ESTRO 2022 - Abstract Book

S1367

Abstract book

ESTRO 2022

DIBH data from 10 left sided breast cancer patients treated with four to six conformal treatment fields delivering 26Gy in five fractions were selected for this study. All breathing traces were acquired using a 3D real-time patient position monitoring (Varian, UK) system during CT scanning process and subsequently used to drive a 0.015cc pin point chamber (PTW, Germany) inside a thoracic phantom (CIRS, USA). Two sets of CT Scans were acquired for the phantom, one in static mode and the other in dynamic mode driven by each patient’s breathing trace. Treatment planning parameters for all patients were transferred to the respective CIRS phantom image sets and dose calculations were performed using collapsed cone algorithm in Raystation (Ver.9B, Raysearch Laboratories, USA). The treatment plans were delivered to the CIRS dynamic phantom in static and dynamic mode from TrueBeam (Varian, USA) linear accelerator. Results At the point of measurement, the average calculated dose is 97% for the static phantom plans and 95% for the dynamic phantom plans from the prescribed clinical dose. From static phantom measurements, the measured doses agree with the treatment planning estimated values within ±3% for 7 plans, for three plans the agreement was between -4.0% to - 8.0%. Whereas, for the dynamic phantom measurements the agreement was within ±4% for 6 plans and within ±9% for the remaining 4plans. Conclusion In this study, individual patient’s breathing traces were used in a dynamic thoracic phantom to simulate exactly the planning CT Scanning and treatment process used with the DIBH technique. Though the patients breathing traces acquire the movements in 3 axes (Left-Right, Ant-Post, Sup-Inf) , due to design limitations of the CIRS phantom the ant-post and left- right movements were combined into a single rotational movement by the motion control software. This limitation restricts the replication of the real transfer of patients 3 axis movements during phantom based dose verifications. But, this is not having any impact on the gated treatment delivery as it is managed through the sup-inf motion only. Though most of the measured single point doses from this study agreed within +/- 4.0% from the treatment planning doses, either 2-d or 3-d dose verification using suitable phantom capable of replicating the motions in all 3 axes will possibly support to co-relate the accuracy of dose delivered to nearby normal organs. 1 Humanitas, Istituto Clinico Catanese , Department of Medical Physics, Misterbianco (CT), Italy; 2 Humanitas, Istituto Clinico Catanese, Department of Medical Physics, Misterbianco (CT), Italy; 3 University of Catania, School of Medical Physics, Catania, Italy; 4 Humanitas, Istituto Clinico Catanese , Department of Medical Physics , Misterbianco (CT), Italy Purpose or Objective The aim of this work is to find a simple and effective mathematical method to predict the output variations of TrueBeam (Varian Medical Systems, Palo Alto, CA, USA) linear accelerators (Linacs), using the daily output measurements performed with Machine Performance Check (MPC) tool. Materials and Methods Three TrueBeam Linacs version 2.7, all placed in Humanitas ICC radiotherapy department, were equipped with the aS1200 portal imager. The MPC application is an automated tool, integrated into all three Linacs, allowing to perform a check of beam output, uniformity and various geometric parameters, utilizing a dedicated phantom and the imaging system. All daily output data, measured with MPC from April 2020 to October 2021, were exported and plotted for all energies to visualize the trend. The first step was to identify the dates of absolute dose calibration, performed in accordance with the IAEA TRS-398 protocol. Then, in order to generalize the results, the measurement period was divided into five sub-periods, common to the three Linacs, taking into account the calibration dates. Finally, a trend line was applied to the data of the series corresponding to each sub-period. Moreover, daily data were compared with monthly output measurements performed by means of ionization chambers and RW3 phantom. Results The output of all the TrueBeam Linacs under examination, measured with both MPC and ionization chambers, showed an increase for all photon and electron energies (on average +4% per year), in agreement with the literature findings. Output data for the 6 MV energy are representative of all the energies and all the Linacs. Output calibrations and subsequent MPC re-baselines resulted in sudden drops of output towards baseline. The regression line fitted the data quite well and represented the average Linac output. The angular coefficient of each line characterized the rate of output increase in the time interval considered. We found that the difference in output increase for all three Linacs and for each sub-period was not statistically significant (p>0.3), so it was possible to generalize using a universal angular coefficient that indicated the daily amount of output drift (on average 0.016%). Applying the linear mathematical model to our daily output data evaluated with MPC, the trends of Figure 1 were obtained. The overall output increase found with the model is 0.34% per month, comparable with the literature data PO-1586 Output prediction for TrueBeam linear accelerators G.R. Borzi' 1 , E. Bonanno 1 , N. Cavalli 2 , G. Stella 3 , M. Pace 3 , Z. Lucia 3 , C. Marino 4