One‐Dose dosimeters, and a TG‐21 calibrated parallel‐plate. All four systems showed agreement within a maximum difference of 3.5%. The agreement between the TG‐21 method and the TG‐51 method was 2.3 %. Conclusion: We do not claim that our TSE absolute calibration is as accurate as a conventional 100 TSD therapy electron beam calibration done with all TG‐51 recommended parameters satisfied. We estimate an additional 2% error on the potential absolute dose calculation accuracy of a TG‐51 measurement. However, with the high degree of uncertainties involved in TSE therapy, our TSE calibration method offers a reasonably accurate absolute dose calibration involving only a few modifications and estimates and allows for the use of commonly used TG‐51 calibrated chambers.
SU‐FF‐T‐301: Clinical Implementation and Commission of Volumetric Modulated Arc Therapy
Authors: Wu QJ , McMahon R , Chang Z , Zhuang T , O'Daniel J , Yoo S , Zhu X , Yin F
Published: April 2010

Purpose Volumetric modulated arc therapy (VMAT) is a rotational IMRT modality that combines dose rate modulation with gantry and MLC motion, thereby increasing the complexity of IMRT delivery. Effective implementation of this technology is still quite new to radiation therapy clinics. This study reports our clinical experience for acceptance testing, commissioning, and routine QA procedure development. Materials and Methods VMAT was implemented on a Varian 21EX™ with a Millennium120 MLC and a Novalis Tx™ with a 2.5mm High‐Definition MLC. The Eclipse™ treatment planning system was used with the associated ARIA record‐and‐verify system. Clinical implementation involves (1) measurements of linac parameters and performance such as MLC leaf positioning and motion accuracy, VMAT dosimetric accuracy, and treatment safety checks; (2) planning algorithms and parameters; (3) patient‐specific QA studies; and (4) routine QA baselines and procedures. Results The difference between planned and measured MLC leaf positions at stationary gantry positions and during VMAT delivery were < 1mm, as measured with film and EPID. The dosimetric accuracy of VMAT delivery was tested with different combinations of gantry speed, dose rate, and MLC motion. Measured variations between these deliveries were < 2%. Guidelines for safe couch positions and placement of immobilization devices were established for both CBCT and VMAT isocenters. The Anisotropic Analytical Algorithm (AAA) dose calculation algorithm was commissioned, with measured point‐dose accuracy of better than 2% at central axis and 5% off‐axis. Patient specific QA data showed that, for prostate and head‐neck cases, point dose accuracy was within 2.5%. Gamma agreements (3mm/3%) > 90% were observed for planar dose distributions measured with an ion chamber array and film. Conclusion A methodology for clinical implementation of VMAT is developed. A critical component for implementation is the design of acceptance tests and commissioning protocols which can be used as the baseline for routine QA procedures.
SU‐FF‐T‐304: Calculation of the Cavity Correction Factor, Pcav, for Cylindrical Ionization Chambers in Clinical Electron Beams by Monte Carlo Simulation
Authors: Araki F
Published: April 2010

Purpose: To calculate the cavity correction factor, Pcav, for the cylindrical ionization chambers in clinical electron beams by Monte Carlo simulation. Methods and materials: The Pcav values for cylindrical chambers were calculated with the EGSnrc C++ user code cavity for 6, 9. 12, 15, and 18 MeV electron beams. The air cavity for cylindrical chambers was replaced with “low density water (LDW)” material. LDW is an artificial material that has all the dosimetric properties of nominal water except its density is equal to that of air. The dose to water was calculated with a 0.1 mm thick slab at a reference depth, dref. The point of measurement for the LDW cavity was taken to be 0.5r (r is the radius of the chamber's cavity) deeper than dref. Pcav was calculated for the cavities with diameters of 7 mm, 6 mm, 5 mm, 4 mm, 3 mm, and 2 mm and lengths of 20 mm, 10 mm, and 5 mm. The calculated Pcav values were compared with those from IAEA TRS‐398. Results: Pcav values calculated for cylindrical chambers at dref in clinical electron beams were typically higher than those of TRS‐398, which are based on experimental data of Johansson et al. For a Farmer chamber with the cavity of diameter 6 mm and length 20 mm, calculated Pcav values were higher from 2% to 0.8% than the TRS‐398's data, in a range of 6 MeV to 18 MeV. The dependence on the cavity lengths for Pcav was found at grater or equal to a diameter of 5 mm, but it was not sufficient at diameters of 4 mm and 3 mm. Conclusions: It was found that the calculated Pcav values of cylindrical chambers in electron beams are sufficiently different from the TRS‐398 data.
SU‐FF‐T‐315: Determination of the Energy Correction Factor for TLD‐100 in Electron Beams Relative to 60Co
Authors: Junell S , Kunugi K , DeWerd L
Published: April 2010

