ESTRO 2020 Abstract Book

S863 ESTRO 2020

Table: Nominal dose parameters for all ten patients

treatment by applying treatment margins. However, larger margins inevitably means irradiating more healthy tissue. Also, motion in free breathing (FB) is highly irregular and not reproducible over multiple treatment fractions. Applying non-invasive ventilation in unsedated subjects may enable regularized (rapid shallow) breathing with smaller tidal volumes, hence smaller respiratory motion. In this study, we investigated amplitude and reproducibility of the right hemidiaphragm motion between FB and regularized breathing (RB) induced by non-invasive ventilation. Material and Methods Six healthy volunteers underwent two magnetic resonance imaging (MRI). Prior to these sessions, the volunteers were introduced to non-invasive ventilation and were trained in letting the machine dictate their breathing pattern at 22 breaths per minute. 4DMRIs were obtained by acquiring a T2W turbo-spin echo scan, resolution (1.3x1.6x5 mm 3 ). Prior to each coronal slice acquisition, the position of the right hemidiaphragm was determined using a 1D navigator. The 2D slices were sorted into 10 amplitude bins after outlier rejection (discarding images corresponding to the 5% outmost navigator positions). This resulted in 10 respiratory correlated 3D images. From the 4DMRIs, the breathing amplitude was obtained by manually registering (in cranio-caudal direction only) each of the 10 image to a reference (i.e. end-exhale image), based on the right hemidiaphragm (Figure 1). Breathing amplitudes during FB and RB obtained in the two MR sessions were compared to investigate the reproducibility of the distribution of RB. Breathing amplitudes of FB and RB from both MR session combined were compared to investigate the effect of breathing regularization by ventilation. Results From the 12 MRIs (six volunteers, two sessions), three breathing motion amplitudes could not be reconstructed due to poor image quality and a missing acquisition. Overall, RB was well tolerated by all volunteers. The breathing amplitude was smaller for RB than for FB for each of the 9 measurement pairs (Figure 2). We pooled the data of both MRI sessions to analyze differences between FB and RB, demonstrating a significant smaller median amplitude of 11.8 (range 8.8–19.0) mm for RB as compared to 20.3 (range 10.6–24.3) mm for FB, tested with a Wilcoxon signed-rank test, p<0.01. The median relative reduction of amplitude was 42% (range 15–54%). Between the two MRI sessions, the variation of amplitude differed from 0.4-13.2 mm for FB and 1.3-5.3 mm for RB (n=4). Conclusion Regularized breathing by non-invasive ventilation significantly reduced the motion of the right hemidiaphragm compared with free breathing. Reproducibility of regularized breathing needs further investigation.

Figure 1 Dose distribution (a) and robustness analysis results (b) for patient 4.

Conclusion The effect of breathing motion on the robustness of proton therapy treatment plans for this patient group is minor and not of clinical significance. The mean heart dose was 0.41 Gy(RBE) with a standard deviation of 0.31 Gy(RBE) of proton therapy plans. Therefore, a deep-inspiration breath hold is not required to improve robustness for these patients . PO-1589 Comparison of diaphragm motion amplitude during free versus regularized breathing measured with MRI Z. Van Kesteren 1 , M.J. Parkes 2 , M.F. Stevens 3 , P. Balasupramaniam 4 , J.G. Van den Aardweg 5 , G. Van Tienhoven 1 , A. Bel 1 , I.W.E.M. Van Dijk 1 1 Amsterdam UMC - location AMC, Department of Radiotherapy, Amsterdam, The Netherlands ; 2 University of Birmingham, School of Sport- Exercise & Rehabilitation Sciences, Birmingham, United Kingdom ; 3 Amsterdam UMC - location AMC, Department of Anesthesiology, Amsterdam, The Netherlands ; 5 Amsterdam UMC - location AMC, Department of Pulmonology, Amsterdam, The Netherlands Purpose or Objective 4D imaging is current practice in radiotherapy of the upper abdomen, as it depicts the target motion during breathing. Motion due to breathing can be accounted for during