ESTRO 2020 Abstract book

S1113 ESTRO 2020

translations and RTT confidence scores for each registration were used to determine inter-observer variations. The exercise was repeated following the creation of a registration guide (including a case study atlas). This guide was developed collaboratively between RTTs and diagnostic MR radiographers, in consultation with clinicians, to identify the most relevant anatomic regions for registration. The guide was used as a reference for repeat registrations several weeks after the initial exercise. The mean, standard deviation (SD) and range of absolute difference among the three observers in right-left (RL), superior-inferior (SI) and anterior-posterior (AP) directions for each registration were calculated. Results Less variability was observed for MR-CT registrations when radiographers referenced a detailed registration protocol compared to matching the cervix alone. The inter- observer mean, SD and range improved for all directions. Details can be found in table 1. Comparison by paired t-test of results before and after the use of the registration guide showed statistically significant differences for both the SI ( p = 0.003) and AP directions ( p = 0.001). Additionally, RTT confidence in registrations, using a Likert scale (1-5), improved from a mean score of 3.2 prior to the intervention to 3.9 following the use of the registration protocol. Conclusion This study has demonstrated the importance for radiographers operating in MGgRT to develop and evaluate robust consensus driven image registration procedures. These protocols aim to minimise uncertainty in image matching and ensure consistent and accurate treatment. The results confirm that the application of registration guidance results in a significant improvement in inter- observer agreement for MR-CT image registration in the SI and AP directions. This strategy is expected to facilitate efficient integration of image registration for cervix plan of the day MGgRT and of other treatment sites following further validation. PO-1900 Is the 4D planning CT representative for breathing motion of esophageal tumors during treatment? Q. Roos 1 , Z. Faiz 2 , M. Dieters 1 , H.P. Van der Laan 1 , L.A. Den Otter 1 , J.T.M. Plukker 2 , S. Both 1 , J.A. Langendijk 1 , A. Knopf 1 , C.T. Muijs 1 , N.M. Sijtsema 1 1 University Medical Centre Groningen, Radiation Oncology, Groningen, The Netherlands ; 2 University Medical Centre Groningen, Surgical Oncology, Groningen, The Netherlands Purpose or Objective Radiotherapy for distal esophageal cancer (EC) is challenging because of high mobility of the tumor due to respiratory motion. When using a bony anatomy match for position verification, the tumor can be missed. Also, the position of the diaphragm with respect to the treatment beams may have a major influence on the radiological equivalent path lengths and therefore, on the delivered dose. Therefore, the aim of the current study was to evaluate the magnitude and inter-fractional variation of the breathing motion and position of EC with respect to

images were then acquired for ten patients and the resulting translations and pitch applied before CBCT acquisition. The other 9 patients were imaged with CBCT directly after positioning based on surface imaging only. The 2DkV-based corrections were analyzed as well as the CBCT-based 6 degrees of freedom corrections obtained with or without prior 2DkV-based corrections. A particular attention was paid for correction values out of our tolerances of 3mm/1.5°. Results 2DkV-based translation corrections in lateral direction were 0.55 mm on average (± 1.75 mm SD), longitudinal corrections: 0.51 (± 1.91 mm SD), vertical corrections: 0.67 (± 1.56 mm SD). These corrections exceeded the tolerances of +/- 3 mm in 9.6% of all fractions for lateral, 13.0% for longitudinal and 8.5% for vertical directions. 2/3 rd of these corrections were in only one direction. 2DkV pitch corrections were less than 1° for 91.5% of all fractions. CBCT translation corrections after 2DkV corrections were on average less than 0.8 mm compared to less than 1.3 mm without 2DkV corrections. Standard deviations were similar for both workflows: and between 1.7 mm and 2.2 mm. With 2DkV corrections, CBCT translation corrections exceeded +/- 3 mm in 15.3% of all fractions laterally, 13.6% longitudinally, 15.3% vertically and respectively 14.4%, 16.1% and 14.4% without 2DkV corrections (figure 1). Corrections found with CBCT were in 49.2% in the opposite direction from corrections with 2DkV for lateral direction. In 22.6% there was > 3mm difference between 2DkV and CBCT. For longitudinal direction 37.9 % of corrections were in opposite direction and resulted in > 3 mm difference in 21.5% of all fractions. While for vertical direction 48.0% of corrections were in opposite direction and exceeded 3mm difference in 25.4 % of the cases (figure 2).For rotation corrections, the number of CBCT corrections superior or equal to 1.5° after 2DkV were 11.4% of all fractions for yaw, 4.5% for pitch and 8.5% for roll and 6.8% for yaw, 5.9% for pitch and 16.9% for roll without 2DkV corrections. Conclusion Excluding 2DkV-based corrections from proton breast positioning workflow did not impact CBCT based corrections with a similar proportion of translations corrections superior to 3 mm/1.5°. PO-1899 Quantification of inter-observer variation before and after an educational intervention in MRgRT J. Rodgers 1 , R. Hales 1 , L. Whiteside 1 , J. Parker 1 , L. McHugh 1 , A. Cree 2 , M. Van Herk 3 , C. Eccles 1 1 Christie Hospital NHS, Radiotherapy, Manchester, United Kingdom ; 2 Christie Hospital NHS, Radiotherapy Related Research, Manchester, United Kingdom ; 3 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom Purpose or Objective Accurate image registration is essential for magnetic resonance (MR) guided online treatment adaptive workflows (MRgRT). On-treatment MR-computed tomography (CT) image registration is a novel method for radiographers (RTTs) familiar with more established imaging platforms such as conebeamCT. In treatment sites demonstrating significant interfraction anatomical variation, such as the cervix, image registration can be particularly challenging. This work aims to validate the use of a guide developed by RTTs to improve inter- observer variation for on-treatment image registration for cervix plan of the day MRgRT. Material and Methods Off-line, retrospective soft tissue image registration was undertaken by 4 RTTs for 10 cervical cancer patients undergoing radiotherapy on an ethics-approved imaging study. RTTs used the cervix to guide the registration of T2w MR images to the planning CT in Monaco (v5.19.05 Research, Elekta, Stockholm, Sweden). Resulting