bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐299: The Use of Beam Fluency to Assure Image‐Guided Radiosurgery Accuracy
Authors: Dieterich S , Nelson BK
Published: April 2010

Purpose: The purpose of this study was to accelerate and improve the measurement process of Cyberknife individual beam accuracy using a novel detector (XRV‐100, Logos Systems, Scotts Valley, CA) consisting of a three‐dimensional scintillator screen geometry and real‐time CCD camera monitoring. Method and Materials: The XRV‐100 hardware consists of a 60 degree cone laminated with conventional Gadox X‐ray scintillator phosphor. As an x‐ray beam passes through the cone, two spots of visible light are formed by Compton scattering at the entry and exit points. These two spots of light define not only the path of the radiation through three‐dimensional (3D) space but also the energy profile of the beam fluence. While QA treatment plans are administered by the radiosurgery system, the CCD camera and monitoring software view the interior of the scintillator cone and record the position of each beam in real‐time. Fiducial spheres mounted in proximity to the scintillator allow the Cyberknife system imaging system to precisely target treatment plans within the cone volume. Several Cyberknife beam collimators were profiled with the XRV system and the fluency data was compared with historical dose data taken with traditional water tank systems. AQA and isocenter end‐to‐end treatment plans were also performed on the XRV detector. Results: Beam fluency field widths and penumbras captured with the XRV using the Cyberknife 10 mm collimator at a SAD of 75 cm compare within 2% of water tank dose data taken at a depth of 50 cm. The AQA vector results confirmed Cyberknife submillimeter accuracy. Conclusion: The XRV results demonstrate that 3D beam fluency metrology can be accurate, fast, and a powerful adjunct to existing radiosurgery QA systems. This detector system is independent of the laser‐beam CAX alignment and improves the measurement methodology by measuring the delivery accuracy of the x‐ray beams directly.
SU‐FF‐T‐305: The Use of TomoDose for TomoTherapy MLC Alignment Verification
Authors: Bichay T
Published: April 2010

Purpose: To develop a method of using TomoDose (Sun Nuclear Corp.), a daily QA device for TomoTherapy, to verify MLC lateral alignment after service or replacement. Method and Materials: Baseline TomoTherapy beam profiles of 1.1 cm, 2.5 cm and 5.0 cm × 40.0 cm were obtained by carrying out TomoDose measurements at the standard isocenter position. The profiles were modified by using an alternating MLC pattern of three open then three closed leaves thereby generating a fingerprint of peaks. The profiles were normalized to detector ♯51 located at one centimeter lateral to the central axis. TomoDose peak heights corresponding to the open leaf positions were then evaluated. Lateral offsets, simulating an MLC replacement error, were set at 0.0, −0.2, −0.4 and −0.6 mm from central axis. Standard EDR2 film was also used for the same verification. Results: Prior to any offsets the central axis detector ♯53 value was 49.12 +/− 1.88 (standard deviation). After a −0.2 mm lateral offset the value dropped to 43.66. Offsets of −0.4 mm and −0.6 mm resulted in further drops to 40.41 and 36.28 respectively. Parallel film measurements were not as sensitive in determining offsets. TomoDose peak height at detector ♯53 changed by approximately 5 percentage points for every 0.1 mm offset in the lateral MLC position. Conclusion: MLC service or replacement is common on TomoTherapy. Film‐based lateral MLC alignment is difficult to determine with better than 0.5 mm resolution due to processor/film instability. We have determined that TomoDose used with an alternating leaf pattern is sensitive enough to determine a lateral MLC replacement error as small as 0.2 mm.
SU‐FF‐T‐306: Optically Stimulated Light Dosimetry: Commissioning of An Optically Stimulated Luminescence (OSL) System for Remote Dosimetry Audits, the Radiological Physics Center Experience
Authors: Aguirre J , Alvarez P , Followill D , Ibbott G , Amador C , Tailor A
Published: April 2010

For the last 30 years the Radiological Physics Center (RPC) has used TLD dosimetry to conduct remote audits of beam output of photon and electron beams and energy checks for electron beams. Acrylic blocks containing capsules of TLD powder were sent for each beam. Powder was used as a single‐use disposable dosimeter. The RPC has previously described the use of the remote audits to identify units with beam measurements exceeding 5% and 5 mm. The system uncertainty is 1.5% (1 standard deviation) indicating high confidence in the 5% threshold for acceptability.

OSL dosimeters have been available since 1992. Several publications have described their use for dose measurements at therapeutic levels. After a promising initial evaluation, the RPC decided to purchase and commission OSL dosimeters and instrumentation with the goal of implementing an OSL‐based system into the remote audit program.

As part of the commissioning procedure, the depletion characteristics of OSL dosimeters were studied as they relate to the number of readings, the reader and the changes after annealing. Dark current and standard signal characteristics of the reader were evaluated and compared against factory specifications. The mechanical properties of the reader positioning characteristics were also tested.

Because OSL dosimeters have unique sensitivities, response factor was determined and analyzed in terms of its variability with each reader, annealing cycle and cumulative dose. Dose dependence was also tested against annealing and accumulated dose. Energy dependence factors and fading characteristics were determined.

A group of dosimeters from a single production was purchased and commissioned for use. Dose response, energy and fading characteristics for the batch were defined together with the design of a reading session and a quality assurance program.

