ESTRO 38 Abstract book

S975 ESTRO 38

For potential future use in reference dosimetry using a single proton pencil beam instead of a broad field, the observed inhomogeneities in the chamber response across its sensitive area require a correction map. The presented in air scanning of the sensitive chamber area provides a quick check of its response but with a limited accuracy. The observed -3% to +6% inhomogeneities had no significant effect on the measurement of IDDCs. References: 1. Bäumer et al. 2015 J. Appl. Clin. Med. Phys. 16 151. 2. Gomà et al. 2017 Phys. Med. Biol. 62 4991. 3. Kuess et al. 2017 Phys. Med. Biol. 62 9189. EP-1800 An Evaluation of Techniques for Dose Calculation on Cone Beam CT V. Giacometti 1 , R.B. King 2 , C.E. Agnew 2 , D.M. Irvine 2 , S. Jain 1 , A.R. Hounsell 2 , C.K. McGarry 2 1 Centre for Cell Biology and Cancer Research- Queen's University Belfast, School of Medicine- Dentistry And Biomedical Sciences, Belfast, United Kingdom ; 2 Northern Ireland Cancer Centre, Radiotherapy Physics, Belfast- Northern Ireland, United Kingdom Purpose or Objective This investigation assessed efficiency, accuracy and feasibility of four different techniques adopted to recalculate the dose delivered to the patients based on cone beam CT (CBCT) images acquired prior to treatment. Material and Methods Four established techniques (standard planning CT calibration curve, CBCT site specific calibration curve, HU density override and deformable registration) were investigated. Each technique was applied to 15 CBCT patients’ images (5 prostate and pelvic nodes, 5 lung and 5 head and neck), and treatment planning dose calculations were performed in order to assess accuracy and efficacy of each technique. The patients were receiving volumetric modulated arc therapy to one of three treatment sites. Dose volume histogram metrics and 2.0% / 0.1 mm volumetric gamma analysis were employed to provide a quantitative analysis of the differences between planning CT and CBCT dose distributions. Results Dose volume histogram analysis indicated that all techniques yielded differences from expected results between -5.4% and +3.8% for both target volumes and organs at risk: -1.4% to 1.8% for prostate and pelvic nodes, -5.4% to 1.0% for head and neck, and -1.8% to 3.8% for lung. With volumetric gamma analysis, the median pass- rates at 10% threshold were 95.5%-97.2%, 92.6%-95.1%, and 85.2%-88.2% for prostate, head and neck and lung patients, respectively. Figure 1 shows the median pass- rates for standard planning CT calibration curve, CBCT site specific calibration curve, HU density override and deformable registration at 10% threshold for each treatment site. Deformed images yielded the highest pass- rates for prostate and head and neck patients, while site- specific calibration curve yielded the highest median pass- rate for lung patients.

Figure 1: Median 2%/ 0.1 mm gamma pass-rates (%) for dose thresholds of 10% for thorax, head and neck, and pelvis acquisitions. Note that the median pass-rate (y-axis scale) starts from 70%. Conclusion All four investigated techniques were identified as dosimetrically accurate and efficient methods to perform dose calculation based on CBCT images. The differences observed were treatment site dependent. EP-1801 Automatic EPID based Beam QA : measurements become pleasure A. Sors 1 , D. Perez 1 , P. Dudouet 1,2 , D. Franck 2 , M. Macé 1 , P. Boucarut 1 , C. Boutry 1 1 Clinique du Pont de Chaume - Groupe Oncorad Garonne, Service de Radiothérapie, Montauban, France ; 2 Clinique Pasteur - Groupe Oncorad Garonne, Service de Radiothérapie, Toulouse, France Purpose or Objective To demonstrate interest of automation for photon and electron beams Quality Assurance (QA) using EPID with a simple approach of absorbed dose to water reconstruction: the EpiDream method [1], integrated in the ARTISCAN Beam QA software (AQUILAB, France). The final aim of this study is to validate the use of EPID as a potential substitute to ionization chamber (IC) for routine beam QA. Material and Methods The study was performed on Clinac 2100CD and 2100EX (Varian Medical Systems, Palo Alto, CA) respectively equipped with on board As500 and As1000 EPID imager (pixel size of 0.784×0.784mm² and 0.392×0.392mm²). EPID has been calibrated based on EpiDream method and ARTISCAN software used to convert Grey Level integrated images into dose to water matrix for 6-25MV photons and 6-9-12-15-18MeV electron beams. A 4-month study was conducted to compare output constancy (including 5- 10mm sliding windows (SW) for photons beams) and beam parameters (flatness, symmetry, penumbra and field size) obtained with EPID method and IC. Comparison was performed using MATLAB (MathWorks Inc., Natick, MA) between radial and transverse profiles measured with a 0.125cm3 IC (PTW31010) and extracted from EpiDream converted matrices. Robustness of EPID vs. IC QA was estimated using standard deviation of each parameter. Time dedicated to daily and monthly QA for all energies using both methods was also evaluated. Results Table 1 reported the mean deviation for 6-25MV and 6-9- 12-15-18MeV between EPID and IC for output constancy results (a), the mean deviation of EpiDream profiles with IC profiles (b) and robustness of EPID vs IC QA results (c).