ESTRO 2020 Abstract book

S954 ESTRO 2020

For γ mean OOT results were obtained for: 1,3% of IMRT and 7,35% of 3DCRT, 1,3% of IMRT and 6,25% of 3DCRT fields and for 6,25% IMRT, 32,5% 3DCRT and 6,25% IMRT, 27,5% 3DCRT fractions. Based on these results, the 2%, 2mm criteria have been chosen for further application. The procedure allowed to detect and correct set-up problems and did not yet result in re-planning. However, sensitivity of the IVD test allowed us to develop a protocol for ART, presented in fig. 1. Fig. 1. ART protocol algorithm.

PO-1645 Development of protocol for EPID IVD based adaptive radiotherapy in a small RT unit. P. Borowska 1 , M. Paluszyńska 1 , U. Sobocka-Kurdyk 1 , M. Peszyńska-Piorun 1 , S. Kubicka 1 1 Wielkopolskie Centrum Onkologii, Zakład Radioterapii III, Kalisz, Poland Purpose or Objective This study was designed to develop a procedure allowing implementation of an adaptive radiotherapy (ART) protocol in a radiotherapy unit with limited staff and technical resources. Due to these limitations, the method should neither be time-consuming, nor require purchase of additional equipment or computer software. This work presents the combination of EPID transit dosimetry with existing IGRT protocol in development of an ART protocol. Material and Methods 15 patients, undergoing RT for prostate (7), lung (3) and breast (5) cancer were enrolled in the study. 722 treatment fields (450 IMRT, 272 3DCRT) delivered in 104 fractions (64, 40, IMRT, 3DCRT, respectively) were tested. Planned dose distributions were calculated with Eclipse TPS. Measurements were performed with EPID integrated with TrueBeam linac, the main therapeutic accelerator in the unit. The γ evaluation method was used to compare a reference EPID image and an IVD EPID image obtained in a daily treatment fraction. Reference image selection algorithm has been determined in course of this study. By means of Portal Dosimetry application, the γ analysis was performed for each field for two sets of the pass criteria: SD (signal difference) and DTA 2%, 2mm and 3%, 3mm, respectively, in order to select the criteria and tolerance levels to be implemented in the developed protocol. In existing IGRT protocol patient set-up is controlled, depending on tumor localization, by the following imaging procedures: • breast: kV + MV during 1st fraction followed by daily kV and MV every 3 days, • prostate, H&N: kV during 1st fraction and onward, CBCT on the 2nd fraction and every 3 days, Results For γ tests the tolerance levels were fixed for the area of γ<1 >80%, γ mean <0,5 and area of γ<1 >90%, γ mean <0,34 for 2%, 2mm and 3%, 3mm pass criteria respectively. For 2%, 2mm and 3%, 3mm criteria respectively, out of tolerance (OOT) results for the area of γ<1 were obtained for: 0,9% of IMRT and 4,04% of 3DCRT, 1,1% of IMRT and 4,41% of 3DCRT fields and for 6,25% IMRT, 17,5% 3DCRT and 4,7% IMRT, 17,5% 3DCRT fractions. • lung: kV daily.

Conclusion Detection by means of EPID IVD of set-up errors overlooked in daily IGRT indicates that basing on the sole IGRT protocol may be insufficient in ART procedure as important indications for re-planning may be missed in the heavy workload. Combining the existing protocol with EPID dosimetry provides powerful algorithm for the decision- making process. PO-1646 Evaluation of the feasibility of EPID-based in vivo dosimetry system for prostate cancer patients N. Kadoya 1 , T. Matsumoto 1 , K. Sato 2 , K. Ito 1 , T. Chiba 1 , Y. Katsuta 1 , S. Dobashi 3 , K. Takeda 3 , K. Jingu 1 1 Tohoku University Graduate School of Medicine, Radiation Oncology, Sendai, Japan ; 2 Tohoku University Hospital, Department of Radiation Technology, Sendai, Japan ; 3 Tohoku University, Department of Radiological Technology- School of Health Sciences- Faculty of medicine, Sendai, Japan Purpose or Objective This study aimed to characterize EPID-based in vivo dosimetry system (PerFRACTION, Sun Nuclear Corporation, USA) for the daily VMAT quality assurance using a EPID exit image. Its ability to identify the main clinical errors was examined using the phantom and patient data. Material and Methods A total of 33 patients with prostate cancer treated with VMAT were examined. TureBeam STx or Clinac 23EX (Varian Medical Systems, USA) were used. Firstly, the sensitivity of the main clinical errors was validated using