ESTRO 2020 Abstract book

S796 ESTRO 2020

confronted with the MPC system in terms of quality and quantity. Material and Methods Optimization of quality control of beam stability and correctness of SX2 collimator operation was based on MyQA application algorithms. Tests for the radiation beam included checking performance stability and energy constancy. The second group of beam-related tests are geometric tests involving the analysis of parameters such as symmetry, flatness, penumbra, size and center of the radiation beam. SX2 collimator tests are tests of stability, repeatability and accuracy of MLC leaf positioning, as well as correctness of MLC collimator operation for complex systems, for IMRT and VMAT techniques. MyQA software is based on predefined algorithms based on the guidelines of the AAPM 142 task group report. In order to adapt the guidelines of the AAPM-142 report to the needs of the Halcyon design solutions, a database of independent tests based on the myQA application algorithms was built and compared with the tests proposed by Varian in MPC software. Results Analysis of the beam parameters for the built QA system turned out to be deeper than in the case of tests for the MPC algorithm. MPC tests regarding beam quality are consistency of performance and homogeneity. The results of both tests are presented in the form of a single value, with fixed tolerance levels of 4% and 2%. The proposed control system, in addition to the performance constancy test (image 1a) with two levels of tolerance (warning 1.0%, error 1.5%), extends the beam controls with energy constancy and symmetry, flatness, halftone, size and center position (image 1b). Tolerance levels for beam quality tests, unlike the MPC system, are a series of values based on the guidelines of the IAEA TRS 389 report. Collimator SX2 control, in the MPC system is an analysis of the accuracy and reproducibility of settings for single leaves, for static systems. The proposed QA system allows testing the stability, repeatability and accuracy of MLC leaf positioning for dynamic systems.

Material and Methods Polymer dosimetry gel was filled into the 3D printed prostate organ shell of the phantom. A stack of nine thermoluminescent detectors (TLDs) mounted onto a specifically designed holder was placed into the rectum posterior to the prostate. The phantom was scanned in treatment position with an in-room CT (Somatom Emotion, Siemens Healthineers). Using Raystation8A (Raysearch Laboratories), a treatment plan simulating the clinical situation with seven beams aiming at a prescribed dose of 4.5 Gy to the whole prostate volume including a 4 mm margin was calculated. Phantom irradiation was performed on a 6 MV Linac (Artiste, Siemens). The dosimetry gel was evaluated using MRI and the TLDs were read out with a harshaw hot gas reader. For the dosimetry gel, the measured 3D dose distribution was compared to the calculated dose distribution with a 3D gamma analysis (Verisoft, PTW, passing criteria: 3% dose difference and 3 mm distance to agreement). The TLDs were used to evaluate punctual doses in the rectum and the measured doses were compared to calculated doses by the treatment planning software on a point-by-point basis. Results The 3D gamma index comparing measured and calculated dose distributions in the prostate was 97.7%. For the TLDs, the average dose difference between calculated and measured doses was 0.08 Gy (range: 0.02 – 0.21 Gy, with 0.08 Gy referring to 1.78 % of the prescribed dose). Table 1 summarizes the calculated and measured doses in the rectum.

Conclusion Empirical validation of dose distributions in prostate and rectum is feasible using 3D dosimetry gel and TLD dose measurements. This study describes a novel method for absolute 3D dosimetry in a complex anthropomorphic phantom. PO-1409 Optimization of the Halcyon accelerator quality control system M. Raczkowski 1 , M. Janiszewska 1 , T. Siudziński 1 , A. Maciejczyk 2 1 Lower Silesian Oncology Center, Medical Physics Departmen, Wroclaw, Poland ; 2 Lower Silesian Oncology Center, Department of Radiotherapy, Wroclaw, Poland Purpose or Objective The purpose of the work is to present a method for optimizing beam quality control and correct operation of the Halcyon MLC collimator. The starting point for optimizing the control system was the MPC algorithm (Machine Performance Check). The development of independent methods for verifying the operation of the SX2 collimator and checking the beam stability of the Halcyon accelerator was based on algorithms independent of MPC. The developed quality control system was

Conclusion