ESTRO 2020 Abstract Book

S707 ESTRO 2020

Quirónsalud- Barcelona- Spain & 2.Servicio de Oncología Radioterápica- Hospital Universitari Dexeus- Barcelona- Spain, Servicio de Oncología Radioterápica, Barcelona, Spain ; 2 1. Department of Radiation Oncology- Calvary Mater Newcastle Hospital- Newcastle- NSW 2298- Australia & 2.School of Mathematical and Physical Sciences- University of Newcastle- Newcastle- NSW 2300- Australia, 1. Department of Radiation Oncology- Cal, ; 3 Servei de Física i Protecció Radiològica. Hospital Vall d'Hebron. Barcelona Spain, Servei de Física i Protecció Radiològica. Hospital Vall d'Hebron. Barcelona Spain, Barcelona, Spain ; 4 1.Servicio de Oncología Radioterápica- Hospital Quirónsalud- Barcelona- Spain & 2.Servicio de Oncología Radioterápica- Hospital Universitari Dexeus- Barcelona- Spain, 1.Servicio de Oncología Radioterápica- Hospital Quirónsalud- Barcelona- Spain & 2.S, Purpose or Objective Pre-treatment dosimetric verification of multi-target SRS plans is a challenging issue that would require 3D matrix devices to get full information about the dose delivery accuracy for all targets. The aim of this study is to verify the accuracy of a virtual water phantom –based method to perform this kind of verification. Material and Methods The VIrtual Phantom Epid dose Reconstruction (VIPER) software has been used for pre-treatment IMRT/VMAT verification [Phys Med Biol 2017; 62:4293-4299]. VIPER back-projects in-air acquired images from EPIDs into a virtual water phantom and converts the signals to dose to water within the phantom. The conversion is performed based on a model developed at Calvary Mater Newcastle Hospital. Fifteen multi-target SRS cases treated in our department were retrospectively selected for this study. SRS plans consisted of multiple non-coplanar IMRT fields treating multiple brain lesions (range: 2-35) with a single- isocenter. Target diameters ranged from 0.4 cm to 4.0 cm. Plans were computed using the AAA algorithm of the Eclipse TPS (v. 13.7), using 6 MV beams from a Varian Clinac equipped with a Millennium 120 MLC and an aSi-500 EPID. Each SRS plan was mapped to a virtual cylindrical water phantom of 20 cm-diameter (VCP20) and re-computed in Eclipse using the same monitor units as the original plan. This verification plan was delivered onto the EPID without any phantom. The verification plan and its air-acquired EPID images were imported into the VIPER software (v. 3.10 beta, May 2019) that reconstructs the 3D dose distribution within the VCP20. This VIPER plan was then sent to the PRIMO Monte-Carlo software (v. 3.1.0.1772) to be simulated. PRIMO simulations were done including the actual beam delivery reported by the Dynalogs files recorded for each SRS plan. PRIMO was validated for dose calculations and dose accuracy within 2.8% was found [Radiat Oncol. 2018 Aug 7;13(1):144]. Agreement between VIPER doses and PRIMO doses was assessed using the 3D gamma tool of the PRIMO software. Gamma analysis was done in the regions (ROIs) of the VCP20 receiving at least 10%, 20%, 30%, 50%, 70%, 80% and 90% of the maximum dose (100%) computed by the VIPER software. Criteria of 3% and 5% (global) and 2 mm (distance-to-agreement) were used. Results Table 1 shows the gamma passing rates (GPRs) for each region. GPRs greater than 92% and 97% were found in all analyzed ROIs for 3%/2 mm and 5%/2 mm, respectively. Figure 1 illustrates the 5%/2 mm-GPRs for the 15 cases enrolled in this study. Statistical uncertainty (k= 2) of the dose calculated by PRIMO ranged from 1.6% to 3.7%.

Conclusion 1.

Criteria of 5% (global)/2 mm are more adequate than 3% (global)/2 mm to be used due to the statistical uncertainty attained for the PRIMO simulations. The VIPER software is a reliable method to do a pre-treatment dosimetric verification of multi- target SRS plans in a straightforward way. phantom-based measurements for multi-target SRS plans have been published. Novelty: no virtual

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PO-1338 Validation of the rotation induced couch shift with the Machine Performance Check (MPC) tool M. Parisotto 1 , M. Valenti 1 , G. Mara 1 , R. Luca 1 , M. Vittoria Emanuela 1 , G. Kukiriza 2 , D. Aramini 1 , S. Maggi 1 1 Ospedali Riuniti, Medical Physics, Ancona, Italy ; 2 International Center of Theoretical Physics, Medical Physics, Trieste, Italy Purpose or Objective To provide an assessment of the beam central axis offset induced by couch rotation and compare the outcomes against the Machine Performance Check (MPC) tool integrated in Truebeam (TB) LINAC. Material and Methods Measurements were conducted on two LINACs: a TB STX, equipped with PerfectPitch Couch with six degrees of freedom (DoF), and a TB with a four-DoF couch. A Gafchromic EBT3 film was located on the couch at SSD 100 cm and irradiated using an asymmetrical field collimated by jaws with X1=-0.5, X=0.5, Y1=-6, Y2=7 cm, with couch rotation of 270°, 315°, 0°, 45° and 90° around the Y-axis (IEC 61217) and collimator angles of 240°, 0° and 120°. After exposures, the film showed fifteen dark not- overlapping regions of about 1x1 cm 2 , visually arranged in a circle with radius of 6.5 cm (fig. 1). An Epson 10000XL