Abstract Book

S101

ESTRO 37

Purpose or Objective MRI-Linacs enable real-time imaging for advanced in- room motion management in radiotherapy. However, the constraint of acquisition time and image quality suggests 2D cine-MRI centered in the tumor as state-of-the-art imaging for motion detection. Bridging the gap between 2D and 3D images is therefore required, with patient- specific motion models being the most viable solution. Standard global motion models are built using a single surrogate, thus assuming a linear correlation between surrogate and changing anatomy, independently from the anatomical location. To date, no global motion model based on 4DCT and cine-MRI images able to provide regional adaptation has been reported. In this work, we present a novel 4DCT global motion model based on Regions Of Interest (ROIs), aiming at accurately compensating for changes during treatment. Material and Methods 4DCT, 3DT1-weighted MRI and 2D sagittal cine-MRI data were generated with a digital CT/MRI phantom (resolution 1×1×1mm3) animated by patient-derived signals. In the simulated planning phase (9mm motion), a motion model was built on 3D motion fields derived from a 3D deformable image registration (DIR) between breathing phases in the 4DCT dataset. In this step, three regions of interest (ROIs) were defined (upper, middle and lower lung) and correlated with surrogate motion fields in correspondent ROIs. Surrogate motion fields were derived by a 2D DIR between sagittal CT slices centered in the tumor (reference 0% exhale: CTslice0%). In the simulated treatment phase (16mm motion), the model was corrected by 3DMRI to compensate for inter- fraction motion and applied to in-room cine-MRI data to compensate for intra-fraction motion. The application was performed in each ROI by deriving a pseudo cine-CT during treatment (i.e. the in-room surrogate motion fields were obtained by 2D DIR between CTslice0% and cine-MRI). FigureA shows the workflow of the ROI-based motion model. Analysis of the method was carried out by considering geometrical differences of target and diaphragm between the estimated 3DCT and the ground truth provided by the phantom. A comparison with a conventional motion model based on single surrogate (tumor or diaphragm) was also performed. Results For the tumor, the proposed method resulted in a difference (median-IQR) with respect to the ground truth of 0.87-0.59mm, against 0.97-0.48mm and 1.15-1.10mm when using tumor and diaphragm alone as surrogates. For the diaphragm, the ROI-based method resulted in an error of 0.69-0.93mm, whereas 2.82-2.09mm and 1.66- 2.11mm were measured for tumor and diaphragm, respectively. As a result, the ROI-based motion model was able to compensate on both tumor and diaphragm together (FigureB). Conclusion A novel ROI-based motion model was proposed with improved image guidance results with respect to conventional strategies. Future studies will rely on the application of the method to patient data and on a dosimetric evaluation to enable closed-loop adaptive radiotherapy.

OC-0189 First MLC-tracking on the 1.5T MR-linac system M. Glitzner 1 , P.L. Woodhead 1 , J.J.W. Lagendijk 1 , B.W. Raaymakers 1 1 UMC Utrecht, Department of Radiotherapy, Utrecht, The Netherlands Purpose or Objective MRI-guided radiotherapy promises better treatment options due to the greatly improved observation capabilities of MRI as compared to other on-line modalities. In addition to fine contrast in anatomy and pathology, MRI offers relatively fast imaging frequencies, enabling real-time feedback control of the delivery beam for motion-compensated treatments. Currently, MRI- guided treatment devices support respiratory gating, triggered by the anatomy as seen in the MR-images. In this work, we present first results of MRI-guided MLC- tracking using a clinical 1.5T MR-linac sytem. We determine tracking latency and investigate its sensitivity to MRI sampling schemes and control settings. Material and Methods All experiments were performed on the clinical prototype of the Elekta Unity MR-linac (Elekta AB, Sweden). The machine features a 1.5T high-field MRI and a 7MV linac. The beam is shaped using an MLC with 80 leaf pairs moving in fixed y-direction (IEC 1217). For all experiments, a circular aperture with 40 mm diameter was applied. Every 40 ms, leaf position updates were sent to a prototype linac control system using an in-house developed tracking software. In parallel to steering the leaves, the tracking software processes a real-time stream of images from the MRI- machine with 4 Hz and 8 Hz, respectively. For each image, the position of a moving object was determined using a center of gravity algorithm. Accordingly, the MLC was steered to reach the new position. The object consisted of cast agarose gel and a circonium oxide ball bearing immersed in the gel and was thus fully visible in both MRI and EPID. The non-metallic bearing guaranteed minimal MR-image distortions. The object was moved using a QUASAR MRI4D motion phantom (Modus QA, Canada), driving a sinusoidal motion with 15 mm aplitude and 10s period. Motion was applied in