Emeritus Professor John O'Connor

Purpose: An ideal commissioning and quality assurance (QA) program for Volumetric Modulated Arc Therapy (VMAT) delivery systems should assess the performance of each individual 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. Methods: 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 individually 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. Results: 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 individual 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. Conclusion: QA test plans and analysis methods have been developed to assess the performance of each dynamic component of VMAT deliveries individually 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.

Multileaf collimator (MLC) positions should be precisely and independently measured as a function of gantry angle as part of a comprehensive quality assurance (QA) program for volumetric-modulated arc therapy (VMAT). It is also ideal that such a QA program has the ability to relate MLC positional accuracy to patient-specific dosimetry in order to determine the clinical significance of any detected MLC errors. In this work we propose a method to verify individual MLC trajectories during VMAT deliveries for use as a routine linear accelerator QA tool. We also extend this method to reconstruct the 3D patient dose in the treatment planning system based on the measured MLC trajectories and the original DICOM plan file. The method relies on extracting MLC positions from EPID images acquired at 8.41fps during clinical VMAT deliveries. A gantry angle is automatically tagged to each image in order to obtain the MLC trajectories as a function of gantry angle. This analysis was performed for six clinical VMAT plans acquired at monthly intervals for three months. The measured trajectories for each delivery were compared to the MLC positions from the DICOM plan file. The maximum mean error detected was 0.07 mm and a maximum root-mean-square error was 0.8 mm for any leaf of any delivery. The sensitivity of this system was characterized by introducing random and systematic MLC errors into the test plans. It was demonstrated that the system is capable of detecting random and systematic errors on the range of 1-2mm and single leaf calibration errors of 0.5 mm. The methodology developed in the work has potential to be used for efficient routine linear accelerator MLC QA and pretreatment patient-specific QA and has the ability to relate measured MLC positional errors to 3D dosimetric errors within a patient volume.

Purpose: To demonstrate a new method for real-time geometric and dosimetric verification of IMRT and VMAT using synchronization between predicted and measured EPID images. Methods: Predicted EPID images were calculated using a comprehensive physics-based model. Each predicted image represents the integrated signal expected from the delivery between control points. The measured images are acquired in cine mode and compared to the set of predicted images in real-time. The system performs geometric verification prior to dosimetric verification. When the measured image is acquired, the algorithm automatically detects the MLC leaf positions. A comparison between the leaf positions of the measured image and control points in the MLC file is made using the cosine similarity technique. The similarity index(SI) provides geometric MLC verification and synchronization between the measured and predicted images, as a uniform dose-rate cannot be assumed for IMRT or VMAT deliveries. The SI threshold was based on a series of experiments including 21 dynamic-IMRT fields defining pass/fail boundary(5 brain, 8 H&N, and 8 prostate cases).If geometric verification is successful, dosimetic verification is performed with the Gamma comparison(3%,3mm).The system reports the verification Result in real-time. Results: The system was simulated by MATLAB/SIMULINK and detected geometric and dosimetric errors during delivery. Both artificially introduced errors and clinical data were used for testing and analysis of the system performance. For a tested prostate field, the cumulative dose comparisons showed the minimum and maximum number of points with Gamma index<1 as 93.5% and 98.5%, respectively. For individual dose comparisons on the same field, the values were 87% and 97%, respectively. Conclusion: This method includes automatic MLC leaf positioning, synchronization, and dosimetric verification. The pass/fail boundary of geometry was calculated based on the experiments. This system is a useful approach to detect unexpected possible errors occurring in the clinical setting and to prevent patient overdoses during radiotherapy especially in complex deliveries such as arc-IMRT. © 2012, American Association of Physicists in Medicine. All rights reserved.