Purpose: Thermoluminescent dosimeters (TLDs) are used to measure absorbed dose to water by calibrating them to a known dose from , a NIST traceable standard. Energy correction factors were measured that enables the conversion of TLD measured absorbed dose from to absorbed dose from 6, 9, 12, 15, and 18 MeV electron beams. Method and Materials: Energy correction factors for LiF TLD‐100 (LiF:Mg, Ti) have been determined experimentally as the ratio of thermoluminescent response in electron beams from a linac over the response from . Annealed TLD‐100 chips (3 mm × 3 mm × 1mm) were irradiated using and 6, 9, 12, 15, and 18 MeV electron beams from a linac. TLD irradiations were carried out in liquid water with water tight Virtual Water™ TLD holders. Using Ion chamber measurements following the AAPM's TG‐51 protocol, doses delivered to the TLDs for both and linac irradiations were verified. Results: The experiments showed a average energy correction factor of 0.95 for TLD‐100 chips for 6, 9, 12, 15, and 18 MeV electron beams relative to . TLD's measured the energy correction factors for electron beams with good precision in a single experiment, with the standard deviation of the mean for the energy correction factors found in each experiment ranging from 0.3% to 0.5%. However, the TLDs were found to be less precise across multiple experiments. The standard deviation of the energy correction factor from experiment to experiment ranged from 2% to 5%. Conclusions: The ability to verify and maintain strict quality control is a fundamental necessity in a radiation treatment facility. External secondary audits are an essential part of a quality assurance program. Energy correction factors allow for TLD's to provide an inexpensive and convenient method to perform external secondary audits of high energy electron beam absorbed dose calibrations.
SU‐FF‐T‐322: Geometric Calibration of a Dual Cone Beam CT System with Large Flat Panel Detectors
Authors: Li H , Bowsher J , Giles W , Roper J , Li T , Yin F
Published: April 2010

Purpose: To assess errors in a simple visual alignment of a dual large‐flat‐panel, cone‐beam‐CT (CBCT) system, and to obtain misalignment parameters that can be incorporated into image reconstruction for accurate target localization in image‐guided radiation therapy (IGRT). Method and Materials: A dual CBCT system with large flat panels has been developed. Two tube‐detector sets are put in the orthogonal position on one optical bench. The designed source‐to‐detector distances (SDDs) and source‐to‐isocenter distances (SIDs) are 150 cm, and 100 cm, respectively. The detectors are 40 × 30cm 194μm‐pixel Varian Paxscan 4030CB flat panels. A ball bearing phantom was scanned with the system. The centroids of the ball bearing projections were measured on both detectors, and geometrical misalignments were calculated firstly for each tube/detector subsystem. Then misalignments between the two subsystems were determined. Results: The effects of out‐of‐plane tilt and skew on the image quality are very limited and have been ignored in our calibration. Calculated in‐plane rotations for both detectors were found to be small. Horizontal and vertical shifts off the detector center were 0.06 and 0.02 cm for subsystem I, and 0.2 and 0.02 cm for subsystem II, respectively. Calculated SDDs and SIDs differed from designed values by 0.5 and 0.2 cm for subsystem I, and 0.2 and 0.5 cm for subsystem II. The vertical shift between the two subsystems was 0.12 cm, and the angle between them was 87.6 degrees. Conclusion: The measurements show misalignments that are sufficient to warrant further mechanical adjustment and/or modeling of misalignment within image reconstruction. Notably the more substantial misalignments include those between the two subsystems.

