ESTRO 2021 Abstract Book

S694

ESTRO 2021

Scatter plots with mean and 95% confidence regions of translational positioning shifts for all 280 couch kicks: uncorrected (a-c) and corrected (d-f) resulting in tolerable deviations (0.5 mm, 0.5°)

Conclusion We conclude that initial setup verification is not enough for frameless LINAC-based SRS and may lead to considerable positioning error due to intra-fractional patient motion and geometrical uncertainties unless this is monitored and corrected for during treatment. Especially for single-isocenter treatments of multiple targets, the use of a 6D positioning system is highly recommended in order to minimize unfavorable off-isocenter displacements leading to insufficient coverage and treatment outcome.

Poster discussions: Poster discussion 23: MR-guided radiotherapy

PD-0860 Kidney motion and motion velocity during inhale and exhale prolonged breath-holding (~5min) L. Ewals 1 , Z. van Kesteren 1 , M. Parkes 1 , M. Stevens 2 , J. van den Aardweg 3 , G. van Tienhoven 1 , A. Bel 1 , I. van Dijk 1 1 Amsterdam UMC - location AMC, Radiation oncology, Amsterdam, The Netherlands; 2 Amsterdam UMC - locations AMC and VUmc, Anaesthesiology, Amsterdam, The Netherlands; 3 Amsterdam UMC - location AMC, Pulmonology, Amsterdam, The Netherlands Purpose or Objective In thoracic and abdominal radiotherapy, short breath-holding (SBH; ~30s) is commonly used to minimize respiratory motion. To eliminate position variations between subsequent SBHs, prolonged breath-holding (PBH; ~5min) is investigated. Residual organ motions still occur during PBH due to gradual lung volume decrease, caused by oxygen uptake. We examined the usability of PBH by quantifying kidney motion during inhale and exhale PBH (PIBH and PEBH) in two MRI sessions. Materials and Methods Eight healthy volunteers were mechanically hyperventilated with 60% oxygen to reach stable hypocapnia, after which they held their breath as long as possible. In two sessions, both including a PIBH and PEBH, 3D cine-MRI scans (balanced turbo field echo sequence, TE=1.51ms, TR=3.0ms) with 3D images (dynamics) of 13–16s were acquired. Seven dynamics, distributed over the PBH duration, were used for evaluation. The first dynamic was registered to the six subsequent dynamics for the left and right kidney separately. The transformations were applied to the center of mass (COM) of the segmented kidneys in the first dynamic. The motions of the COM of both kidneys during all PBHs were computed, expressed as vector lengths, and plotted over time with a fitted linear line. The slope of this line was defined as mean motion velocity (mm/min). The mean motion velocities of both kidneys and both sessions during PIBH and PEBH were plotted, as well as the mean motion velocities of both kidneys during both PBH strategies during sessions 1 and 2. Considering all data of the left and right kidneys and both sessions, the median PBH duration, median motion, and median mean velocity during PIBH and PEBH were computed. The median mean velocity during sessions 1 and 2 were computed based on data of both kidneys and both PBH strategies. Differences between mean motion velocities per kidney (PEBH minus PIBH, session 1 minus 2) were computed of which the median was calculated. The differences were analysed using the Wilcoxon signed-rank test (α=0.05). Results The median PIBH and PEBH durations were 6.9 (range 2.0–8.0) and 5.6 (range 1.8–7.9) min respectively. Median left and right kidney motion was 23.0 (range 10.8–71.4) mm during PIBH and 24.6 (range 6.8–51.3) mm during PEBH. The median mean velocity was 4.6mm/min during PIBH and 4.9mm/min during PEBH, and 4.8mm/min during session 1 and 4.6mm/min during session 2. Figure 1 shows the mean motion velocities during PIBH and PEBH; the median difference between mean velocities during PIBH and PEBH was 0.2 (range - 4.4–3.4) mm/min (p=0.30). Figure 2 illustrates the mean motion velocities of sessions 1 and 2; the median difference was 0.0 (range -3.8–3.6) mm/min (p=0.60).