ESTRO 2020 Abstract book

S1037 ESTRO 2020

PO-1769 Multicenter end-to-end test for lung SBRT treatments S. Pallotta 1 , L. Marrazzo 2 , S. Calusi 1 , C. Talamonti 1 , S. Russo 3 , M. Esposito 3 , C. Fiandra 4 , F.R. Giglioli 5 , M. Pimpinella 6 , V. De Coste 6 , A. Stravato 7 , M. Stasi 8 , P. Mancosu 7 , R. Lisci 9 1 Azienda Ospedaliero Universitaria Careggi - University of Florence, Department of Biomedical Experimental and Clinical Sciences "Mario Serio", Florence, Italy ; 2 AOU Careggi, Medical Physics Unit, Florence, Italy ; 3 Firenze Azienda Sanitaria USL Toscana Centro, Fisica Sanitaria, Florence, Italy ; 4 University of Turin, Oncology Department, Turin, Italy ; 5 AOU Città della Salute e della Scienza di Torino, Medical Physics Unit, Turin, Italy ; 6 ENEA-INMRI, National Institute of Ionizing Radiation Metrology-, Rome, Italy ; 7 Humanitas Cancer Centre, Radiotherapy, Rozzano MI, Italy ; 8 AO Ordine Mauriziano, Health Physics, Turin, Italy ; 9 University of Florence, Department of Agricultural- Food and Forestry System, Florence, Italy Purpose or Objective To support medical physicists in handling technical and dosimetric aspects of Stereotactic Body Radiation Therapy (SBRT), the Italian Association of Medical Physicists created, in 2013, a working group (WG) on this issue. The current project, born in this WG framework, aims to set- up and test a lung SBRT end-to-end (e2e) protocol, using local expertise and devices, to propose to Italian Centers- The Italian institute of ionizing radiation metrology and four Hospitals using devices of different vendors (linacs, treatment planning systems, and 4DCT breathing synchronization devices) were involved in this study. Phantom- ADAM (1,2) was selected for this test due to its anthropomorphic shape, realistic breathing, and capability to interface with any respiratory synchronization device. Instructions and video tutorial for the phantom usage were provided to the involved hospital. Detectors - A diamond detector (PTW- Freiburg Germany) and radiochromic films were selected to perform point dose and dose distribution measurements. Dose calibration of the diamond detector and of the Farmer chamber used to calibrate radiochromic films were both checked at the “metrology” institute. Film reading and analysis, performed with FILMQA™ (Ashland Inc., Wayne, NJ) according to a shared procedure (3), were centralized in a single Hospital. Detectors and phantom delivery were made by express courier. Results e2e protocol – According to all centers specifications, a shared e2e protocol was defined. Users are required to image, plan, and irradiate ADAM, following the procedures adopted in their centre but according to dose planning and irradiation protocols synthesized in fig.1. Static and/or dynamic tracks can be followed delivering dose to LT1 (right lung) and LT4 (left lung). Users are also required to upload in a shared drive ADAM CT study, RTDose, RTStructure and RTplan for all prepared plans together with a copy of daily output check and plans QA measurements. radiotherapy centers. Material and Methods

The new Accuray platform for helical tomotherapy, Radixact, delivery has been introduced at our department in june 2018. The objective of the present work is to analyze the results obtained from a large dataset of pretreatment measurements for this new machine, to determine which parameters has to be optimized to obtain a more robust delivery. At the date of the submission this is the first study on performances evaluation of the Radixact platform and Precision TPS. Material and Methods More than 350 clinical plans, delivered between June 2018 and September 2019, were measured with ArcCHECK (SunNuclear, Melbourne, FL) diode array. All the plans were optimized and calculated with the new TPS Accuray Precision v 1.1.1. All the plans were delivered on a Radixact Helical Tomotherapy unit. According to the AAPM's report TG-119 (updated by TG-218 report), the gamma index passing rate (GP%) chosen for the analysis is: 3% dose difference, 2 mm Distance to Agreement (DTA), Van Dyk criteria of 10% threshold, absolute and global dose comparison. The plans were considered deliverable if the universal tolerance limit, GP% passing rate ≥ 95%, is met. In all the other cases we replanned the case in order to produce a less complex plan. The values of the actual modulation factor (MF), pitch, field width (FW), gantry period (Trot), dose per fraction (D/fr), dose per fraction on pitch (D/p), maximum, median, mean and standard deviation of Leaf Open Time (Max-LOT, m-LOT, M-LOT and SD-LOT respectively) and site of the disease were collected. A multivariate analysis of variance (mANOVA) was performed to test the influence of these parameters on GP%. A post-hoc test was carried out to assess the significance of the quantities identified as predictors. Results An average GP% (±SD) of 97.5 (±2.5) was found. All the results are reported in figure 1. Only 2% of the plans fails to meet the action level defined by TG-218. They were replanned to meet the GP%>90% criteria. Max-LOT, M-LOT and pitch resulted significant predictors of dosimetric accuracy of the plans, defined as GP% (3%/2mm). No correlation with the treatment site nor with other parameters was found.

Figure 1: Box Plot of GP% 3%/2mm. Horizontal lines indicates the TG-218 tolerance and action limits. Conclusion The results here presented suggest that the calculation accuracy of the new Precision TPS and the delivery accuracy of Radixact unit is adequate, with respect to the international guidelines and reports. Despite the general and positive conclusion on the performances of the presented pre-treatment QA, the predictive value of the analyzed parameters on QA failures is still under investigation.