Conflict of Interest: This work is supported in part by Varian Medical Systems and Carestream Health Inc..
SU‐FF‐T‐335: Dose Uncertainty Due to Motion‐Induced Depth Changes in Energy‐Stacked Proton Beams
Authors: Slopsema R
Published: April 2010

Purpose: To measure the dose uncertainty due to motion‐induced depth changes in energy‐stacked proton fields. Method and Materials: In energy‐stacked proton beams the spread‐out Bragg peak (SOBP) is generated by delivering the energy layers sequentially. In the presence of motion the depth of a point can change between the different energy layers, generating uncertainty compared to the planned dose. The magnitude of the uncertainty depends on the time‐structure of the dose delivery, and the frequency and amplitude of the depth variations. We add wax strips to the outside of a 4D Dynamic Thorax Phantom (CIRS). The strips are 1.5cm wide and 0.5 or 1.0cm thick. A cc04 ionization chamber (IBA Dosimetry) is moved perpendicularly behind the strips, at various amplitude and frequency. The proton field is delivered using both energy‐stacking and ‘quasi‐instantaneous’ for comparison. For each combination of motion and field delivery the dose is measured ten times to determine the statistical variation. A Matlab program determines the expected error in dose based on the time‐structure of both the motion and dose delivery. Results: Motion at 5.0cm depth behind 0.5cm strips, at a frequency of 0.25Hz, in a field with a range of 8.0g/cm2, modulation width of 6.0g/cm2, dose rate of 2Gy/min, total dose of 50cGy, gives a maximum error in dose of 7.4%. Decreasing the dose rate to 1Gy/min reduces the error to 5.3%. Conclusion: Motion‐induced variations in depth can lead to clinically significant dose uncertainties.
SU‐FF‐T‐337: Use of GAFCHROMIC® EBT Film for In‐Water Measurements of Surface Dose of Build Up Region, and Other Dosimetric Parameters Using 6, 9, 12, 16, and 20 MeV Electron Beams
Authors: Gonzalez N Ruiz , Stathakis S , Waggener R , Esquivel C , Papanikolaou N
Published: April 2010

Purpose: Use of GAFCHROMIC® EBT film for in‐water measurements of surface dose of build up region, and other dosimetric parameters of 6, 9, 12, 16, and 20 MeV electron beams. Materials and Methods: PDD measurements were acquired using a radiation field analyzer and a parallel‐plate ionization chamber. Linear interpolation and the fit‐generated data were used to estimate depths of 100%, 90%, 85%, 80%, 50% dose, surface dose, percent dose due to bremsstrahlung, practical range, and Rq. The normalized dose gradient, mean energy at the surface, and most probable energy at the surface were also calculated. A single energy value at the surface was also calculated. EBT film was cut in rectangular strips each measuring 2.54 × 7.0 cm2 or 2.54 × 12.0 cm2 and placed in a water phantom, parallel to the beam, with the upper edge flush to the water level, and exposed to 150 MU. The beam central‐axis crosshairs were carefully aligned and centered on the film's edge. The electron beam energies used were 6, 9, 12, 16, and 20 MeV with electron cones of 6 × 6, 10 × 10, 15 × 15, 20 × 20, and 25 × 25 cm2. The SSD was 100 cm and a dose rate of 600 MU/min. The films were immersed in water for no more than 3–5 minutes and were carefully dried after exposure. The films were protected from excessive ambient light or adverse environmental conditions before and after exposure. The EBT films were scanned after at least 12 hours post‐exposure. Results: The EBT film measurements in water, for all electron beam dosimetric parameters investigated, were in agreement with the ionization chamber results. Conclusion: Using a simple set up, EBT film can be used, in water, to perform accurate PDD measurements of electron beams with high resolution.
SU‐FF‐T‐351: Using EPID Images to Verify the Dose Delivered From 3D Conformal Or Step‐N‐Shoot IMRT Fields
Authors: Chen H , Huang K , Jacob D , Sarkar A , Simpson L
Published: April 2010

