ESTRO 36 Abstract Book

S781 ESTRO 36 2017 _______________________________________________________________________________________________

Fourteen H&N and ten prostate VMAT plans were created and their respective QA plans were developed using Monaco 5.1 treatment planning system and delivered by an Elekta Synergy Linac equipped with an Agility 160 MLC system. The cylindrical phantom Octavius 4D was used to measure the dose distribution. The 2D-array Octavius 729 consists of 729 vented ionization chambers arranged in a 27x27 matrix with a spatial resolution of 10mm.The chamber volume is 0.125cm 3 . The Octavius 1500 array has the same layout and dimensions but with 1405 ionization chambers with a chamber volume of 0.06cm 3 . In order to reconstruct and analyze the measured 3D dose from each plan, the PTW VeriSoft 6.2 patient plan verification software was used and a volumetric 3D gamma index analysis (max dose of calculated volume, suppress dose below 10% of max dose of calculated volume) for both 3%3mm and 2%2mm criteria was performed to compare and evaluate the measured and calculated doses for both Table I summarizes the results for both cases. The mean pass rate of global 3D gamma index for all prostate cases was superior to 99% with 3%3mm and 95% with 2%2mm criteria. The Octavius1500 achieves higher results for both criteria. The mean difference was 2.9% for the gamma 2%2mm and 0.6% for the 3%3mm. For the H&N cases, the mean passing rate was lower than prostate cases. Similarly, the Octavius1500 obtain better results for both criteria. The mean difference was 4.2% for the gamma 2%2mm and 0.8% for the 3%3mm. arrays. Results

Conclusion A split field model was generated for the whole range of field sizes of a beam modulator linear accelerator from 0.8x0.8 cm 2 to 16x21 cm 2 using field size dependent photon beam energy spectra. The model was successfully validated independently and was found have a good agreement with measured doses and relative output factors. EP-1479 Gamma 3D analysis for VMAT treatments using two detector arrays E.M. Ambroa Rey 1 , D. Navarro Jiménez 1 , A. Ramirez Muñoz 1 , R. Gómez Pardos 1 , D. Amat de los Angeles 1 , A. Gibert Serrano 1 , A. López Muñoz 1 , M. Parcerisa Torné 1 , M. Colomer Truyols 1 1 Consorci Sanitari de Terrassa, Medical Physics Unit- Radiation Oncology Department, Terrassa, Spain Purpose or Objective The development of advanced radiation therapy techniques, such as volumetric modulated arc therapy (VMAT), requires a patient-specific pre-treatment quality assurance (QA). Two-dimensional array detectors are widely used for dose distribution verifications and the 3D gamma index is one of the metrics which have been extensively used for clinical routine patient specific QA. The aim of this study is to evaluate the 3D gamma index for different VMAT plans, such as head and neck (H&N) and prostate, with the Octavius 4D system using two 2D-arrays (PTW Octavius4D 1500 and PTW Octavius4D 729). Material and Methods

Expectedly, the greatest differences between Octavius 1500 and 729 are shown in the gamma 2%2mm criteria. Figure 1 shows a comparison between the gamma 2%2mm for both detectors for a H&N case.

Conclusion As we expected, the Octavius 1500 achieves a better result for pre-treatment VMAT plan verification and the results are more remarkable for the gamma 2%2mm criteria. In addition, plans with a higher complexity such H&N can benefit from the superior results of the Octavius 1500. Moreover, the Octavius 1500 present the possibility to increase the spatial sampling frequency and the coverage of a dose distribution by merging two measurements. We can conclude that Octavius 1500 outperformed Octavius 729 for VMAT pre-treatment QA. EP-1480 Patient-specific QA for CyberKnife MLC plans using Monte Carlo