John OConnor

Measurement and modeling of the effect of support arm backscatter on dosimetry with a Varian EPID

Publisher: Wiley

Date: 28-04-2010

DOI: 10.1118/1.3369445

Abstract: Amorphous silicon EPIDs have been used for planar dose verification in IMRT treatments for many years. The support arm used to attach some types of EPIDs to linear accelerators can introduce inaccuracies to dosimetry measurements due to the presence of metallic parts in their structures. It is demonstrated that this uncertainty may be as large as approximately 6% of maximum image signal for large fields. In this study, a method has been described to quantify, model and correct for the effect of backscattered radiation from the EPID support arm (E-Arm type, Varian Medical Systems). Measurements of a support arm backscatter kernel were made using several 1 x 1 cm2 6 MV pencil beam irradiations at a s le of positions over the sensitive area of the EPID in standard clinical setup and repeated with the EPID removed from the support arm but at the same positions. A curve-fit to the subtraction of EPID response obtained on and off the arm was used to define the backscatter kernel. The measured kernel was compared with a backscatter kernel obtained by Monte Carlo simulations with EGS/BEAM code. A backscatter dose prediction using the measured backscatter kernel was added to an existing EPID dose prediction model. The improvement in the agreement of the modified model predictions with EPID measurements for a number of open fields and IMRT beams were investigated by comparison to the original model results. Considering all functions tested to find the best functional fit to the data points, a broad Gaussian curve proved to be the optimum fit to the backscatter data. The best fit through the Monte Carlo simulated backscatter kernel was also found to be a Gaussian curve. The maximum decrease in normalized root mean squared deviation of the measured and modeled EPID image profiles for open fields was 13.7% for a 15 x 15 cm2 field with no decrease observed for a 3 x 3 cm2 (the smallest) field as it was not affected by the arm backscatter. Gamma evaluation (2%, 2 mm criteria) showed the improvement in agreement between the model and measurement results when the backscatter was incorporated. The average increase in Gamma pass rate was 2% for head and neck and 1.3% for prostate IMRT fields investigated in this study. The application of the backscatter kernel determined in this study improved the accuracy of dosimetry using a Varian EPID with E-arm for open fields of different sizes: Eight head and neck and seven prostate IMRT fields. Further improvement in the agreement between the model predictions and EPID measurements requires more sophisticated modeling of the backscatter.

Deposition and characterization of Ti2AlC MAX phase and Ti3AlC thin films by magnetron sputtering

Publisher: Elsevier BV

Date: 09-2016

DOI: 10.1016/J.MATLET.2016.05.086

An EPID-based system for gantry-resolved MLC quality assurance for VMAT

Publisher: Wiley

Date: 09-2016

DOI: 10.1120/JACMP.V17I5.6312

The damage evolution of He irradiation on Ti3SiC2 as a function of annealing temperature

Publisher: Elsevier BV

Date: 04-2018

DOI: 10.1016/J.JEURCERAMSOC.2017.11.041

Commissioning and quality assurance for VMAT delivery systems: An efficient time‐resolved system using real‐time EPID imaging

Publisher: Wiley

Date: 10-07-2017

DOI: 10.1002/MP.12387

Abstract: An ideal commissioning and quality assurance (QA) program for Volumetric Modulated Arc Therapy (VMAT) delivery systems should assess the performance of each in idual dynamic component as a function of gantry angle. Procedures within such a program should also be time-efficient, independent of the delivery system and be sensitive to all types of errors. The purpose of this work is to develop a system for automated time-resolved commissioning and QA of VMAT control systems which meets these criteria. The procedures developed within this work rely solely on images obtained, using an electronic portal imaging device (EPID) without the presence of a phantom. During the delivery of specially designed VMAT test plans, EPID frames were acquired at 9.5 Hz, using a frame grabber. The set of test plans was developed to in idually assess the performance of the dose delivery and multileaf collimator (MLC) control systems under varying levels of delivery complexities. An in-house software tool was developed to automatically extract features from the EPID images and evaluate the following characteristics as a function of gantry angle: dose delivery accuracy, dose rate constancy, beam profile constancy, gantry speed constancy, dynamic MLC positioning accuracy, MLC speed and acceleration constancy, and synchronization between gantry angle, MLC positioning and dose rate. Machine log files were also acquired during each delivery and subsequently compared to information extracted from EPID image frames. The largest difference between measured and planned dose at any gantry angle was 0.8% which correlated with rapid changes in dose rate and gantry speed. For all other test plans, the dose delivered was within 0.25% of the planned dose for all gantry angles. Profile constancy was not found to vary with gantry angle for tests where gantry speed and dose rate were constant, however, for tests with varying dose rate and gantry speed, segments with lower dose rate and higher gantry speed exhibited less profile stability. MLC positional accuracy was not observed to be dependent on the degree of interdigitation. MLC speed was measured for each in idual leaf and slower leaf speeds were shown to be compensated for by lower dose rates. The test procedures were found to be sensitive to 1 mm systematic MLC errors, 1 mm random MLC errors, 0.4 mm MLC gap errors and synchronization errors between the MLC, dose rate and gantry angle controls systems of 1°. In general, parameters measured by both EPID and log files agreed with the plan, however, a greater average departure from the plan was evidenced by the EPID measurements. QA test plans and analysis methods have been developed to assess the performance of each dynamic component of VMAT deliveries in idually and as a function of gantry angle. This methodology relies solely on time-resolved EPID imaging without the presence of a phantom and has been shown to be sensitive to a range of delivery errors. The procedures developed in this work are both comprehensive and time-efficient and can be used for streamlined commissioning and QA of VMAT delivery systems.