This work was supported by PHS CA010953 awarded by NCI, DHHS
SU‐FF‐T‐309: Quality Assurance for Imaging Guided Stereotactic RadioSurgery with Novalis Tx™ System
Authors: Chang Z , Wang Z , Wu Q , Bowsher J , Yoo S , Yin F
Published: April 2010

Purpose: To present QA measurements over extended periods at our institute and recommend a QA program for image‐guided radiosurgery on Novalis Tx system. Method and Materials: Novalis Tx system is developed based on a Trilogy linear accelerator featuring BrainLAB 6D‐ExacTrac, HD120‐MLC, MV‐EPID and KV‐OBI&CBCT. Utilization of any new technology and equipment necessitates a comprehensive QA program to maintain and monitor system performance characteristics. In this study, we carried out a systematic QA program, including linear accelerator QA, treatment planning QA, planning MRI and CT QA, treatment imaging QA, patient‐specific QA, and system setup procedures for image‐guided radiosurgery. Methodology, frequencies and criteria of QA tests were based on AAPM TG40, TG42, TG51, TG66, and ACR MRI QC. Results: Comprehensive QA was performed from March to December 2008. 1) Daily Winston‐Lutz tests of 199 measurements showed that isocenter of gantry rotation was 0.44±0.21mm; isocenter of couch rotation was 0.34±0.20mm, within 1mm tolerance. Other linear accelerator QA results were within TG40 criteria. 2) Treatment planning QA was conducted with spot‐checks and a RPC SRS head phantom test. In spot‐checks, dosimetric discrepancies were within 3% (3.5% for the 4mm cone). RPC radiosurgery test was passed with less than 1% dosimetric deviation, and 100% geometric match. 3) Daily MRI QA showed that average geometric distortion was −0.2±0.4mm, −0.3±0.4mm, and −0.9±0.1mm along vertical, lateral, and longitudinal directions, within 1mm on phantom, 50% of ACR specifications. 4) OBI QA showed that imaging isocenter displacement was 0.46±0.30mm at AP and 0.76±0.19mm at RLAT. 5) Over ten months, 128 patients with intracranial lesions were treated with radiosurgery. All second MU checks were within 3%, and there were none treatment errors associated with the SRS treatments. Conclusion: Comprehensive QA program for image‐guided radiosurgery has been developed with proper methodology, frequency and criteria on Novalis Tx system, demonstrated with extensive measurements.
SU‐FF‐T‐311: Calibration of Lot 47207‐01I GAFCHROMIC EBT Film Using the Xoft Axxent X‐Ray Source
Authors: Kelley L , Axelrod S , Rusch T
Published: April 2010

Purpose: To determine the absolute dose delivered for a given exposed optical density of GAFCHROMIC® EBT film over a range of 0.25 to 14 Gy and over a range of distances from 1cm to 4cm in water for a Xoft Axxent® Model S700 X‐ray Source. Method and Materials: GAFCHROMIC film is used for dose delivery validation for the Xoft x‐ray source and associated applicators used to deliver prescription dose distributions. Custom Gammex RMI 457 Solid Water™ film fixtures were designed to locate both a PTW34013 ionization chamber and a film coupon at precise distances from the x‐ray source. The fixtures complete integral rotations around the x‐ray source during exposures to minimize azimuthal effects. Three sets of 12 exposures were performed for the dose range at each distance. Film coupons were scanned with an Epson® Expression® 10000XL scanner and the maximum pixel value was determined for each coupon corresponding to the calibrated ionization chamber measured dose. Results: The measured maximum pixel values for each data set were fit to a 5th order polynomial. Calibration coefficients were determined at different distances from the x‐ray source to investigate spectral differences. All fits were well behaved with residuals within ±4%. Differences between the fitted dose as a function of distance were small, indicating little sensitivity of the film to the range of source spectra. This calibration was applied to Xoft Axxent Vaginal Applicator validation study films, producing good agreement with the isodose contours predicted by BrachyVision™ treatment planning software. Conclusion: A precise calibration of GAFCHROMIC EBT film was performed for the Xoft x‐ray source using a PTW 34013 ionization chamber. It showed no significant difference in the measured optical density for a given measured dose over a distance range of 1cm to 4cm.

Conflict of Interest: Research sponsored by Xoft, Inc.
SU‐FF‐T‐312: Dosimetry for Radiobiological Evaluation of the Active Beam Scanning System at Loma Linda University Medical Center
Authors: Wroe A , Rightnar S , Ghebremedhin A , Schulte R , Lesyna D , Patyal B , Slater J , Gridley D , Slater J
Published: April 2010

Purpose: Active beam scanning is a new and important innovation in proton therapy, potentially allowing for a more conformal proton treatment through dynamic manipulation of proton pencil beams. Active beam scanning thus requires accurate online dosimetry to track the proton pencil beam both in space and time, whilst at the same time monitoring dose delivered to a given point within the patient. Radiobiological validation of this new technology is equally important. Currently, with passive proton beam delivery a uniform RBE of 1.1 is assumed, and this has lead to satisfactory clinical results both in terms of tumor control and side effects. However, it is unclear whether this RBE value is also appropriate for active beam scanning treatments. Methods and Materials: To investigate this, researchers at Loma Linda University Medical Center are utilizing a multilayered system to evaluate radiobiological parameters at multiple positions within a water tank along the depth dose profile. The results obtained are being compared with identical experiments completed with Co‐60 photons and passively delivered protons. As a prerequisite for accurate and meaningful comparisons, precise dosimetry ensuring compatibility of dose rate, depth dose profile, dose uniformity and absolute dose have to be performed for all three radiation modalities. This presentation describes in detail the dosimetry methodology associated with the project, including film and ion chamber evaluation of the three radiation modalities. Results: Initial results indicate that the active beam scanning system provides proton radiation fields comparable to that utilizing passive beam delivery with a flatness and symmetry within 3% (TG45) at all positions along the Bragg peak. Further, dose rates experienced with this system are also comparable to that for passive beam delivery. Conclusions: This preclinical work will provide a unique opportunity to discuss both dosimetry‐ and radiobiology‐associated issues of automatic beam scanning in proton therapy.
SU‐FF‐T‐318: Software Quality Assurance with and Beyond TG‐53 for the Cyberknife MultiPlan System
Authors: Antony J , Dieterich S , Wang L , Lo A
Published: April 2010