Purpose: A cine-EPID based method to separately measure the mechanical motion characteristics of gantry, jaws and multi-leaf collimator as a function of gantry angle during VMAT has been developed. Methods: Irradiations were performed using 6 MV beams of a Varian Trilogy linear accelerator with an aS1000 EPID. Images were acquired using 360 MU irradiations at 600 MU/min in cine acquisition mode at 2 Hz frame-rate. To establish the gantry isocentre, a Winston-Lutz technique was used with a circular collimator rigidly attached to the gantry head. The displacement of the centre of a fixed tungsten ball at isocentre from the field centre was determined on each image with a sub-pixel thresholding technique. EPID coordinates were transformed to room coordinates. Jaw and MLC sag relative to the gantry head were determined from the displacement of a gantry-mounted tungsten ball relative to static jaw and MLC positions. MLC speed constancy was determined by segmenting MLC positions on each image for a constant leaf-speed test pattern. Gantry speed constancy was assessed with an independent liquid-based inclinometer and the linac gantry angle potentiometer. Results: The gantry isocentre was ~ 1 mm amplitude with changes in isocentre occuring over time suggesting frequent measurement is required. Jaw sag was found to be very small ~ 0.2 mm amplitude, with MLC sag ~ 0.6 mm. Average leaf speed was found to be consistent for the MLC leaves however the variation in speed varied systematically across the leaf bank which requires further investigation. Gantry speed was consistent although the inclinometer was found to lag ~ 2 degrees in reading from the potentiometer. Conclusions: The cine-EPID based method quantifies the mechanical motion characteristics of the linac as a function of gantry angle during arc-therapy with improved precision and efficiency of quality assurance over previous methods. © 2011, American Association of Physicists in Medicine. All rights reserved.

The backscattered radiation from the support arm of Varian a-Si EPIDs can affect the accuracy of dosimetric measurements using these devices. In this study the effect of insertion of lead sheets between the EPID and the arm has been investigated for the E-type arms. The optimum lead thickness was determined by comparison of the imager response on and off the arm with increasing lead thicknesses and 2 mm of lead was selected as the optimal thickness considering the reasonable extra weight added to the imager. It reduced the arm backscatter from a maximum of about 6% and 3.5% higher than the off-arm signal in 6 MV and 18 MV beams to about 2% for both energies. On-axis EPID response measurements for different field sizes showed a considerable decrease in arm backscatter with the addition of lead. The symmetry improved for the largest field from about 105% and 103% to 101% and 100% using 2 mm lead. Changing the SDD did not affect the backscatter component more than 1%. The addition of lead decreased the contrast-to-noise ratio and resolution by 1.3% and 0.8% for 6 MV and by 0.5% and 0.4% for 18 MV beams. The root mean square deviation for the difference in EPID central pixel position with and without lead during a whole gantry rotation was one pixel at maximum. In conclusion a 2 mm thick lead layer seems sufficient for acceptable dosimetry results with no major degradation to the routine performance of the imager. © 2010, American Association of Physicists in Medicine. All rights reserved.

Over the past 25-30 years, Australia has experienced a significant decline in the percentage of senior secondary school students electing to study the enabling sciences and higher level mathematics that are the career pre-requisites for Engineering and Science. Without arresting this decline and increasing the size of this 'pool' of students, there can be no significant improvement in the present situation. The Australian "Science and Engineering Challenge" has grown from small beginnings in New South Wales in 2000 as a secondary school outreach program for the Faculties of Engineering and Science at the University of Newcastle. Supported by the Australian Federal Government and industry with 27 partner universities, numerous Rotary Clubs and an army of thousands of volunteers (including retired professional scientists and engineers) it reaches 18,000+ year 9 and 10 secondary students each year in 40 venues across the nation. It is the largest national outreach program for secondary students and is managed by the University of Newcastle. Whilst the underlying goal is to increase the national pool of science and mathematics 'enabled' year 12 students, the immediate goal of the program each day is to implant a seed in the minds of the students who compete in fun-filled activities based on sound science and mathematics principles. The 'seed' is that, scientists and engineers actually do interesting and challenging things and to instill a positive mental image that "I too could become a scientist or an engineer!" The range of activities available in a typical Challenge event are demonstrated as is the operational structure that sees regional competitions leading annually to a national competitive event for outstanding secondary schools. The real impact on the decisions students make in their final year 12 subject selections in the enabling sciences are also demonstrated.