Defect behavior and radiation tolerance of MAB phases (MoAlB and Fe2AlB2) with comparison to MAX phases

Publisher: Elsevier BV

Date: 09-2020

DOI: 10.1016/J.ACTAMAT.2020.07.002

Reduction of the effect of non-uniform backscatter from an E-type support arm of a Varian a-Si EPID used for dosimetry

Publisher: IOP Publishing

Date: 20-10-2010

DOI: 10.1088/0031-9155/55/22/003

Abstract: Backscatter from the metallic components in the support arm is one of the sources of inaccuracy in dosimetry with Varian amorphous silicon electronic portal imaging devices (a-Si EPIDs). In this study, the non-uniform arm backscatter is blocked by adding lead sheets between the EPID and an E-type support arm. By comparing the EPID responses on and off the arm, with and without lead and considering the extra weight on the imager, 2 mm of lead was determined as the optimum thickness for both 6 and 18 MV beam energies. The arm backscatter at the central axis with the 2 mm lead in place decreased to 0.1% and 0.2% for the largest field size of 30 × 30 cm(2) using 6 and 18 MV beams, from 2.3% and 1.3% without lead. Changes in the source-to-detector distance (SDD) did not affect the backscatter component more than 1%. The symmetry of the in-plane profiles improved for all field sizes for both beam energies. The addition of lead decreased the contrast-to-noise ratio and resolution by 1.3% and 0.84% for images taken in 6 MV and by 0.5% and 0.38% for those in 18 MV beams. The displacement of the EPID central pixel was measured during a 360° gantry rotation with and without lead which was 1 pixel different. While the backscatter reduces with increasing lead thickness, a 2 mm lead sheet seems sufficient for acceptable dosimetry results without any major degradation to the routine performance of the imager. No increase in patient skin dose was detected.

Damage accumulation and recovery in C+-irradiated Ti3SiC2

Publisher: Elsevier BV

Date: 03-2014

DOI: 10.1016/J.ACTAMAT.2013.11.019

Modeling and measurement of Al interlayer diffusion in Pd(100): A low-energy ion scattering study

Publisher: American Physical Society (APS)

Date: 28-04-2004

DOI: 10.1103/PHYSREVB.69.165418

Surface damage of Ti3SiC2 by MeV iodine bombardment

Publisher: Elsevier BV

Date: 07-2013

DOI: 10.1016/J.NIMB.2013.03.021

Improvement of Varian a-Si EPID dosimetry measurements using a lead-shielded support-arm

Publisher: Elsevier BV

Date: 06-2012

DOI: 10.1016/J.MEDDOS.2011.06.003

Radiation damage of MoAlB at elevated temperatures: Investigating MAB phases as potential neutron shielding materials

Publisher: Elsevier BV

Date: 04-2022

DOI: 10.1016/J.JEURCERAMSOC.2021.11.017

Investigation of the sag in linac secondary collimator and MLC carriage during arc deliveries

Publisher: IOP Publishing

Date: 30-05-2012

DOI: 10.1088/0031-9155/57/12/N209

Near Surface Changes Due to 700 keV Si⁺Irradiation of Titanium Silicon Carbide

Publisher: Wiley

Date: 06-08-2015

DOI: 10.1111/JACE.13793

Deuterium retention in the carbon co-deposition layers deposited by magnetron sputtering in D2/He atmosphere

Publisher: Elsevier BV

Date: 05-2013

DOI: 10.1016/J.JNUCMAT.2013.01.327

Detection and correction for EPID and gantry sag during arc delivery using cine EPID imaging

Publisher: Wiley

Date: 11-01-2012

DOI: 10.1118/1.3673958

An independent system for real-time dynamic multileaf collimation trajectory verification using EPID