Purpose: To evaluate the application of AAPM‐TG53 to the Cyberknife MultiPlan™ treatment planning software with additional use of an inhomogeneity phantom to QA the new Monte Carlo algorithm, and using motion phantom for QA of the 4D treatment planning algorithm. Method and Materials: An inhomogeneous Quasar phantom (Modus Medical Devices Inc) and a 4D‐motion phantom (Accuray Inc) with known density inserts were used in this study. Both phantoms accommodate film loading. The Quasar phantom also has an insert for a PTW 0.6cc farmer chamber. Both phantoms were scanned using GE PET/CT scanner with 1.25 mm thickness. The 4D phantom was scanned and planned with a full 4D scan. The Quasar phantom was scanned in 3D with a variety of density inserts with defined volumes. These known volumes were compared with the calculated volumes which were contoured using different drawing tools. The treatment plans for the inhomogeneous MC plan and the 4D plan was done using both the pencil beam and Monte Carlo algorithms. Results: The volumetric information differed from the actual volume based on the contouring tools. The contours made with the ellipse, bumper and rescale tool showed <25% error in volume in 8 out of 9 structures. For the MC plan with a prescription dose of 10 Gy, the measured dose with the farmer chamber was 9.99 Gy. The dose measured in the 4D delivery agreed with the 4D plan dose, whereas the 3D plan overestimated the dose to a static critical structure. Conclusion: TG‐53 offers basic guidance for software QA, but additional QA must be added to cover new software components. The choice of contouring tools affects the calculated volume of the structures contoured in MultiPlan™. Both MC algorithm QA and 4D planning QA can be implemented in a comprehensive clinical software QA program with commercially available phantoms.
SU‐FF‐T‐321: Megavoltage Electron Beam Energy Estimation Using Thermoluminescent Dosimeters
Authors: Nelson V , McLean I , Holloway L
Published: April 2010

Purpose: To explore the possibility of using a combination of thermoluminescent (TL) materials to estimate electron beam energy from a range of TL materials which exhibit energy dependent TL response to electrons. Method and Materials: Five different types of thermoluminescence dosimeters (TLD100, TLD100H, TLD200, TLD400 and TLD500) were irradiated with five electron energies available from a Siemens Primus linear accelerator. All types of TLDs were irradiated at depth of maximum dose to a dose of 1 Gy and the process was repeated three times. TL Response per Gy dose delivered was calculated for all the materials. Ratios of TL sensitivity of different combinations of TL materials for all five electron beams were determined. Results: All TL materials showed different magnitudes of variation in electron beam energy dependent response. The reproducibility of each type of TLD was; TLD100 2%, TLD200 3%, TLD400 1%, TLD500 5% and TLD100H 2%. The sensitivity ratios of combination TLD400‐TLD100 and TLD400H‐TLD100H showed the highest variation with electron energy. Conclusion: Lithium Fluoride (LiF) based TL materials are commonly employed in dosimetry audits conducted by IAEA/WHO. In electron dosimetry intercomparison and audits where the beam energy may not be accurately known, error in dose estimation can occur where LiF based materials are employed. However, if a combination of two TL materials is employed, the energy of the electron beam can be estimated. The present study has demonstrated that the combination of TLD100 and TLD 400 has the highest sensitivity to variation in electron energy in the nominal electron energy (E0) range of 5–14 MeV. In IAEA/WHO dosimetry audits when a combination of these two TL dosimeters (instead of a single TL dosimeter) is used, it has the potential to improve the accuracy of electron dosimetry by providing an energy dependent correction factor.
SU‐FF‐T‐326: A Systematic Methodology in Dosimetric Verification of Anisotropic Analytical Algorithm in Eclipse Treatment Planning System
Authors: He R , Yang C
Published: April 2010

