ESTRO 2020 Abstract book

S193 ESTRO 2020

mm/min). We tested for differences in median velocity during in- and exhale PBHs (Wilcoxon signed-rank test, α=0.05). To investigate the reproducibility of PBHs recorded in respectively MR sessions 1 and 2, differences of the velocity per volunteer were calculated. Results Overall, in- and exhale PBH durations ranged from 2min2s to 8min2s, and from 1min45s to 6min21s, respectively (Figure 1), with diaphragm motion varying from 10–45mm. V02 was excluded from analysis due to PBH duration <3min. For the remaining volunteers who performed PBH of >3min, velocity of diaphragm motion during inhale PBHs (n=8; median 3.1 mm/min; range 1.0–4.5 mm/min) was significantly slower than during exhale PBHs (n=8; median 5.9 mm/min; range 1.8–7.9 mm/min) (Table 1). Per volunteer, the difference in velocity of diaphragm motion between the two MR sessions varied from 0.1–1.6 mm/min during inhale PBHs, and from 0.4–2.5 mm/min during exhale PBHs.

Conclusion We successfully applied our motion modelling framework for the first time to lung cancer patient data acquired on an MR-Linac. Our method allows for real-time imaging of the tumour in the surrogate slice while also providing volumetric motion information. Our framework displays a constant model performance over multiple treatment days. OC-0339 First MRI based quantification of diaphragm motion during prolonged breath-holds up to 8 minutes I. Van Dijk 1 , M.J. Parkes 2 , M.F. Stevens 3 , P. Balasupramaniam 1 , J.G. Van den Aardweg 4 , G. Van Tienhoven 1 , Z. Van Kesteren 1 , A. Bel 1 1 Amsterdam University Centers – location AMC, Dept. of Radiation Oncology, Amsterdam, The Netherlands ; 2 School of Sport Exercise & Rehabilitation Sciences, University of Birmingham, Edgbaston- Birmingham, United Kingdom ; 3 Amsterdam University Centers – locations AMC and VUmc, Dept. of Anesthesiology, Amsterdam, The Netherlands ; 4 Amsterdam University Centers – location AMC, Dept. of Pulmonology, Amsterdam, The Netherlands Purpose or Objective During radiotherapy in the thoracic-abdominal region, patients may be instructed to repeatedly perform short breath-holds (SBH; 20-30s) to diminish respiratory motion. However, the effectiveness depends on patients’ compliance with instructions. Also, tumor and organ position vary between repeated SBHs, and residual motion still occurs. Applying noninvasive mechanical hyperventilation (causing hypocapnia) with pre-oxygenation enables prolonged breath-holds (PBHs; ~5min). A complete radiation fraction could be delivered during a single PBH. However, information on motion during PBHs is lacking.The aim of this pilot study is to assess in- and exhale PBH durations and to quantify diaphragm motion during PBHs. Material and Methods Six healthy volunteers were trained to be non-invasively hyperventilated through a face mask causing safe hypocapnia to perform PBHs of at least 3min. In each of two MR sessions volunteers were asked to perform one inhale and one exhale PBH. During each PBH, a 3D cine- MRI was acquired (balanced turbo field echo: TE, 1.51ms; TR, 3.0ms; resolution, 1x1x1.6mm3; field-of-view, 450x400x220mm3) yielding a 3D dynamic image every 8s. In- and exhale PBH durations were recorded. Dynamics were registered to the first dynamic (i.e. the reference) using the right diaphragm dome as region of interest, to obtain diaphragm motion in craniocaudal direction. Three dynamics over the course of PBHs and three dynamics towards the end of PBHs were used to linearly fit the velocity of diaphragm motion, with the diaphragm dome position at the start of the MRI acquisition defined as (0mm), and velocity defined as slope of the fit (in

Conclusion Performing in- and exhale PBHs of 3min and more during MRI is feasible. The velocity of the right diaphragm dome motion was slower during inhale than during exhale PBHs. To make PBHs effective in clinical radiation therapy and to