ESTRO 2020 Abstract book

S761 ESTRO 2020

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. 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