ESTRO 36 Abstract Book

S904 ESTRO 36 _______________________________________________________________________________________________

organ (CTV) and global percentage gamma pass (%GP) 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 node oligometastases. Material and Methods

constraints.

Figure 1: a) A schematic representation of the planning strategy applied in this study to reduce optimization times. b) Resulting dose distribution with isodoses (cGy). Results For 30 (91%) of the 33 cases no clinical dose constraints are violated in combination with sufficient PTV dose coverage. In the other 3 (9%) cases PTV coverage is reduced by 5.4 ± 3.0 % to meet all dose constraints of the OAR. The average time required for optimization is 158 ± 95 s. The estimated dose delivery time, as reported by Monaco, is 198 ± 32 s. This leads to a total average optimization and delivery times of 357 ± 124 s, which fits well within the proposed 30 minute time limit for treatment on the MR-linac. Both the optimization and delivery time are dependent on the volume of the PTV and increases with increasing PTV. The average PTV is 6.4 ± 5.1 cc (range, 1.8 – 28.3 cc). Conclusion We have shown that automated full-online replanning for the MR-linac to account for inter-fraction motion is feasible for SBRT of lymph node oligometastases. With the planning strategy as applied in this study we are able to automatically generate treatment plans, suitable for clinical use, within a timespan which is clinically acceptable for treatment on the MR-linac. EP-1664 Two-step verification of dose deformation in presence of large inter-fraction changes during LACC RT A. Gulyban 1 , M. Baiwir 1 , S. Nicolas 1 , M. Enescu 2 , V.P. Nguyen 1 , M. Gooding 2 , T. Kadir 2 , J. Hermesse 1 , V. Baart 1 , P.A. Coucke 1 , F. Lakosi 3 1 Liege University hospital, Department of Radiation Oncology, Liege, Belgium 2 Mirada Medical Ltd., Department of Research, Oxford, United Kingdom 3 University of Kaposvar, Health Science Center, Kaposvar, Hungary Purpose or Objective Dose accumulation is one of the most challenging parts of modern radiotherapy, especially in the presence of large inter-fraction motion. Determining actual dose to a given organ during external treatment of locally advanced cervical cancer (LACC) is one of the most prominent examples. In our current investigation we aimed to evaluate the residual dose deformation errors during the summation of dose for clinical target volume (CTV), bladder and rectum.