Purpose: The electronic portal imaging device (EPID) has particular advantages as a dosimeter such as high resolution, large detection area, real‐time acquisition capability and linear dose response. The doses from 3D or IMRT fields can be verified by the images acquired on EPID. This study demonstrates a calibration and calculation method to verify the doses from 3D conformal or step‐and‐shoot IMRT fields. Method and Materials: To use EPID for dosimetric purpose, a series of calibrations images would need to be acquired. The uniformity of the dose response, dose linearity, and field size dependence were measured and the results were used to correct the subsequent clinical measurements. A few 3D conformal and step‐and‐shoot IMRT fields were delivered directly to EPID. The pixel values were calculated to project the delivered doses and the images were processed to determine the MLC positions. The ion chamber measurements were also performed to be the benchmark of delivered dose. Results: The dose accuracy of this method was investigated by comparing the ion chamber to EPID results. The MLC positions from EPID images were compared to the treatment plans. Conclusion: There are some basic corrections needed when the EPID is used for dose measurements. By applying the proposed method, the MLC positions and delivered doses from 3D conformal or step‐and‐shoot IMRT fields can be verified with EPID images.
SU‐FF‐T‐352: Characterization of Small Volume Ion Chambers for Absolute Dosimetry
Authors: Calvo O , Stathakis S , Gutiérrez A , Mavroidis P , Moral S , Esquivel C , Shi C , Papanikolaou N
Published: April 2010

Purpose: To perform a dosimetric response characterization amongst the following chambers: two Slimline Microchambers (A16 and A14P, Standard Imaging, Middleton WI), a PinPoint, Semiflex and MicroLion chambers (PTW, New York City, NY). Sensitivity, stability, bias voltage dependence, dose rate dependence, energy dependence, spatial resolution and angular dependence are tested for each chamber. Method and materials: Measurements were performed using both a Co‐60 irradiator and a Varian 2300 C/D linac accelerator. The Slimline microchambers (A14P and A16) have collecting volume of 0.002 cm3 and 0.007 cm3, respectively, with the effective point of measurement (EPM) located 1.5 and 1.7mm from tip of chamber. The pinpoint and farmer type possess sensitive volume of 0.015 cm3 and 0.125 cm3 with the EPM located at 3.4 and 4.5mm from the tip of the chamber. The MicroLion possesses a sensitive volume of 0.002 cm3 with the EPM at 0.975 mm from the entrance window. Most of the measurements were evaluated in water using the Blue Phantom (Scanditronix) and associated software. A Unidoswebline electrometer (PTW) was used to measure the charge and current. An external high voltage supply was used in the case of the MicroLion to function at the recommended bias voltage of 800V. Results: Results show similar photon energy characterization among the chambers. For all chambers, electron energies response differences of <2%, dose rate percent differences <2% and good spatial resolution was observed. Overall, the MicroLion chamber demonstrates better responses in comparison to the other chambers while the A14P was shown to be the most difficult to characterize due to the small volume. Conclusion: The preliminary results show that all chambers are suitable for clinical use for both relative and absolute dosimetry. Small variations exist among the chambers.
SU‐FF‐T‐357: Clinical Implementation of the OneDosePlusTM MOSFET Detector for in Vivo Dose Measurement
Authors: Ainsley C , Both S
Published: April 2010

Purpose: To demonstrate the clinical implementation of the OneDosePlus™ MOSFET detector for in vivo dose measurement. Method and Materials: The ratio of the mean dose reported by OneDosePlus™ MOSFETs placed on the surface of a 90 cm × 30 cm × 9 cm water‐equivalent plastic phantom to the mean dose measured by an ionization chamber placed at dmax within the phantom, was determined as a function of (i) gantry angle and detector orientation, (ii) square‐field size, (iii) SSD, (iv) wedge angle, (v) monitor units, and (vi) dose rate, over clinically relevant ranges, and separately for the 6 MV and 15 MV beams of our clinical linear accelerator. A least‐squares straight‐line fit was performed to the response ratio as a function of each of these parameters in turn, and normalized to the ratio evaluated under reference conditions ((i) 0°, (ii) 10 cm, (iii) 100 cm, (iv) 0°, (v) 100, (vi) 400 MU min−1). From these, a formalism was derived to predict the dose reading by OneDosePlus™ and its uncertainty for arbitrary treatment fields for comparison with the treatment planning system prediction. Results: At 6 MV (15 MV), in order to achieve agreement with the ionization chamber, corrections of <3.5% (<11.5%) to the reported OneDosePlus™ dose are required at 0° gantry angle across the full clinical range of all other variables, increasing rapidly as the gantry rotates beyond 50° to 27.5% (22.1%) at 80°. For non‐zero gantry angles, OneDosePlus™ was found to exhibit a stronger angular dependence when its contacts pointed towards the linac head. Conclusion: The OneDosePlus™ MOSFET is well‐suited to in vivo dose measurement, providing a highly convenient alternative to TLDs and diodes. Provided that appropriate corrections are made, verification of the expected dose should be possible to within 3.3% (6 MV) and 3.7% (15 MV) for arbitrary treatment fields.
SU‐FF‐T‐360: Correction for Angular Dependence of the MatriXXEvolution Ionization Chamber Array
Authors: Wagar M , Wolfsberger L , Nitsch P , Bhagwat M , Zygmanski P
Published: April 2010