Purpose: A series of practical phantoms were constructed to investigate the accuracy of photon dose calculations performed by the Anisotropic Analytical Algorithm (AAA) in homogeneous and inhomogeneous media. Method and Materials: A total of seven heterogeneous and homogeneous phantoms with solid water (SW) and cork, SW with MapCHECK or MatriXX were constructed to investigate the difference between dose calculated by AAA in Eclipse treatment planning system (TPS) and measured dose using Varian 23 IX LINAC. Ion Chamber was used for point dose measurements and film, MapCHECK, MatriXX were used for 2D dose measurements. A detailed analysis of data computed by the AAA algorithm was carried out and data were compared against measurements. To better appraise the performance of AAA, data obtained from the pencil beam convolution (PBC) algorithm implemented in Eclipse were also added in the comparison. Results: AAA calculation has showed better than PBC in agreement with measurement from both ion chamber and 2D dosimeters. As planar dose evaluation, MapCHECK yielded a pass rate ranging from 88.5% to 95.5% for AAA, and 69.7% to 81.9% for PBC based on 1% difference and 1mm distance to agreement for different phantoms. The film dose distribution showed as well that AAA calculation is better than PBC in agreement with measurements. Generally, the better agreement of AAA than PBC calculation with measurement is more obvious in heterogeneous than homogeneous phantoms. Conclusions: Eclipse TPS dose calculation AAA showed better agreement with dose measurement in both heterogeneous (more significant) and homogeneous phantom than PBC using three different types of 2D dosimetry system and ion chamber. The design of measurements and construction of phantoms represent a simple, efficient, and accurate means for verifying dose calculation algorithms in TPS.
SU‐FF‐T‐327: Effect of Rectal Balloon Filling Content On Rectal Wall Dose Distribution in Prostate Radiotherapy
Authors: Scheuermann R , Both S
Published: April 2010

Purpose: To estimate the dose difference at the level of the rectal wall for air‐ and water‐filled rectal balloons for conformal and IMRT prostate treatment plans. Method and Materials: A Radiadyne rectal balloon was filled with 100 cc of either air or water and placed in water equivalent phantom. CT simulations of the phantom were performed with air and water‐filled balloons. Eight marked points along the circumference of the balloon were identified. Treatment fields for both conformal and IMRT prostate plans were copied and calculated on both phantom image sets using the Varian Eclipse TPS v8.1 (AAA). Thermoluminescent detectors (TLDs) were then positioned at each of the 8 marked positions. A treatment fraction was delivered to the phantom using a Varian 2300 IX, 15MV photon beam. The dose measured at corresponding positions was then compared between water vs. air and against the Eclipse calculated dose. Results: The measurements confirm a difference in dose deposited along the rectal wall for air‐ and water‐filled rectal balloons. Points along the anterior rectal wall saw an increase in dose for water‐filling relative to air‐filling, while points along the posterior rectal wall exhibited a reduction in dose for water‐filling relative to air. The average dose increase along the anterior wall for IMRT and conformal plans was 12% (range 10–15%; IMRT) and 7% (range 3–13%; conformal) respectively, while the average dose decrease along the posterior wall was 7% (range 3–11%; IMRT) and 11% (range 0–18%) respectively. Conclusion: This study demonstrates the need for careful consideration when implementing rectal balloons clinically, as a dose increase of 12% could result along the anterior rectum for water‐filling vs. air‐filling.
SU‐FF‐T‐329: Measurements of Delivered Dose in Tissue Inhomogeneity Using a MOSFET Detector for An Anthropomorphic Phantom
Authors: Kohno R
Published: April 2010

Purpose: An accurate estimate of the radiation dose is important for verifying that the expected dose of radiation has been delivered to the patient. To verify delivered dose in tissue inhomogeneity, in‐vivo dosimetry by a metal oxide‐silicon field effect transistor (MOSFET) detector as a pinpoint dosimeter was performed for an anthropomorphic phantom. Method and Materials: The RANDO anthropomorphic phantom was used in this study. The MOSFET detectors were placed within cavities in the phantom. In‐vivo dosimetry using the MOSFET detector for simple square field sizes of 10×10, 5×5, 3×3 cm2 with a 6 MV photon beam was measured in various tissue inhomogeneities such as the abdominal, thoracic, and head and neck regions. On the other hand, doses for them were calculated using the superposition (SP) algorithm with inhomogeneity correction in the XiO radiotherapy treatment planning system. Three‐dimensional dose distributions were calculated using 0.2 cm resolution. In addition, dose calculation on megavoltage cone‐beam CT (MVCBCT) was demonstrated to improve the precision of the in‐vivo dosimetry, because the MVCBCT can monitor the dosimetric affect of changes in anatomy or position compared with the reference CT by applying the initial treatment plan to the MVCBCT images. Results: The results of the SP algorithm for the abdominal, thoracic, and head and neck regions agreed with the doses measured by the MOSFET detector to within 3%. Conclusion: We performed in‐vivo dosimetry using the MOSFET detector in the abdominal, thoracic, and head and neck regions of the anthropomorphic phantom. The doses measured by the MOSFET detector agreed well with the dose calculations by the SP algorithm to within 3%. The MOSFET detector was useful for pinpoint absolute dosimetry even in inhomogeneous media.
SU‐FF‐T‐331: Dose to the Body Organs From IMRT and 3D‐CRT Breast Radiotherapy: Prone and Supine
Authors: Becker S , Elliston C , Jozsef G , Wyngaert J De , Chang J , Brenner D , Formenti S
Published: April 2010

