ESTRO 36 Abstract Book

S891 ESTRO 36 2017 _______________________________________________________________________________________________

evaluation (criteria: 3%/3mm, 2%/2mm and 1%/1mm). For each patient mean %GP, standard deviation (σ) and angular coefficient of linear fit of %GP where evaluated. In particular σ is used to monitor random set- up and preparation errors while the angular coefficient is used to monitor the target size variation and tumor response during treatment course. The results were correlated to treated pathologies. Results The phantom results showed a sensitivity equal to 100% in detecting all simulated errors. The obtained results are described in the table ANOVA analysis pointed out that the significance of the difference between %GP and pathologies exists only when calculating mean %GP, s, and angular coefficient with the 1%/1mm CTV criterion, obtaining respectively p<0.006, p<0.04, and p<0.04. Applying the other criteria, the obtained results were p>0.05. The results showed that gynecological patients, followed by lung, head and neck, and rectum pathologies are the most responsive patients. Conclusion Based on these results, we can state a general correlation law between angular coefficient of %GP and treated pathology to search a quantitative parameter to help predicte adaptive radiotherapy. This methodology could provide an important element toward informed decision- making for adaptive radiotherapy. EP-1663 Automated full-online replanning of SBRT lymph node oligometastases for the MR-linac D. Winkel 1 , P. Kroon 1 , J. Hes 1 , G. Bol 1 , B. Raaymakers 1 , I. Jürgenliemk-Schulz 1 1 UMC Utrecht, Department of Radiotherapy, Utrecht, The Netherlands Purpose or Objective Diagnostic imaging on the MR-linac most probably provides better visibility of lymph nodes compared to CBCT on conventional linacs. While commercially available plan adaptation methods are feasible, full-online replanning is the preferred method to reach good plan quality. The aim of this study is to investigate the feasibility of fast online replanning on the MR-linac to account for inter-fraction motion for stereotactic body radiotherapy (SBRT) of lymph Patient imaging data and delineations from seven advanced cervix cancer patients with a combined total of 33 lymph nodes in the abdominal and pelvic region were included. A planning simulation study was performed on these lymph nodes with a 7-field IMRT technique and a prescription dose of 5x 7Gy to 95% of the PTV. Treatment plans were automatically generated using the research version of Monaco by Elekta AB (Stockholm, Sweden) with the use of their research automation API and in-house developed automated treatment planning software. A CTV-PTV margin of 3mm in all directions was applied. To decrease optimization time an additional margin of 5mm around the PTV was applied and only the parts of the OAR’s within this margin were considered as OAR during the optimization (Figure 1). All plans were generated using the MR-linac machine model and a 1.5T magnetic field in superior-inferior patient direction. Dosimetric outcomes were evaluated against clinical dose constraints and optimization time was measured. When required, PTV coverage (V 100% >95%) was sacrificed to meet all OAR dose constraints. node oligometastases. Material and Methods

Four pts had a second adaptation during RT. For these pts, changes above tolerance were solely observed for SS. The median decrease in CTV V95% was 0.2% [0-2.1%] and PTV V95% was 4.5% [0.6-7.2%]. None of the twice adapted patients showed changes in anatomy which justified reverting to the original treatment plan – either the changes were further in the same direction or in a different region. Conclusion Target coverage during the chemoRT in EC patients was compromised in some cases due to interfractional anatomical changes. Changes observed during RT persisted and in some cases they increased, making adaptation of the RT plan necessary. EP-1662 Interfractional trend analysis of sinograms: a decision-making for adaptive radiotherapy S. Bresciani 1 , A. Garello 1 , A. Miranti 1 , A. Maggio 1 , A. Di Dia 1 , P. Gabriele 2 , M. Stasi 1 1 Candiolo Cancer Institute - FPO- IRCCS, Medical Physics, Candiolo TO, Italy 2 Candiolo Cancer Institute - FPO- IRCCS, Radiotherapy, Candiolo TO, Italy Purpose or Objective The aim of this study is to investigate how geometric and anatomical changes can be detected in daily sinogram informations and how this information can be used to examine interfractional trends, building toward a methodology to optimize treatment and support adaptive replanning. Material and Methods Sensitivity of detectors and sinograms complex to detect shift errors and anatomical variations was previously tested on thoracic phantom. In particular systematic variations in shifts (1-5 mm in lateral direction), anatomical variations (adding 1.25-2.5 cm bolus over phantom) were applied. Subsequently, a total amount of 106 patients treated with Tomotherapy and their related 1573 sinograms were analyzed. The sinograms, measured using Xenon detectors integrated in Tomotherapy unit, were compared with a reference one (usually the first fraction) using both organ (CTV) and global percentage gamma pass (%GP)