Purpose: The MatriXXEvolution ionchamber array (IBA Dosimetry) is an attractive tool for plan verification of both conventional IMRT and rotational therapies (RapidArc, VMAT, TomoTherapy, etc). However, the response of this array has a significant angular dependence. We have quantified this dependence, and developed a correction method suitable for plan verification. Method and Materials: A MatriXX phantom (MatriXXEvolution ionchamber array + solid water) and a reference phantom (A12 ionchamber + solid water) were irradiated from different gantry angles (0°–360°, steps of 10°, with finer steps of 3° for lateral angles). Reference measurements were taken using a phantom of the equivalent dimensions. The calibration factor was calculated as the ratio of the MatriXX dose to the A12 dose for a given gantry angle. The water equivalent thickness of the MatriXXEvolution reported by the manufacturer was also examined in the TPS and by measuring 6MV attenuation due to MatriXXEvolution vs. solid water for variable total thickness of the phantom. Three patient‐specific IMRT and two VMAT cumulative plan verifications were delivered to the MatriXX phantom. Dose measurements collected by MatriXXEvolution were then corrected with in‐house software. Measurements were then repeated to the reference phantom. Corrected MatriXX, uncorrected MatriXX and A12 measurements were compared to the treatment planning system. Results: The angular response of the MatriXX array was found to vary as much as 11%, depending on the irradiation angle. The most variation was found to occur around 91–110° and 260–269°, and also close to 180°. The corrected MatriXX doses were found to agree within 1.0% of the A12 ion chamber dose, vs. 2.5% for uncorrected. Conclusion: The MatriXXEvolution ionchamber array has an angular dependent response which is clinically significant, and must be corrected. We have developed and tested the use of an angular‐dependent correction factor which corrects for these response variations.
SU‐FF‐T‐363: An Investigation of the Accuracy of Head‐And‐Neck IMRT Dose Distribution Using Three‐Dimensional Dosimetry Techniques and Monte Carlo Simulation
Authors: Wu D
Published: April 2010

Purpose: Complex dose delivery techniques like intensity‐modulated radiation therapy (IMRT) require dose verification in three dimensions. This work investigates the accuracy of dose distribution of head‐and‐neck IMRT plans using three‐dimensional measurement and Monte Carlo simulation. Method and Materials: Eleven head‐and‐neck cases delivered by Varian 23EX were planned with Pinnacle 8.0. The plans were recalculated in the three‐dimensional measurement phantom Delta4 with the structures of every patients and accurate plans parameters. The plans were also recalculated by Monte Carlo using leaf sequences and MUs for individual plans of every patients and Delta4 phantom. All plans of Delta4 phantom were delivered and measured using Delta4. The dose distribution of iso slice, dose profiles, gamma maps and dose‐volume histograms were used to evaluate the agreement. Results: The dose distribution of iso slice and dose profiles from Delta4 measurement were in excellent agreement with both the Monte Carlo simulation and the Pinnacle calculation at all points. Gamma maps comparison show that all three distributions mutually agreed to within a 3% (dose difference) and 3mm (distance‐to‐agreement) criteria. A 93.2% gamma pass ratio was obtained between the Delta4 measurement and Pinnacle distributions with 3mm/3% gamma criteria. A 96.8% gamma pass ratio was obtained between the Delta4 measurement and Monte Carlo simulation with 2mm/2% gamma criteria and 99.2% gamma pass ratio with 3mm/3% gamma criteria. The DVH plot have slightly differences between Pinnacle and Delta4 measurement as well as Pinnacle and Monte Carlo simulation, but have excellent agreement between Delta4 measurement and Monte Carlo simulation. Conclusions: It was shown that Delta4 and Monte Carlo simulation can be used very efficiently to verify head‐and‐neck IMRT delivery and no data is missed. The primary advantage of Delta4 is the fact it can measure true 3D dosimetry while Monte Carlo can simulate in patients CT images but not in phantom.
SU‐FF‐T‐365: Dosimetry QA of Scanned Proton Beam with An Ion Chamber Array
Authors: Zhao Li , Simon W , Das IJ
Published: April 2010

