ESTRO 2020 Abstract Book

S713 ESTRO 2020

Material and Methods The beam data was acquired on a 6FFF x-ray beam from a Varian Halcyon using the 3D-Scanner w.p. from Sun Nuclear (SN). Depth dose curves, crossline and diagonal dose profiles at different depths and fixed SSD=90cm were measured using a SNC125c ion chamber from 4x4 cm 2 to the maximum allowed field size of 28x28 cm 2 .Output factors were measured at SSD=95cm and 5 cm depth. The nominal linac output and the beam quality index were determined according to TRS-398. A total of seven modulated plans were verified using the ArcCheck solution from SN, in combination with the preconfigured portal dosimetry system. Absolute point dose measurements were carried out on a solid wp at discrete points using a pinpoint ion chamber. Results The measured depth dose curves, dose profiles and diagonal profiles showed very good agreement with the preconfigured beam data, differences were within 1%- 1mm. Field output factors differences were less than 0.5% for all the field sizes measured with a mean dose difference of - 0.1%. Measured linac output for the 10x10 cm 2 square field size at SSD=90cm and 10cm depth was 1cGy per MU, in concordance with machine calibration specifications from the vendor.The output was also verified calculating the dose on a virtual water phantom on the TPS with a 0.4% difference against reference value. The difference between the in-water measured quality index TPR 20, 10 of 0.627 and the one calculated on a virtual water phantom from isocentric doses at 10 cm and 20 cm depth was 0.77%. The ArcCheck measured data from modulated plans were compared to the planned dose using the SNCPatient software leading to a mean value of 99.8% points passing 3%-3mm gamma evaluation with a 10% threshold. The portal dosimetry for the same tests gave a mean value of 99.8% points passing 3%-3mm gamma evaluation in absolute dose mode. The mean difference of the point doses measured with a pinpoint chamber 31016, inserted into the Easycube phantom, to the calculated doses was -0.12% with a maximum difference of -1.13%.

Conclusion The mean measured surface dose is 141±22 cGy, with a deviation of -30% from prescribed dose. If the clinical goal is to have a better dose coverage at the evaluated depth (0,15mm for EBT3) alternative approaches must be considered, as the use of bolus material [4,5] or the use of a virtual bolus strategy for planning [2,6]. Bibliography [1] Lewis et al. - Med Phys. 2012 Oct;39(10):6339-50 [2] Zibold et al. - Strahlenther Onkol. 2009 Sep;185(9):574- 81 [3] Ramsey et al. - Med Phys. 2007 Aug;34(8):3286-93 [4] Cheek et al. - Med Phys. 2008 Aug;35(8):3565-73 [5] Galle et al. - Technol Cancer Res Treat. 2016 Jun;15(3):411-5 [6] Moliner er al. - J Appl Clin Med Phys. 2015 Nov 8;16(6):164–176 PO-1347 Halcyon commissioning using a 3D water phantom: beam data validation and initial end-to-end tests D. San José Olmedo 1 , A. Villacé Gallego 1 , D. Crelgo Alonso 1 , Z. Martín Rodriguez 1 , J. Fernández García 1 1 Hospital Universitario Central Asturias, Radiofísica, Oviedo, Spain Purpose or Objective Basic beam data validation was carried out for the new Varian Halcyon platform using a 3D water phantom (w.p.). The data acquired was compared with the preconfigured AAA reference data set provided by the vendor to validate the algorithm clinically. In addition, end-to-end tests with modulated plans were carried out to compare planned against delivered dose in common clinical localizations with different measurement systems.