ESTRO 2020 Abstract book

S803 ESTRO 2020

check that these parameters followed a normal distribution. EWMA control charts were constructed and correlated to major machine service (MLC/linac replacement, jaw actuator, output, energy). A weekly end-to-end test was performed for constancy check and also after a TPS upgrade. Results Measured to calculated dose ratios follow a gaussian distribution with estimated 1.013 mean value and 0.007 std. deviation. Lower control limits (LCL) and upper control limits (UCL) were determined (1.006 and 1.020 respectively). LOG of GI failing rate was fitted to a normal distribution. The resulting 95% confidence limit was 2% failing rate using 3%/3mm global gamma pass/fail criteria Conclusion Analysis of these data demonstrated that our process is currently stable and under control. However, we were not able to reduce the number of DQAs as a consequence of frequent machine service that include major components as linac or MLC replacements. Trending analysis through EWMA control charts is a powerful tool for risk assessment. It is a robust method to anticipate drifts and detect out-of-control deviations with clinical impact PO-1420 2%/2mm Gamma index analysis in routine EPID verification of conventional fractionated VMAT plans A. Zaleska 1 , A. Zawadzka 2 , D. Bodzak 2 , P. Kukołowicz 2 1 Masovian Oncology Hospital, Medical Physics Department, Wieliszew, Poland ; 2 Maria Sklodowska- Curie Memorial Cancer Center and Institute of Oncology, Medical Physics Department, Warsaw, Poland Purpose or Objective Gamma index analysis is currently the standard method of comparing the planned and measured dose distribution. This study was made to consider the possibility of narrowing plan acceptance threshold from more than 95% of measuring points meets the criterion of dose compliance Gamma 3%/3mm (G3%3mm), to 98% and 2%/2mm (G2%/2mm). Study was performed for Electronic Portal Image Devices (EPID) dosimetry. Material and Methods Retrospective results of 255 treatment plans with conventional fractionation and various treated locations were analyzed. TrueBeam accelerators: T1, T2 and T3 (Varian) and EPID were used. Plans were prepared in Eclipse Treatment Planning System (Varian) with Anisotropic Analytical Algorithm (ver. 15.6) and PDIP (ver. 13.) algorithm. Analysis was performed with 5% threshold. The region of interest was set as none. Evaluation was done with gamma parameters set to 2% and 2 mm and was accepted if 98% points passed the evaluation. For 22 plans with negative results of G2%/2mm analysis, the new plans were created using the Aperture Shape Controler (ASC) tool witch increase the segments opening in dynamic fields. For all these 22 new plans measurements were performed again. Additionally, to assess the method accuracy, stability for all 3 TrueBeam machines were tested. The same plan was measured every day on each machine for 3 weeks. Results Over 75% of the 255 analyzed plans passed G2%/2mm and 98% acceptance criterion. 12% met the G2%/2mm compliance condition for 97% of measuring points. In all 22 re-prepared plans more than 97% measuring points met the 2%/2mm dose criterion. The use of ASCs did not reduce the quality of treatment plans from a clinical point of view. The mean results of stability measurements were T1: 96,9% ± 1,5%, T2: 98,1% ± 0,5% and T3: 98,3% ± 0,8 % points passed the 98% criterion of G2%/2mm in plan composite. The T1 result differs from the other machines. Conclusion Narrowing the tolerance criteria in the Gamma analysis assessment allows better indication of treatment plans for

previous studies have shown some detrimental effects of RT on the left ventricular diastolic functions. The aim of our study was to calculate the dose in the left ventricle and compare it with the dose of the whole heart. Material and Methods We performed a retrospective dosimetric study by contouring the left ventricular for 50 patients treated with radiotherapy for left breast carcinoma. Seven patients had a history of cardiac disease. Among the patients, 8 patients (16%) were classified luminal A, 22 were luminal B (44%), 17 were HER2-enriched (34%) and only 3 patients were tripe negative (6%). Radiotherapy was 3D conformational. The delivered dose was 50 Gy in 25 fractions for 43 women (86%) and 40 Gy in 15 fractions for 7 women (14%). Fourty six patients had locoregional radiotherapy and the remaining 4 patients had a whole breast or wall chest radiotherapy. Results The breast was irradiated by two tangential fields and the supraclavicular nodes by a direct field. Mean doses for the the whole heart were ranged from 1.3 Gy to 5.9 Gy with an average mean dose of 3.7 Gy. The doses of left ventricle were different. The average dose in the left ventricle was 4.9 Gy with extremes of 1.5 Gy and 9.4 Gy. A dose strictly inferior than 5 Gy was respected for 45 patients for the whole heart, and for 26 patients the left ventricular, respectively. A dose strictly inferior than 3 Gy was respectect for 15 Patients for the whole heart, and for 7 patients the left ventricular, respectively. Conclusion Doses in the heart and left ventricle can be high. Clinical and ultrasound cardiac examination may be necessary to detect radiation-induced cardiotoxicity, especially for patients with a history of heart disease or who have received Trastuzumab. In addition to mean heart dose, breast cancer RT treatment planning should also include constraints for cardiac subvolumes such as left ventricle to ensure adequate heart protection. PO-1419 Is it possible to reduce the number of DQAs with Tomotherapy HD using Statistical Process Control? S. Losa 1 , P. Francois 2 , N. Pierrat 1 , P. Poortmans 1 1 Institut Curie Ensemble Hospitalier, Radiothérapie, Paris cedex 05, France ; 2 CHU Poitiers, Radiation Therapy, Poitiers, France Purpose or Objective In our institution, patient pre-treatment control (DQA) is currently performed for every patient plan prior to treatment delivery with helical tomotherapy. Keeping machine QA within tolerance levels in terms of output, energy, field width and MLC is mandatory to ensure beam matching to the TPS. The purpose of this work is to (1) evaluate the performance of our DQA process over time based on SPC (2) review our DQA pass/fail criteria tolerance levels and action limits (TG 218) and (3) determine a baseline to eventually reduce the number of DQAs Material and Methods Since 2014, major hardware (Dose Servo System or DCS and dynamic jaws) and software upgrades were performed on our two Tomotherapy machines. SPC has been implemented as part of our QA program. DQA measurements were performed with a 2D array of ion chambers (IBA Matrixx) in most cases. For small PTVs , Std. Imaging A1SL ion chamber combined with Gafchromic films was preferred because of its higher spatial resolution. A large series of data (n=968 and n=1100 patients in each machine) were followed up using process capability indicators calculated from absolute dose differences (measured/calculated ratios) in the high dose- low dose gradient regions (3% local dose difference) and LOG (Gamma Index failing rate) with 3%/3mm global analysis criteria. Data was plotted as an histogram and Chi-squared and Kolmogorof-Smirnof statistical test was calculated to