Purpose: To investigate an IC Profiler for clinical proton dosimetry QA. Methods and Materials: An IC Profiler (model 1122) (Sun Nuclear Corporation, Melbourne, FL) consisting of four linear arrays (X, Y, and two diagonals) with 251 vented ion chambers was investigated. The vented ion chamber has 2.6 mm effective collection width and 0.9 g/cm2 inherent buildup. It was irradiated with a proton beam with 16 cm penetration depth at a depth of 11 cm corresponding to the middle of a 10 cm spread‐out Bragg peak (SOBP) for a 10‐cm diameter circular field using uniform scanning proton beam delivery system. Lateral dose profiles measured with ICP was compared with results obtained from a Markus chamber (MC) positioned in water tank, a large multi‐pad ionization chamber (MPIC) transverse detector, and a MatriXX detector (Scanditronix / Wellhofer). Field width (50%–50%), lateral penumbra (20%–80%), field flatness and symmetry were analyzed. Results: The measured field widths were 10.92 cm, 10.70 cm, 10.21 cm and 10.51 cm from ICP, MC, MPIC and, Matrixx detector, respectively. The measured lateral penumbras were 5.6 mm, 6.0 mm, 8.2 mm and 8.1 mm for ICP, MC, MPIC, and Matrixx, respectively. The flatness values obtained with ICP, MC, MPIC and Matrixx were 107.40%, 104.03%, 108.05%, and 104.55%. The symmetries obtained with ICP, MC, MPIC and Matrixx are 0.62%, 0.16%, 0.15%, and 0.12%, respectively. Conclusions: Results obtained from the IC Profiler are similar to those obtained from the other three detectors. From these investigations it can be concluded that the newly available IC PROFILER is a suitable device for quality assurance and dose verifications in proton therapy either in double scattering or scanned beam.
SU‐FF‐T‐367: Surface Dose Measurements with OneDose MOSFET System
Authors: Lehmann Joerg , Sydlowski Kristen
Published: April 2010

The purpose of the study was to evaluate the performance of the MOSFET based OneDose system in comparison with an established TLD system for clinical surface dose measurements for electron and photon irradiation. The impact of an oblique beam angle was investigated for electron beams. Three electron (6, 12, 22MeV) and two photon (6, 20MV) energies have been investigated. For each energy OneDose and TLD detectors were irradiated at reference condition in a solid water phantom and on the surface of the phantom (100 cm SSD). For the electron beams additional surface measurements were performed with an incident beam angle of 30°. Four OneDose detectors and two TLDs were irradiated in each measurement. Three such measurements were performed for each data point. Comparisons were made between OneDose and TLD data as well as with established machine depth dose data. The average standard deviation of the OneDose dose measurements over 13 sets of 12 measurements was 1.8% with higher standard deviations for the photon beam surface measurements (5.7%). Without correction for energy and beam type, which is generically offered by the system, the measured OneDose dose agreed within 2% with the TLD measured data for the photon beams and was 4–7% low for the electron beams. Applying these deviations as corrections for the surface measurements, the OneDose system was found to be in good agreement with the reference data (3%) and the TLD measurements (0–8%). OneDose underestimated the photon surface dose by 22%, TLD by 14%. Changing the beam angle to 30 degrees did not impact the OneDose electron beam surface measurements. The OneDose is fit for use for electron surface patient measurements with applied beam correction factors. Surface measurements for photon beams are more challenging as with TLD.