Purpose: The potential carcinogenic effect of low dose scattered radiation has been a recent concern in breast cancer radiotherapy. The widespread use of IMRT has heightened this concern, due to increased monitor units. Method and Materials: An anthropomorphic phantom with customized prone and supine breasts modeled off patient CT scans, was imaged, planned, and treated with 3D conformal and IMRT whole breast plans, in both the prone and supine positions. All plans used a pair of opposed tangent beams. The 3D and IMRT plans used the same field arrangements, but the IMRT plan involved 3.5 times the monitor units. MOSFETs were inserted in various organs throughout the phantom, including both lungs, regions of the heart, the thyroid, and both breasts. They were kept in these locations for all treatments. Results: The dose to the contralateral (CL) breast and thyroid showed little difference between the prone and supine measurements. Prone plans delivered less dose to the lung than supine plans, especially for the part of the lung near the chest wall (3.3% of the prescribed dose, compared to 9.5%). The heart received less dose prone versus supine. IMRT delivered less dose than the 3D plan in all organs measured. Conclusion: For all organs, the dose measured mainly depended on the distance from the field edge. The prone position conveyed better lung and heart sparing because the treatment field was more distant from these organs. IMRT delivered less dose than 3D to the heart, lungs and CL breast. The reduction in scatter dose to these proximal organs outweighed any increase in leakage dose. More measurements and Monte Carlo simulations are necessary to explore the differences in anatomy in the prone and supine positions, and from patient to patient, and to assess the very low dose levels far away from the treatment volume.
SU‐FF‐T‐332: The Effect of Metal Implants On Proton Transport
Authors: Wroe A , Schulte R , Lingampally V , Patyal B
Published: April 2010

Purpose: Proton therapy treatment planning in the presence of high‐density metal implants has considerable uncertainty. This uncertainty arises from artifacts produced through the imaging process, uncertainty in the CT density of the implant itself and the effect which both of these have on the range of protons. Because of this uncertainty, beam directions which pass through such implants are avoided if possible. However, in some cases such beams are unavoidable in order to preserve normal tissue and treat the tumor volume effectively. The goal of this ongoing project is to investigate proton transport through high‐density implants and to determine the accuracy of treatment planning software in reproducing isodose distributions when such materials are present. Methods and Materials: We have constructed a phantom with titanium and medical grade stainless steel plates of 1–4 mm thickness embedded within polystyrene. The phantom has been CT‐scanned and then planned using the Odyssey treatment planning system (PerMedics Medical Systems), which is used in routine treatment planning at Loma Linda University Medical Center. The plan generated will be compared to physical dose measurements using ion chambers inserted into the polystyrene phantom. In addition, GEANT4 Monte Carlo simulations of the experimental setup will be completed using CT data as the basis for geometry and material definitions. Additional simulations will be performed with geometry defined based on manufacturers' specifications. Results: Treatment plans of the experimental setup provide an indication of the perturbation of the proton radiation field caused by the metallic implants and indicate areas where measurements and comparisons with GEANT4 should be completed. Conclusions: This study will provide useful data on the accuracy in treatment planning in the presence of metallic implants in proton therapy, and may indicate areas in which improvements to current treatment planning methods can be made.
SU‐FF‐T‐333: Onyx Embolization Effect On Diagnostic Images for Radiosurgical in AVM Patients
Authors: Shtraus N , Alani S , Mimon S , Schifter D , Kanner A
Published: April 2010

Background: Treatment of arterio‐venous malformations (AVM) of the brain can be a challenge due to the complexity of location, size and their proximity to the cerebral vascular circulation. stereotactic radiosurgery (SRS) recent advanced in catheter technique and new embolization materials, in particular Onyx have increased the success rate of total and near‐total obliteration. The use of Onyx cause distortion of the MRI and CT images and there for has to be considering in any radiation treatment planning. Methods and Material: Between 12/2006 and 12/2008 we treated 13 AVM patients after Onyx immobilization with SRS. A bottle with 1.5ml of onyx been irradiate to find and define the absorption of it per mm of thickness, after doing that we contor the onyx and the projected area in the planning system and apply a homeginity correction to virtual organ that been draw. a pinpoint chamber been used for relative measurement because of the size of the bottle. Results: In particular larger Onyx embolized AVMs showed significant imaging artefacts especially on CT, which rendered this imaging modality useless for planning purposes. The absorption of the onyx with thickness of about 15mm was up to 5.6% higher compare to a 15mm of bolous without the onyx. Conclusion: The use of Onyx caused significant image artefact on MR and more so on CT. this results shows the importance of the correction that need to apply manually in the planning system. For patient with a large area and volume of onyx, this results shows the crucial of the correction that must be applied, if not a dose calculation can be completely wrong especially for radiosurgery patients, who prescribed high dose in single fraction to a completely healthy brain.
SU‐FF‐T‐334: Assessment of the Accuracy of Gafchromic®EBT Film Dosimetry in Inhomogeneous Media
Authors: Jeong H , Kum O , Han Y , Kim C , Suh T
Published: April 2010

Purpose: To accurately verify the dose distributions for IMRT in an anthropomorphic environment, we investigated the accuracies of Gafchromic@EBT film measurements in heterogeneous media by comparing with Monte Carlo calculations. Method and Materials: Measurements were performed in a home‐made multilayer 20 × 20 × 20 cm3 polystyrene phantom with a cubic hole at the center for the inclusion of heterogeneities such as an air cavity or Teflon cube. 6 MV X‐rays (Primus, Siemens, USA) with two beams (3 × 3 cm2 and 9 × 9 cm2) were irradiated to the homogeneous and heterogeneous phantoms: (1) homogeneous polystyrene phantom, (2) polystyrene with a cubic air cavity, (3) polystyrene with a cubic Teflon block. Cube sizes of 1×1×1 cm3, 2×2×2 cm3, and 3×3×3 cm3 were used. The EBT film was sandwiched between the phantom slabs parallel to the beam axis through the inhomogeneity. The percentage depth dose and lateral distributions on the film were obtained by Epson 1680 flatbed scanner. Computations were performed with the BEAMnrc and PMCEPT Monte Carlo codes with a cell size of 0.1 × 0.1 × 0.1 cm3 in the phantom. Results: The errors of percent depth dose (PDD) and lateral profiles were about 2% in homogeneous phantom. With an air cavity, the PDDs were agreed with each other within 2%, but big fluctuations were observed in the air cavity due to the poor convergence of MC (error of ∼48%). The measured depth doses in the Teflon cube were higher than those of MC. The behavior of PDDs on the boundaries of the Teflon cube was very different to each other Conclusion: EBT film predicted accurately the dose distributions with the inclusion of an air cavity, but over estimated them in the high density/atomic number medium such as Teflon. Correction methods are under investigation and will be presented
SU‐FF‐T‐336: Influence of Permanent I‐125 Seeds On the Dose Perturbation to Prostate Using 6MV and 18MV Beams
Authors: Steinman J , Bakhtiari M , Nazareth D , Malhotra H
Published: April 2010