Publisher: IOP Publishing

Date: 12-12-2013

DOI: 10.1088/0031-9155/59/1/61

Abstract: A new tool has been developed to verify the trajectory of dynamic multileaf collimators (MLCs) used in advanced radiotherapy techniques using only the information provided by the electronic portal imaging devices (EPID) measured image frames. The prescribed leaf positions are res led to a higher resolution in a pre-processing stage to improve the verification precision. Measured MLC positions are extracted from the EPID frames using a template matching method. A cosine similarity metric is then applied to synchronise measured and planned leaf positions for comparison. Three additional comparison functions were incorporated to ensure robust synchronisation. The MLC leaf trajectory error detection was simulated for both intensity modulated radiation therapy (IMRT) (prostate) and volumetric modulated arc therapy (VMAT) (head-and-neck) deliveries with anthropomorphic phantoms in the beam. The overall accuracy for MLC positions automatically extracted from EPID image frames was approximately 0.5 mm. The MLC leaf trajectory verification system can detect leaf position errors during IMRT and VMAT with a tolerance of 3.5 mm within 1 s.

Phason mechanism for diffusion in quasicrystalline structures

Publisher: Elsevier BV

Date: 11-2020

DOI: 10.1016/J.SSC.2020.114017

Step mediated surface alloy formation of Pt/Cu(111)

Publisher: Elsevier BV

Date: 08-2005

DOI: 10.1016/J.SUSC.2005.05.034

Damage tolerance of Ti3SiC2 to high energy iodine irradiation

Publisher: Elsevier BV

Date: 06-2012

DOI: 10.1016/J.APSUSC.2012.03.022

Intelligent methods for solving inverse problems of backscattering spectra with noise: A comparison between neural networks and simulated annealing

Publisher: Springer Science and Business Media LLC

Date: 03-12-2009

DOI: 10.1007/S00521-008-0219-X

Growth and alloying of submonolayer Cu on Pt(121211)

Publisher: Elsevier BV

Date: 03-2004

DOI: 10.1016/J.VACUUM.2003.12.028

Real-time in vivo rectal wall dosimetry using MOSkin detectors during linac based stereotactic radiotherapy with rectal displacement

Publisher: Springer Science and Business Media LLC

Date: 27-02-2017

DOI: 10.1186/S13014-017-0781-4

A system for EPID-based real-time treatment delivery verification during dynamic IMRT treatment

Publisher: Wiley

Date: 09-08-2013

DOI: 10.1118/1.4817484

Abstract: To design and develop a real-time electronic portal imaging device (EPID)-based delivery verification system for dynamic intensity modulated radiation therapy (IMRT) which enables detection of gross treatment delivery errors before delivery of substantial radiation to the patient. The system utilizes a comprehensive physics-based model to generate a series of predicted transit EPID image frames as a reference dataset and compares these to measured EPID frames acquired during treatment. The two datasets are using MLC aperture comparison and cumulative signal checking techniques. The system operation in real-time was simulated offline using previously acquired images for 19 IMRT patient deliveries with both frame-by-frame comparison and cumulative frame comparison. Simulated error case studies were used to demonstrate the system sensitivity and performance. The accuracy of the synchronization method was shown to agree within two control points which corresponds to approximately ∼1% of the total MU to be delivered for dynamic IMRT. The system achieved mean real-time gamma results for frame-by-frame analysis of 86.6% and 89.0% for 3%, 3 mm and 4%, 4 mm criteria, respectively, and 97.9% and 98.6% for cumulative gamma analysis. The system can detect a 10% MU error using 3%, 3 mm criteria within approximately 10 s. The EPID-based real-time delivery verification system successfully detected simulated gross errors introduced into patient plan deliveries in near real-time (within 0.1 s). A real-time radiation delivery verification system for dynamic IMRT has been demonstrated that is designed to prevent major mistreatments in modern radiation therapy.

Structural changes of Ti3SiC2 induced by helium irradiation with different doses

Publisher: Elsevier BV

Date: 03-2018

DOI: 10.1016/J.APSUSC.2017.11.170

Verification of the linac isocenter for stereotactic radiosurgery using cine-EPID imaging and arc delivery

