ESTRO 2022 - Abstract Book

S194

Abstract book

ESTRO 2022

Z. van Kesteren 1 , J. Veldman 1 , M. Parkes 1 , G. Tienhoven 1 , J. van den Aardweg 2 , M. Stevens 3 , A. Bel 1 , I. van Dijk 1

1 Amsterdam UMC, Radiotherapy, Amsterdam, The Netherlands; 2 Amsterdam UMC, Pulmonology, Amsterdam, The Netherlands; 3 Amsterdam UMC, Anesthesiology, Amsterdam, The Netherlands Purpose or Objective Radiotherapy in the thoracic-abdominal region, requires generous margins to ensure tumor coverage due to respiratory motion, at the expense of healthy tissue dose. We investigated the influence of high frequency mechanical ventilation at 60 breaths/minute (brpm), with positive end expiratory pressure (PEEP) and without percussion, on diaphragm motion amplitude using magnetic resonance imaging (MRI). Materials and Methods After two training sessions, six healthy volunteers were scanned at a 3T MRI during free breathing (FB) and during mechanical ventilation at 60 brpm without PEEP and with 5, 10 and 15 cm H2O PEEP. In one sagittal plane of the right lung, three minute cine-MRIs were acquired consisting of 350 2D images with a rate of 0.51 s/frame. The diaphragm motion amplitude was measured at a fixed point on the right diaphragm. The median sagittal lung surface was calculated on these 350 sagittal images per volunteer. Significance of the differences were determined by a Wilcoxon signed rank test (with the Bonferroni correction applied). Results Mechanical ventilation with 60 brpm and several levels of PEEP was easily adopted and well tolerated by volunteers. Compared to FB, the median (range) diaphragm motion amplitude of all volunteers significantly reduced by 76% (46% - 79%) from 37 (22 – 74) mm to 11 (8 – 17) mm during mechanical ventilation at 60 brpm without PEEP ( α =0.013; p=0.008). The reduction was present for each volunteer and for all of the four levels of PEEP (Fig 1). We could not establish a significant further reduction in motion amplitude for the various levels of PEEP. However, applying higher levels of PEEP moved the diaphragm in a more caudal position resulting in an significantly increased median lung area (Fig 2) of 18% for PEEP5, 31% for PEEP10 and 41% for PEEP15, compared to no PEEP ( α =0.017; p<0.001).

Conclusion Compared to free breathing, mechanical ventilation at 60brpm without PEEP significantly reduced the median diaphragm motion amplitude. Increasing PEEP did not further reduce the motion. However, applying higher PEEP significantly increased the lung surface measured on 2D images compared to no PEEP. Since increased lung volume is associated with lower mean lung dose and a lower diaphragm position, altering the position of the heart, applying high pressure PEEP during radiotherapy may reduce toxicity even further.

PD-0234 Temporal accuracy for gated radiotherapy beams

J. Snäll 1 , A. Thoft 1 , M. Kügele 1,2 , A. Edvardsson 1

1 Skåne University Hospital, Department of Hematology, Oncology and Radiation Physics, Lund, Sweden; 2 Lund University, Department of Medical Radiation Physics, Clinical Sciences, Lund, Sweden Purpose or Objective Radiotherapy (RT) can be delivered synchronized to the patient's respiratory motion, i.e. gating. The temporal accuracy for gated beams, i.e. the time from the gate signal to the beam-on/off, should be within 100 ms according to quality