Purpose: Many times a suboptimal dose distribution resulting from I‐125 seeds in prostate brachytherapy is salvaged by giving additional radiation dose using 3DCRT/IMRT. In standard treatment planning, the dosimetric perturbations introduced by the existing seeds are usually ignored. Present study aims at studying these perturbations for 6MV and 18MV beams within a phantom setting in region immediately behind the seed. Methods and Materials: Three Kodak X‐OmatV films were placed on top of 10cm of Solid Water at 100cm SSD. On top of the films a single non‐radioactive (preactivated) seed was placed and aligned parallel in the longitudinal direction under 1cm bolus and 4cm Solid Water for a total buildup of 5cm. A 1cm × 1cm field was setup and irradiated with 10MU of 6MV and 18MV photons. A second set of measurements was obtained using three seeds each separated vertically by 0.5cm bolus material allowing the study of the interseed shielding effect. Control fields were irradiated with no seeds. All the films belonged to the same batch and were processed simultaneously. The films were scanned using a Vidar VXR‐16 scanner and analyzed using RIT 113 Version 5.1 obtaining profiles in the transverse and longitudinal direction. Results: For the single seed measurement, at about 0.5mm from the seed (top film), the maximum change in dose from having no seed was 27.1% (6MV) and 13.4% (18MV). The three seed measurement revealed 24.1% (6MV) and 11.1% (18MV). Conclusion: The dose perturbation caused by the I‐125 seeds is significant locally around the seed. This can be seen by the fact that the change in the dose profile is independent of the number of seeds spaced intermitted above the seed.
SU‐FF‐T‐342: Evaluation of Superficial Dose for Helical Tomotherapy with GAFCHROMIC® EBT Film
Authors: Lee H , Chong N , Tien H , Lin C , Yen H , Shueng P , Wu L , Hsieh C , Chen C
Published: April 2010

Purpose: To measure the surface dose using EBT films for helical tomotherapy. Materials and methods: A cylindrical solid water phantom was scanned on a Siemens Somatom CT scanner with 3mm slice thickness. Four types of tomotherapy plans were studied: 1. The PTV touching the surface, planned and treated with different thicknesses of bolus (0,3,5,8 and 10mm), 2. Same as first, but treated without the bolus, 3. The PTV extended by 5mm outside the surface, planned and treated with and without a 10mm bolus, and 4. The PTV withdrawn from the surface by 3mm to 5mm, planned and treated without bolus. For each case, a plan was created to deliver 2Gy to 95% of the PTV per fraction. Gafchromic EBT films were used to measure the surface and the target doses. We also measured the surface doses of five clinical patients to verify the phantom study results. Results: For type 1 cases, when planned and treated without the bolus, the surface dose was about 65% of the prescribed dose; when different thicknesses of bolus were used, the surface doses were slightly higher than the prescribed dose. For type 2 cases, the surface dose decreased by about 40% to 45% of the prescribed dose, whereas the dose to the rest of PTV only increased by less than 3%. For type 3 cases, the surface doses were similar to the other cases with and without the bolus. For type 4 cases, the surface dose decreased by about 40% relative to the prescribed dose. Conclusion: When the PTV touches the surface, using bolus to ensure sufficient dose coverage of the tumor volume is recommended. Conversely, when the target volume is withdrawn from the surface, adequate tumor coverage can be achieved without bolus while reducing the surface dose by about 40% to 45%.
SU‐FF‐T‐345: Agreement of Different Methods of Measuring Relative Electron Cone Output
Authors: Lightfoot D
Published: April 2010

Purpose: To determine suitability of various methods of determining relative output factors of electron cones. Method and Materials: The electron cone factors of a Varian accelerator were measured by several different methods: the daily output check device (Atlas QA3), electron diode, scatter diode, 0.125 cc ion chamber and 0.6 cc Farmer chamber. The ion chambers were used in water and plastic water with both positive and negative polarities. The diodes were used on top of plastic water with added bolus and the scatter diode was also used in a water phantom. Results: All methods had variations in relative output factor of about 0.5%. They agreed with one another within about 1% as long as the size of the detector was small relative to the cone size. The Farmer chamber gave erroneously low results for a 4×4 cone size due to a portion of the ionization chamber being within the penumbra region. Conclusion: The practice of using the most convenient set‐up (daily output check device) for verifying cut‐out factors is justified by the good agreement with other methods. Very small cut‐outs require special considerations and may require diode or film measurements for good results.
SU‐FF‐T‐350: Dose in Buildup Region of Flash: PBC, AAA Calculation Vs Measurement
Authors: Musser S , Mayo C , Ding L
Published: April 2010