Publisher: Wiley

Date: 17-06-2011

DOI: 10.1118/1.3597836

Abstract: Verification of the mechanical isocenter position is required as part of comprehensive quality assurance programs for stereotactic radiosurgery/radiotherapy (SRS/SRT) treatments. Several techniques have been proposed for this purpose but each of them has certain drawbacks. In this paper, a new efficient and more comprehensive method using cine-EPID images has been introduced for automatic verification of the isocenter with sufficient accuracy for stereotactic applications. Using a circular collimator fixed to the gantry head to define the field, EPID images of a Winston-Lutz phantom were acquired in cine-imaging mode during 3600 gantry rotations. A robust MATLAB code was developed to analyze the data by finding the center of the field and the center of the ball bearing shadow in each image with sub-pixel accuracy. The distance between these two centers was determined for every image. The method was evaluated by comparison to results of a mechanical pointer and also by detection of a manual shift applied to the phantom position. The repeatability and reproducibility of the method were tested and it was also applied to detect couch and collimator wobble during rotation. The accuracy of the algorithm was 0.03 +/- 0.02 mm. The repeatability was less than 3 pm and the reproducibility was less than 86 microm. The time elapsed for the analysis of more than 100 cine images of Varian aS1000 and aS500 EPIDs were approximately 65 and 20 s, respectively. Processing of images taken in integrated mode took 0.1 s. The output of the analysis software is printable and shows the isocenter shifts as a function of angle in both in-plane and cross-plane directions. It gives warning messages where the shifts exceed the criteria for SRS/SRT and provides useful data for the necessary adjustments in the system including bearing system and/or room lasers. The comprehensive method introduced in this study uses cine-images, is highly accurate, fast, and independent of the observer. It tests all gantry angles and is suitable for pretreatment QA of the isocenter for stereotactic treatments.

Dose-to-water conversion for the backscatter-shielded EPID: A frame-based method to correct for EPID energy response to MLC transmitted radiation

Publisher: Wiley

Date: 25-07-2014

DOI: 10.1118/1.4890677

Abstract: To develop a frame-by-frame correction for the energy response of amorphous silicon electronic portal imaging devices (a-Si EPIDs) to radiation that has transmitted through the multileaf collimator (MLC) and to integrate this correction into the backscatter shielded EPID (BSS-EPID) dose-to-water conversion model. In idual EPID frames were acquired using a Varian frame grabber and iTools acquisition software then processed using in-house software developed inMATLAB. For each EPID image frame, the region below the MLC leaves was identified and all pixels in this region were multiplied by a factor of 1.3 to correct for the under-response of the imager to MLC transmitted radiation. The corrected frames were then summed to form a corrected integrated EPID image. This correction was implemented as an initial step in the BSS-EPID dose-to-water conversion model which was then used to compute dose planes in a water phantom for 35 IMRT fields. The calculated dose planes, with and without the proposed MLC transmission correction, were compared to measurements in solid water using a two-dimensional diode array. It was observed that the integration of the MLC transmission correction into the BSS-EPID dose model improved agreement between modeled and measured dose planes. In particular, the MLC correction produced higher pass rates for almost all Head and Neck fields tested, yielding an average pass rate of 99.8% for 2%/2 mm criteria. A two-s le independent t-test and fisher F-test were used to show that the MLC transmission correction resulted in a statistically significant reduction in the mean and the standard deviation of the gamma values, respectively, to give a more accurate and consistent dose-to-water conversion. The frame-by-frame MLC transmission response correction was shown to improve the accuracy and reduce the variability of the BSS-EPID dose-to-water conversion model. The correction may be applied as a preprocessing step in any pretreatment portal dosimetry calculation and has been shown to be beneficial for highly modulated IMRT fields.

Submonolayer alloying of copper on vicinal platinum: A combined atom and ion scattering study

Publisher: American Physical Society (APS)

Date: 07-09-2004

DOI: 10.1103/PHYSREVB.70.125403

Structure and deuterium retention properties of tungsten layers deposited by plasma sputtering in a mixed atmosphere of D2 and He

Publisher: Elsevier BV

Date: 03-2014

DOI: 10.1016/J.JNUCMAT.2013.12.008

Annealing effects on the structure and hardness of helium‐irradiated Cr ₂ AlC thin films

Publisher: Wiley

Date: 15-09-2020

DOI: 10.1111/JACE.17469

The University Of Newcastle

Location: Australia

View Organisation

Discovery Projects - Grant ID: DP120100487

Start Date: 06-2012

End Date: 06-2015

Amount: $340,000.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE0453521

Start Date: 2003

End Date: 12-2004

Amount: $508,374.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP0208681

Start Date: 2002

End Date: 07-2005

Amount: $175,000.00

Funder: Australian Research Council

Linkage Projects - Grant ID: LP0562653

Start Date: 09-2005

End Date: 08-2009

Amount: $206,433.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE0883095

Start Date: 2008

End Date: 08-2009

Amount: $750,000.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE170100205

Start Date: 2017

End Date: 08-2018

Amount: $295,000.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE0237874

Start Date: 2001

End Date: 12-2002

Amount: $424,000.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE120100229

Start Date: 06-2012

End Date: 12-2013

Amount: $250,000.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE0668510

Start Date: 06-2006

End Date: 04-2007

Amount: $180,240.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE0668257

Start Date: 06-2006

End Date: 06-2007

Amount: $1,200,000.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE0989861

Start Date: 2009

End Date: 12-2010

Amount: $650,000.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE0775646

Start Date: 2007

End Date: 12-2009

Amount: $636,000.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP0559514

Start Date: 07-2005

End Date: 06-2008

Amount: $300,000.00

Funder: Australian Research Council