ESTRO 2020 Abstract Book

S355 ESTRO 2020

OC-0636 Highly spatially resolved 2D plastic scintillation detector system for small field real-time QA P. Pittet 1 , J. Ribouton 2 , P. Jalade 2 , J. Esteves 1 , F. Blanc 3 , G. Haefeli 3 , P. Hopchev 3 , J. Galvan 4 , G. Lu 1 1 Institut des Nanotechnologies de Lyon, Université Claude Bernard Lyon 1, Villeurbanne, France ; 2 Hospices Civils de Lyon, Centre Hospitalier Lyon Sud, Pierre Bénite, France ; 3 EPFL, Laboratoire de Physique des Hautes Energies LPHE, Lausanne, Switzerland ; 4 Institut des Nanotechnologies de Lyon, CPE Lyon, Villeurbanne, France Purpose or Objective Beam characterization during commissioning and periodic QA verifications for SRS treatment equipment relies nowadays on time consuming procedures. To address this problem, we propose the use of the SciFi detector, a real- time 2D detector (initially developed for the LHCb experiment at CERN) for commissionning and QA of SRS systems. Material and Methods The SciFi detector is made of tissue-equivalent scintillating plastic fibers arranged in a staggered geometry to form a ribbon with a fiber pitch of 275µm as shown in Fig. 1. For TMR measurements, a single SciFi ribbon is placed in a RW3 solid-water plate arrangement and is irradiated from the side of the ribbon. For small field monitoring, up to 6 SciFi ribbons are stacked at the depth of maximum dose in a RW3 phantom with in-plane 30° tilted orientations. The scintillating signal at the detector output which represents the projected irradiation profile along the fibers axis is acquired by either a sCMOS camera or Si phototodiode linear arrays (with an element pitch of 400µm). The acquired images allow an iterative tomographic reconstruction of the field for QA procedures. The detector system has been tested with a 6MV photon beam for measuring TMR curves on the one hand and for characterization of SRS cone collimators on the other.

measurements (within ±0.32mm for both FWHM and penumbra region). The system can thus serve to verify the reproducibility of conical collimator insertion into mount. Measured output factors (OF) with the proposed approach are within ±1.6% for all the tested cone sizes as compared with OF determined independently by IRSN.

Conclusion We have tested a SciFi plastic scintillating detector for beam monitoring during QA and commissioning procedures of SRS treatment. It allows implementation of accurate and time-efficient QA procedures for determining TMR curves, FWHM, penumbra and OF for fields based on SRS cone collimators. As the SciFi detector is tissue equivalent, it could be implemented for full End-To-End testing of SRS treatment.

OC-0637 Identifying links between beam modeling accuracy and IROC phantom performance

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OC-0638 Multivariate log file analysis for MLC failure prediction A. Wojtasik 1 , M. Bolt 2 , C.H. Clark 2 , A. Nisbet 3 , T. Chen 1 1 University of Surrey, Chemical and Process Engineering, Guildford, United Kingdom ; 2 Royal Surrey County Hospital, Medical Physics, Guildford, United Kingdom ; 3 University College London, Department of Medical Physics & Biomedical Engineering, London, United Kingdom Purpose or Objective MLC failures are a relatively common cause of radiotherapy disruption. Where MLC trajectory log file data of a linac is available, it may be beneficial in helping to predict future MLC failure. A multivariable approach, comparing individual leaf behaviour to that of other surrounding leaves/MLC carriage could help distinguish uncharacteristic leaf behaviour from natural variation. This in turn has the potential to allow for proactive MLC maintenance, lessening the potential impact on patient treatment time. Material and Methods 2 months of trajectory log data from an operational Varian TrueBeam linac has been collected, encompassing all 538 treatments delivered. Based on the logs, the average error in each of the 130 parameters for each treatment was calculated. In order to track the changes in parameter behaviour over time, a moving window approach to the analysis was employed. The resultant dataset was then standardised via z-scoring. This was performed to minimise the impact of varying measurement units for different

Results The sCMOS camera is suitable for high resolution readout of the SciFi detector (40µm/pixel), but requires 20MU to acquire images for fields as small as 4mm. The readout based on the photodiode array provides a much higher sensitivity (a few MU are sufficient for a good signal to noise ratio) with a lower 400µm resolution. The SciFi detector system used for simultaneous measurement of the on-axis full TMR curve has a high resolution in depth. The TMR curve shown in Fig. 2 is measured within 2s, which is much shorter than for 3-axis water scanning phantom based procedures. Moreover, the obtained curve is particularly well defined in the build-up region due to the 275µm fiber pitch. The SciFi detector system is also used for SRS field monitoring for 4, 5, 6, 7.5, 10, 12.5 and 15mm cone collimators: results are in good agreement with EBT3 film