Abstract Book

ESTRO 37

S512

3 National Center for Radiation Research in Oncology, Heidelberg Institute for Radiooncology, Heidelberg, Germany 4 Institute of Radiooncology – OncoRay, Helmholtz- Zentrum Dresden-Rossendorf, Dresden, Germany 5 OncoRay – National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden- Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany 6 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Dresden, Germany Purpose or Objective Magnetic resonance (MR)-guided radiotherapy shows high potential to improve the precision and accuracy of radiation therapy. Especially hybrid MR-LINAC devices provide the possibility to perform on-line MR imaging during dose delivery and offer efficient tumour tracking or gating techniques for high-precision treatment of moving tumours. For the commissioning of such systems, the accuracy and reliability of real-time motion tracking through MR-imaging needs to be assessed. In this study we evaluate the dynamic target localization accuracy of a programmable MRI-compatible motion phantom using clinical cine-MRI sequences in all three spatial dimensions. Material and Methods The phantom (CIRS Model 008M MRI-LINAC Dynamic Phantom) has a body which represents a human thorax in shape and proportion and which was filled with a 6,26g/l NaCl water solution. It incorporates an off-centric cylindrical rod with an embedded gel-based target (see Fig. 1) that can be moved and rotated through a programmable actuator. Sinusoidal motion trajectories with patient-oriented breathing frequencies (0.1-0.2 Hz) and motion amplitudes (5 mm – 20 mm) in all three spatial dimensions were programmed in the phantom’s Motion Control Software. Balanced steady-state free precession sequences (TE/TR= 1.11/2.22 ms; FOV=300×300×7 mm³; Res=1.34×1.34 mm 2 , Slice Thickness=7 mm, FA=30°, BW=1944 Hz, SENSE=1) were acquired in cine mode on a 3T MR scanner (Philips Achieva) with a time resolution of 489 ms. The center-of- mass motion of the target was extracted from the cine images using a manual segmentation-based procedure. Both the measured frequency and amplitude were compared to the programmed motion parameters. The frequencies were determined using Fast Fourier Transform (FFT). The phantom was also fed with a real patient’s 1D-navigator-based breathing pattern to evaluate the accuracy of non-regular target motion detection. Results The frequencies ( f ) and amplitudes ( A ) extracted from the cine-MRI are in good agreement with the pre-set values (see Fig. 2). For the sinusoidal motion patterns, we observed 2% deviations between the measured and pre-set frequencies in IS direction for f =0.1 Hz and f =0.2 Hz with A =20 mm. In the AP and LR direction, the frequency deviation is 3% for f =0.2 Hz and A =5 mm. The measured amplitudes agreed to 99% in IS, and 92% in AP/LR direction with deviations smaller than 0.4 mm. For the real patient’s navigator breathing-pattern with main frequency components between 0.14-0.2 Hz and amplitudes between 5-20 mm we observed an amplitude agreement of 98% with a maximum deviation of 1.2 mm

in IS direction. The uncertainties in frequency and amplitude are dominated by the spatial and time

resolution. Conclusion

Localization of a moving phantom target through cine-MRI was demonstrated to be feasible with high spatial and temporal accuracy. The motion parameters of the MRI- LINAC Phantom could reliably be extracted from the cine- MRI in all 3 dimensions.

PO-0942 A superior alternative to the conventional Varian two-marker RPM™ box for respiration monitoring S. Damkjaer 1 , N.K. Jensen 1 , L.S. Fog 1 , M. Josipovic 1 1 Rigshospitalet, Department of Oncology- Section for Radiotherapy - 3994, København, Denmark Purpose or Objective To compare a novel respiratory motion surrogate (NRS) that could be positioned stably on the sternum of any patient regardless of adiposity or chest curvature to the Varian two-marker RPM™ box (TMB). Specifically, surface dose enhancement, detected motion amplitude and clinical performance were evaluated. Material and Methods NRS (Figure 1) was made from 0.25mm thick polyester and consisted of a 9mm tall circular base with either a 60mm or a 72mm tall vertical stalk (XL version). Reflective markers were placed on the stalk with 30mm spacing to emulate TMB. The reflectors on the NRS were placed 21mm (33mm for the XL version) above the base, making them visible to the RPM system on all patients. The narrow base (38mm diameter) enabled the surrogate to be positioned flat on the sternum. Surface dose enhancement was measured with Gafchromic EBT3™ film, placed on a 5cm thick Solid Water™ backscatter block and irradiated with a 10x10cm 2