Purpose: In this paper, results are presented comparing AAA and PBC dose calculation algorithms with dose measured in the buildup region where flash is of concern. Flash is of primary interest for breast, sarcomas, and whole brain radiotherapy treatments. Method and Materials: A cylindrical phantom was scanned on a GE LightSpeed Advantage Sim with slice thickness of 2.5 mm. On an Eclipse TPS (Version 8.2.24), AAA and PBC (Modified Batho inhomogeneity correction) dose calculation algorithms were used to calculate the dose distribution for photon beam energies of 6 MV and 18 MV and field size 12 × 12 cm2 with a calculation grid size of 2.5 mm. The fields were arranged as parallel opposed pair incident on a cylindrical phantom with 3 cm of flash. Dose profiles were compared at 1.5, 4.0, 6.5, and 9.0 mm depths from the end of the cylindrical phantom. Gafchromic EBT film was exposed, in the same dose planes, on a Varian Trilogy Clinac. Measured and calculated dose profiles were compared using RIT113 V5.0 Radiation Therapy Dosimetry Software. Results: Calculations show a difference in excess of 7% in dose between the two algorithms at a depth of 4 mm. The AAA produces dose distributions better resemble the measured dose distribution. Conclusion: Preliminary results indicate that the AAA algorithm more closely calculates dose in the buildup region where flash is of concern.
SU‐FF‐T‐356: Operating Parameters of a Novel CVD Diamond Detector for X‐Ray Dosimetry
Authors: Betzel G , Lansley S , Baluti F , Meyer J , Reinisch L
Published: April 2010

Purpose: To determine optimal operating parameters of a clinical x‐ray detector based on chemical vapor deposition (CVD) diamond while using conventional dosimetry instrumentation. Method and Materials: A prototype diamond detector was fabricated based upon commercially available single crystal diamond (0.5 × 3 × 3 mm3) synthesized via CVD and Perspex encapsulation that was designed to adapt to an existing Solid Water phantom used for routine clinical dosimetry. 6 MV photons from a Varian 600C linear accelerator were measured using a SSD of 90 cm and 10 cm phantom depth with a 10 × 10 cm2 field size in the central axis of the beam. A 2570/1 Farmer Dosemeter and Keithley 6430 SourceMeter were used to measure dose. Response dynamics e.g. rise and fall times, sensitivity, stability, and linearity of dose and dose rate were observed as functions of operating voltage. Results: Photocurrent rise and fall times were observed as a function of operating voltage ranging from 50 to 200 V. Differences in rise times were found to be nearly negligible. A nonlinear decrease in fall times of detector current was observed, ranging from 42 to 5 s for 100 to 200 V. Average current and sensitivity as a function of voltage followed a power‐law relationship, with average sensitivity values of 276 and 474 nC Gy−1 mm3 for 100 and 200 V, respectively. Small deviations were observed in the fitting parameter Δ for photocurrent as a function of dose rate for different voltages. Conclusion: Results indicate that using voltages of 175 V and above minimize response times and provide sufficient sensitivity with negligible dark currents. However, more investigations are needed to improve temporal response, especially below 175 V. Studies of desired properties and parameters are ongoing to evaluate and optimize detector performance.
SU‐FF‐T‐358: Evaluation of the Delta4 ™ 3D Cylindrical Phantom for Delivery Quality Assurance of Stereotactic Body Radiotherapy (SBRT) Treatments Using Helical Tomotherapy
Authors: Gutiérrez A , Calvo O , Stathakis S , Papanikolaou N
Published: April 2010

Purpose: To evaluate the use of the ScandiDos® Delta4 ™ cylindrical 3D phantom for delivery quality assurance (DQA) of stereotactic body radiotherapy (SBRT) treatments using helical tomotherapy. Method and Materials: Five (n=5) patients with single or multiple lesions in either the lung or liver were treated with SBRT techniques using the Hi‐Art II™ unit in our Institution. SBRT plans delivered dose per fraction doses in the range of 10.0–20.0Gy. For DQA plan creation, the Delta4 ™ was scanned using the MVCT of tomotherapy and imported into the tomotherapy database as a phantom. A “fine” dose grid resolution was used for the dose calculation. Patient plan, structures, and dose were exported via DICOM RT protocol and uploaded into the ScandiDos® software platform. Profiles and gamma index analysis were used for plan validation. Results: Good agreement between calculated and measured doses was observed using the Delta4 ™. Results were consistent among the ten treatment plans evaluated over a period of 6 months. No diode calibration drift was noticed. For all the cases, good agreement in the high and low dose regions were recorded. All of the DQA measurements passed with at least 95% of the diode measurements within the gamma analysis criteria of 3% or 3mm. Additionally, a 20–30 minute time saving per DQA was noted using the Delta4 ™ when compared to film and ion chamber techniques. Time reduction primarily occurs during post‐delivery processing since analysis is performed instantly after measurement. Conclusion: Our results indicate that the ScandiDos® Delta4 ™ cylindrical phantom is an effective and efficient method for the delivery quality assurance of SBRT treatments using helical tomotherapy. The capability of measuring large doses per fraction eliminates the necessity of scaling the DQA procedure thus minimizing any possible scaling errors such as those associated with MLC latency.
SU‐FF‐T‐364: Skin Dose Measurement for Partial Breast Brachytherapy Using OneDose MOSFET Dosimeter
Authors: Liu H , Anamalayil S , Doyle L , Harrison A , Cao J , Pursley J , Xiao Y , Yu Y
Published: April 2010

