ESTRO 36 Abstract Book

S282 ESTRO 36 2017 _______________________________________________________________________________________________

Purpose or Objective IROC Houston’s head and neck phantom has been used to verify IMRT dose delivery for over a decade. While the passing rate has seen gradual improvement, at present more than 10% of institution’s that irradiate this phantom still fail to meet the generous acceptability criteria. This study explores the causes of these failures. Material and Methods To pass the head and neck phantom, the TPS-calculated dose must agree within 7% with TLD measurement at 6 assessed locations in two PTVs. Two film planes must show >85% of pixels passing a 7%/4mm gamma criterion. Failed irradiations over the past year were evaluated qualitatively for the cause of the failure: positioning errors, systematic dosimetric errors, and other errors. Based on the finding that most errors were systematic dosimetric errors, further quantitative exploration was done. Potential errors in institutional TPS beam models were evaluated using an independent recalculation of the phantom’s treatment plan. This was done using “reference” beam models constructed from aggregated IROC Houston measurements for different classes of accelerator that were developed in Mobius 3D. 259 phantom plans were recalculated with these models. Results On qualitative evaluation, only 13% of failures were attributed to localization errors, 18% were other non- systematic errors, and the vast majority, 69%, were systematic dosimetric errors: the dose distribution had the right shape and was in the right place, but it was the wrong magnitude. The independent recalculation of 259 phantom plans showed many cases where our reference model was less accurate than the institution, but a shocking number of cases where our recalculation was more accurate, both significantly (based on a 2-sided t- test with a failure detection rate correction applied), and substantially (>2% average improvement across the 6 TLD). The independent recalculation was significantly and substantial better than the institution’s calculation in 18% of all cases, and in 68% of cases where the institution failed the phantom. Conclusion Failures of the IROC Houston IMRT phantom overwhelmingly indicated a deficiency in the beam model. This is concerning because this beam model is applied to all patients, suggesting suboptimal treatment at nearly 1 in 5 radiotherapy facilities. It also indicates that physicists should increase attention to beam modeling. OC-0537 A remote EPID-based dosimetric auditing method for VMAT delivery using a digital phantom concept P. Greer 1 , K. Legge 2 , N. Miri 2 , P. Vial 3 , T. Fuangrod 1 , J. Lehmann 1 1 Calvary Mater Newcastle Hospital, Department of Medical Physics, Newcastle, Australia 2 University of Newcastle, School of Physical and Mathematical Sciences, Newcastle, Australia 3 Liverpool Hospital, Radiation Oncology, Sydney, Australia Purpose or Objective Current methods to perform dosimetric audits for participation in clinical trials are expensive and time- consuming with high failure rates. Currently nearly 2/3 of trial centres in Australia/New Zealand have not had an independent VMAT dosimetric audit. The aim of this work is to develop an inexpensive new method for remote dosimetric VMAT auditing. Material and Methods Remote centres are provided with CT datasets and planning guidelines to produce trial VMAT plans for a head and neck and a post-prostatectomy treatment. The plans are transferred in the treatment planning system (TPS) to two digital water equivalent phantoms, one cylindrical (20

cm diameter) and one flat phantom. EPID cine images are then acquired during the VMAT delivery to the EPID panel in air. The cine images are backprojected to 3D dose in the digital cylindrical phantom using an established conversion method. This dose is then compared to TPS dose at a central site. Individual 2D arc doses (with gantry angles collapsed to zero in the TPS) are compared to EPID derived dose at 10 cm depth in the flat phantom. For Varian systems EPID images are obtained for Clinac systems using cine imaging mode and for Truebeam systems using image processing service which stores individual cumulative frames. Both of these systems store the gantry angle for the image in the header. For Elekta systems the service XIS software was used as the clinical cine mode normalises every image. This software acquires individual frames without a gantry angle. Three methods for gantry angle measurement were investigated, an inclinometer, a video-based method, and the machine iCom log files. The video-based method uses a printed green arrow attached to the gantry, with a mobile phone video recording during delivery. Colour detection and image processing were used to determine the angle in each movie frame. Results To date six Varian centres have been fully analysed. The 3D gamma comparison results (3%,3mm for >10% of global maximum dose) were greater than 95% pass-rate for five centres and 93% for the one centre. For the 2D individual arcs all results were greater than 95% pass-rate for 3%,2mm criteria. Three Elekta centres have had preliminary investigations. Gantry angle comparisons show that the video method is comparable to the inclinometer and is easy to perform using only a printed piece of paper and mobile phone. Challenges for the method include synchronisation of the angle measurement and the EPID frames, and difficulty of use of the XIS software.

Figure 1. Comparison of gantry angles recorded for Elekta system, inclinometer (NG360), iCom log file and mobile phone method (insert)

Figure 2. Comparison of EPID derived dose and TPS dose calculation for a Varian Truebeam system (sagittal dose plane)