Sicel Technologies provided the OneDose detectors used in this study.
SU‐FF‐T‐368: Evaluation of Physical Characteristics of a Prototype Portable Dosimeter System in CT: Comparison Between Portable and Mobile MOSFET Dosimeter Systems
Authors: Toncheva G , Anderson‐Evans C , Yoshizumi T
Published: April 2010

Purpose: The purpose of this study was to evaluate the physical characteristics of a prototype portable MOSFET dosimeter system (TN‐RD‐91, Best Medical Canada, Ottawa, Canada) and compare its performance with the mobile MOSFET dosimeter system (TN‐RD‐70‐W) at clinical diagnostic settings. Method and Materials: Two dosimeters (TN‐1002RD) of each system were placed side by side accompanied by an ion chamber (model 10×5–6) and Radiation Monitor (model 9015, Radcal, Monrovia, CA) on top of a tissue equivalent phantom 150mm×150mm×200mm (CIRS, Norfolk, Virginia). They were exposed to a beam quality of HVL 7.24 mm Al for the GE CT scanners (GE Healthcare, Milwaukee, WI). Results: We evaluated and compared the portable and mobile MOSFET dosimeter system (with high sensitivity reader bias) for sensitivity, linearity, and reproducibility at two CT beam energies: 120 kVp and 80 kVp. The average sensitivity at 120 kVp was 28.8±1.01 mV/cGy for portable dosimeter and 31.61±0.84 mV/cGy for mobile MOSFET, and 32.3±1.3 mV/cGy and 31.62±0.25 accordingly at 80 kVp. The linearity of dosimeters was tested from 0.3 mGy to 14.0 mGy; the goodness of fit at 120 kVp for the portable dosimeter was 0.9982 and for the mobile MOSFET: 0.9973. The goodness of fit at 80 kVp was 0.9992 and 0.9997 accordingly. The reproducibility for both dosimeters was evaluated at 120 kVp and 80 kVp from 1 mGy to 17 mG and showed comparable results at each of dose points. Conclusion: We conclude that the portable dosimeter and mobile MOSFET performance characteristics are basically equivalent and the portable dosimeter offers an alternative to regular mobile MOSFET system. Conflict of Interest: Best Medical Canada provided the prototype portable dosimeter system for the study
SU‐FF‐T‐401: Monte Carlo Simulation of Batch Assay of Iodine‐125 Seeds Preloaded in Unshielded Cartridges
Authors: Chen C , Yaparpalvi R , Kuo H , Tsai S , Guha C , Mah D
Published: April 2010

Purpose: Although the feasibility of batch assay of LDR brachytherapy sources has been proposed, the technique has not gained widespread clinical acceptance due to the uncertainties in the measurement of packaged seeds. Interest in batch assays has been revived with the availability of preloaded seeds in a cartridge, which permits all seeds to be assayed in bulk and limits radiation exposure. In this study, the batch assay of I‐125 seeds was simulated by Monte Carlo method including the evaluation of the correction factor between individual and batch assays. Method and Materials: To validate the initial simulation model, both FLUKA and DOSRZnrc/EGSnrc Monte Carlo code were used to calculate the dose distribution of single I‐125 seed based upon the TG‐43 formulation. Further simulations of seed assay were then performed by FLUKA code including detailed geometries of the seeds, the unshielded cartridge, source holders and a HDR‐1000 Plus well chamber. The sensitivity of the batch scheme was also tested by simulating defective (out of calibration tolerance) seeds in cartridges. Results: The calculated correction factor is 1.482 (s=1.60%), which is comparable with the measured result of 1.496 (s=0.71%). The test of defective seeds shows an average 0.87% error (only 6.64% batches with error >2%) could be found if some seeds in batch with 5%–10% departure from reference activity. An extra and missing seed in cartridge caused 10.45% error in chamber response. Conclusion: A cartridge assay provides an accurate measurement of the mean activity but could mask large individual seed variability. The correction factor for seeds preloaded in unshielded cartridge could be measured and used confidently. To agree with NRC regulation and AAPM recommendations, all the seeds should and could be measured by batch. The individual sample of 10% of the seeds could then be chose from the cartridges with the most divergent reading.