Purpose: To calibrate a new‐designed OneDose MOSFET dosimeter to Ir‐192 HDR radiation and use it to measure the skin dose from partial breast HDR brachytherapy. Method and Materials: A set of cylindrical phantoms of different sizes (from 2cm to 8cm diameter) were made with a central hole to hold the HDR catheter, so that the treatment distance of 1cm to 4cm can be measured. Ten OneDose MOSFET dosimeters were placed outside of the phantom equal‐distance in order to get the average reading and thus remove the inter‐dosimeter variance for each phantom. The dosimeters' build‐up side (1mm water‐equivalent thickness) was facing the HDR source. The dosimeters and phantom were immersed into water in a 30×30×30cm tank to get full scattered environment. Nucletron microSelectron HDR was used to deliver the radiation. Monte Carlo data published on CLRP website http://www.physics.carleton.ca/clrp/seed_database/Ir192_HDR/microSel ectron_v2/ was used to calculate the expected dose. The ratio of the measured dose and expected dose is the calibration factor for each distance. For patient measurement, a CT‐simulation process was performed to find a skin point and a fiducial marker was placed on that point, so that it is shown in the treatment planning CT and the expected dose from the treatment planning system (Nucletron Oncentra Brachy) can be obtained. A skin mark was made on that point so that the dosimeters placement is inter‐fraction reproducible. The measured dose is compared to the TPS calculated dose and the differences are recorded. Results: The calibration factors at different treatment distances were obtained and used for dose correction from the dosimeter reading. Two patients were measured so far and more are expected (work in progress). The expected results are that the measured dose is less than the TPS estimated dose since there are no backscatter as assumed in the TPS.
SU‐FF‐T‐366: Temporal Methods for the Elimination of the Cerenkov and Fluorescence Interference in Optical Fiber Dosimeters for External Beam Radiation Therapy
Authors: Liu T , Stern R , Yang C , Kahilainen J , Purdy J
Published: April 2010

Purpose: To compare previous temporal methods for the elimination of the Cerenkov and fluorescence interference in optical fiber dosimeters, and propose a new temporal method. Methods and Materials: When using optical fiber dosimeters in external beam radiation therapy there is an unwanted interference signal generated in the optical fiber connecting the light output from the actual dosimeter scintillating element to the photodetector readout system. This signal consists of both Cerenkov and fluorescence light both induced in the section of the fiber that is located inside the beam area and it therefore varies with changing beam geometry and intensity distribution. It is created by high energy photons reacting with the different parts of the fiber, mainly the cladding material. The most effective methods for the elimination this signal have been based on temporal i.e. time dependent separation of the interfering signal from the dose signal. In this work we present an analysis of the efficiency and practical usability of the different temporal methods that have been presented in earlier papers. Comparison between the different methods is made with actual dose measurements performed in different field geometries. A new temporal method is introduced. This new method uses new type of connection between an optical fiber and a large area scintillator panel. It eliminates the need for the connection of electrical cable, thus has a simpler configuration. Results: The signals from the scintillator panel measured by a continuous wave (CW) photodetector at 30cm/100cm are 14,000/6,000 CPS. These values are comparable to signals from a trigger detector placed inside the beam area, which typically are 7,000 – 10,000 CPS. Conclusions: New temporal method using new type of connection between an optical fiber and a large area scintillator panel provides potential improvement for the elimination of the Cerenkov and fluorescence interference in optical dosimeters.
SU‐FF‐T‐369: Electron Therapy with Hanging Lens Block for Orbital Lymphoma: Dosimetry Measurements Using Diode Arrays
Authors: Gao W , Mourad W , Chan W , Cheng M , Salloum J
Published: April 2010

Purpose: An effective technique to treat orbital lymphoma is to use an anterior electron beam with a hanging lens block. Traditionally the dose distributions under the lens block are measured with film, TLD or a stereotactic diode. The measurements are labor intensive and time consuming, considering that the dose distribution varies with several parameters, including distance between the block and the skin, electron energy used and the thickness of the beam scatterer. Multiple measurements are therefore required to optimize the dose distribution in each patient. We have developed an efficient technique to determine the optimal treatment parameters by utilizing commercially available diode arrays to instantly monitor the dose distributions of electron fields as physics parameters are varied; Methods: A cerrobend block of 1.8cm long, 1.0cm in diameter was made and placed at distances(gaps) of 0.5–2cm above patient's eye. Dose distributions for different physics parameters were measured interactively by using a two‐dimensional diode array (Mapcheck) with solid water build‐ups to observe the dose changes with varying electron energies and set‐up parameters. Once the energy, SSD and beam scatterer are determined, films, ion‐chambers and a one‐dimensional diode array(Profiler) were used to measure the dose distributions and the results were compared with measurements using the Mapcheck; Results: It was found that the effective thickness of the internal buildup in the Mapcheck device is 4mm less than the specified 2cm water equivalence for the electron beam used. After this thickness‐correction was applied, good agreement was achieved between different methods of measurement for both outside and under the block; Conclusions: The diode arrays have been proven to be the ideal devices in the initial stage of electron treatment planning to determine the optimal physics parameters that produce the desired dose distributions, with substantial savings in time and effort comparing with traditional dosimetry measurements.