ESTRO 2020 Abstract book

S797 ESTRO 2020

The proposed, independent QA system, unlike the MPC system, allows a significant reduction in tolerance levels for performance stability and introduces their differentiation for individual beam geometric parameters. The presented QA system allows testing the accuracy of the MLC collimator for dynamic systems used in IMRT and VMAT techniques. Delivery time and time availability of results, as well as the ability to analyze time trends are the strengths of both systems. PO-1410 Trend lines on patient specific quality assurance in ion beam therapy with protons and carbon ions M. Schafasand 1 , G. Kragl 1 , J. Osorio 1 , S. Vatnitsky 1 , M. Stock 1 , A. Carlino 1 1 MedAustron, Medical Physics, Wiener Neustadt, Austria Purpose or Objective Our center is a synchrotron based ion beam therapy center for protons and carbon ions that started clinical operation with protons. Patients are currently treated in two irradiation rooms (IR), one equipped with a fixed horizontal beam line (IR3HBL) and the other equipped with a fixed horizontal and vertical beam line (IR2HBL and IR2VBL). An active scanning method is implemented in the beam delivery system. The complexity of this technique request high demands in patient specific quality assurance (PSQA). Current state of the art for PSQA at this center is the absorbed dose to water measurements by means of 24 PinPoint (PP) ionization chambers (model TM31015, PTW Freiburg) mounted on a MP3-P (PTW, Freiburg) water phantom. These measurements are carried out beam specific before each patient treatment. The PSQA setup is adaptable for both fixed beam lines. The aim of this work is to report trend lines on the PSQA results acquired during the first years of clinical operation. Material and Methods In order to perform PSQA, the treatment plan generated with the TPS RayStation RS (RaySearch Laboratories, Stockholm, Sweden) was recomputed in a virtual water phantom. Three different dose algorithms (Pencil Beam PBv3.5, PBv4.1 and Monte Carlo MCv4.0) for protons and PBv3.0 for carbon ions were clinically used. The absorbed dose to water was determined with the 24 PP. For very superficial tumors a Range Shifter (RaShi, a 3cm thick PMMA slab) is automatically inserted in the beam line. The performance of the dose algorithms was evaluated, paying special attention to beams with RaShi. PPs positioned in high dose gradient (>0.04Gy/mm) and below a threshold dose (<0.1Gy) were excluded from the analysis. The average of signed and unsigned dose difference between the measured and the planned dose normalized to the maximum dose per beam (global deviations) was calculated for each beam. The data were grouped according to target location (Central Nervous System CNS, head and neck and pelvis), presence of RaShi and the dose calculation algorithm. Results Figure 1 represents the results of 916 beams, corresponding to 145 patients treated during 18 months, starting from December 2016 till April 2018.

Current analysis includes results for protons in a limited time span i.e. not all algorithm versions and beam lines are included. At the time of presentation the analysis will be extended to carbon ions as well. Conclusion For all algorithms, all anatomical sites and all the beams (with and without RaShi), the average deviations are within ±2% and clinically acceptable. From the analysis a trend to slightly over predict the dose for both PB algorithms and in presence of RaShi is evident. Since November 2017, only the MC algorithm is used for protons. Furthermore, it could be confirmed that the two beam lines agree within ±2% which is within the accepted tolerance and that measurements can be performed independent of the irradiation room which leads to beam time sparing. PO-1411 Commissioning of Tomotherapy treatment planning in RayStation K. Schubert 1 , X. Wester 1 , C. Weinmann 1 , S. Erdem 1 , D. Oetzel 1 , J. Debus 1 1 University Hospital Heidelberg, Department of Radiotherapy and Radiation Oncology, Heidelberg, Germany Purpose or Objective Before using RayStation (RS) as an independent treatment planning option for Tomotherapy (Tomo) units, a unique beam modelling and commissioning process has to be performed. All Tomo systems are calibrated according to a gold standard, which contains the basic beam information, while an individual calibration of the MLC characteristic and the output is added at the end of installation. The Tomo machine has a specific treatment couch, which needs to be considered during dose calculation. Since the calculation of open beams configurations is not possible, special attention has to be paid to individual plan verifications. Here measurements with a two dimensional array and calculations with an independent Monte Carlo algorithm were used. Material and Methods After the beam model was imported from Precision 2.0.0.1 (Accuray) into RS 8B (RaySearch) the ‘gold standard