ESTRO 2020 Abstract Book

S1059 ESTRO 2020

PO-1897 CBCT for evaluation setup errors among different fixation in proton therapy to intracranial lesions

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 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.

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PO-1898 Do 2DKV images add value in online position verification of breast ca patients treated with protons? T. Van Faassen 1 , E. Batin 1 , M. Kuijpers 1 , A. Meijers 1 , A. Crijns 1 , J.A. Langendijk 1 1 University Medical Center Groningen, Department of Radiation Oncology / GPTC, Groningen, The Netherlands Purpose or Objective At our institution the initial positioning workflow for breast cancer patients treated with proton therapy consisted of surface imaging, followed by two dimensional kilovoltage (2DkV) images and cone beam computed tomography (CBCT). Tolerances of 3mm/1.5° were used for CBCT and it was assumed that surface imaging and 2DkV images would prevent a need for a second CBCT acquisition, resulting in reduction of the X-ray dose and shorter positioning time. In order to estimate the added value of 2DkV images, a second workflow was introduced using only surface images followed by CBCT. Material and Methods Nineteen breast cancer patients treated with protons were initially positioned using surface imaging (AlignRT TN ). 2DkV 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°.

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.