ESTRO 38 Abstract book

S213 ESTRO 38

(INTRA) which was compared to the original reference (REF) dose.

Results Figure 1 shows the D99% point of the target structures between REF and INTRA dose for these 20 patients. The PTVs (average decrease of 9.4%±6.8%) ensured the full CTV/GTV coverage for 80% patients. The average drop in D99% coverage for CTV_35 (prostate) and CTV_30 (seminal vesicles) was 1.7%±2.8% and 4.1%±4.7% respectively while the GTV dropped by 3.2%±3.6% with a mean dose reduction of 2.8%±2.4%. Figure 2 shows boxplots of the Dmax for rectum and urethra respectively. Rectum Dmax (40 Gy) was exceeded in 4 cases while V35Gy was stable (0.1%±1.1%).

Conclusion In breath-hold adrenal SABR, use of visual MR feedback achieved high geometric tumor coverage, but beam latency effects increased in patients who performed subsequent breath-holds in other phases of respiration. OC-0412 Dosimetric impact of marker-based intrafraction motion from cine-MRI in prostate SBRT C. Kontaxis 1 , D. De Muinck Keizer 1 , L. Kerkmeijer 1 , H. De Boer 1 , B. Raaymakers 1 1 UMC Utrecht, Radiotherapy department, Utrecht, The Netherlands Purpose or Objective To investigate the effect of intrafraction translation and rotation motion, as extracted from 3D cine-MRI based on fiducial marker tracking, on the dose distributions of extremely hypofractionated (SBRT) prostate patients. Material and Methods Twenty prostate patients treated with extremely hypofractionated radiotherapy (4 mm CTV to PTV margin, 5 × 7 Gy, with focal boosts up to 5 × 10 Gy) underwent weekly cine-MRI within an Ethical Review Board approved study. Fiducial marker-based online corrections were applied during treatment. Each cine-MRI session consisted of a balanced 3D gradient echo sequence, acquiring one volume every 11 sec over 10 min. A marker tracking algorithm was used to obtain the prostate marker-based intrafraction motion in subsequent dynamics for each session (ESTRO 37). The fiducial positions from the first cine-MR dynamics were registered to the planning CT, thus transferring the local cine-MRI motion to the CT coordinate system. Then for each patient fraction, the treatment plan delivery consisting of two VMAT arcs was played back, by proportionally splitting the MUs into 11 sec intervals leading to 33 partial plans during a 6 min delivery. For each partial plan, the corresponding volume was generated by applying the cine-MRI rigid transformation to the CT, the partial dose was calculated using our research treatment planning system and was then warped back to the reference volume by using the inverse transformation. The mean±SD translations (mm) were 0.04±0.79 (LR), 0.4±1.9 (AP), -0.5±1.73 (CC) and rotations (degrees) were -0.84±2.91 (LR), 0.13±1.15 (AP), 0.25±1.84 (CC) among all patients and timepoints. For each fraction the 33 partial doses were summed leading to the accumulated fraction and total treatment dose

Conclusion We have simulated the dosimetric impact due to prostate intrafraction motion as extracted by marker-based tracking on cine-MRI for extremely hypofractionated treatments. Evaluating the impact of intrafraction motion with such high spatial and temporal resolution has now become possible with MRI-linacs. The analysis presented is essential to establish the optimal inter/intrafraction adaptation strategies for MRI-guided radiotherapy, which will ensure continuous online target coverage while safely decreasing margins. We are now working towards a full dosimetric analysis of both targets and OARs for more patients. OC-0413 MR-derived signals for respiratory motion models evaluated using sagittal and coronal datasets E.H. Tran 1 , B. Eiben 1 , A. Wetscherek 2 , U. Oelfke 2 , G. Meedt 3 , D.J. Hawkes 1 , J.R. McClelland 1 1 University College London, Medical Physics and Biomedical Engineering, London, United Kingdom ; 2 The Institute of Cancer Research and the Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom; 3 Elekta, Medical Intelligence Medizintechnik GmbH, Schwabmünchen, Germany Purpose or Objective MR-Linac systems can provide 2D cine-MR images to monitor the motion of a tumour during beam delivery, enabling gated or tracked radiotherapy treatments.