ESTRO 36 Abstract Book

S154 ESTRO 36 2017 _______________________________________________________________________________________________

Nanotechnology- Center for Nanomedicine and Theranostics, Lyngby, Denmark 4 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective The purpose of this study was to estimate the intra- and inter-breath-hold tumour position uncertainty in voluntary deep-inspiration breath-hold (DIBH) radiotherapy for patients with locally advanced non-small cell lung cancer. Material and Methods Patients had liquid fiducial markers injected in mediastinal lymph nodes, and, if possible, in the primary tumours. Treatment was delivered during DIBH. Anterior and lateral fluoroscopic movies were acquired in free breathing (FB) and visually guided DIBH at three fractions (start, middle and end) during radiotherapy (33 fractions, 2 Gy per fraction) of nine patients with locally advanced non-small cell lung cancer. Fluoroscopies were acquired post treatment for two perpendicular gantry angles (Figure 1). Marker excursions in free breathing and DIBH, inter-breath-hold position uncertainty, systematic and random errors during DIBH in each of the three cardinal directions were investigated using an image based tracking algorithm, defining the marker template as one of the images from the middle of the first DIBH fluoroscopy. The mean marke r position during each DIBH, relative to a template frame for the first fluroscopy, was regarded as each fractions and markers uncertainty during the DIBH. A systematic error for the patient group was calculated as the standard deviation (SD) of all these mean marker positions. The standard deviation of the markers position within each DIBH was used to quantify the intra-breath- hold uncertainty (Figure 1). A root mean square (RMS) of the intra-DIBH SD was calculated to estimate random errors.

0.6 mm (LR), and 1.0 mm (CC) (Figure 2).

Conclusion Our study showed that the motion of lung tumours could be substantially reduced, but not eliminated, using visually guided DIBH radiotherapy. Intra- and inter-breath- hold position uncertainty of the tumour and lymph nodes were mostly less than 2 mm for visually guided DIBH radiotherapy of non-small cell lung cancer. OC-0302 Dosimetric evaluation of a global motion model for MRI-guided radiotherapy C. Paganelli 1 , S. Albertini 1 , F. Iudicello 1 , B. Whelan 2 , J. Kipritidis 2 , D. Lee 2 , P. Greer 3 , G. Baroni 1 , P. Keall 2 , M. Riboldi 1 1 Politecnico di Milano, Dipartimento di Elettronica- Informazione e Bioingegneria, Milano, Italy 2 University of Sydney, Radiation Physics Laboratory- Sydney Medical School, Sydney, Australia 3 Calvary Mater Newcastle, Department of Radiation Oncology, Newcastle, Australia Purpose or Objective MRI-Linac therapy will enable real time adaption of radiotherapy and is being actively developed by several academic and commercial groups. To acquire images of high spatial and temporal resolution, interleaved 2D imaging is typically used. However, to enable closed loop adaptive radiotherapy, accumulated 3D dose is required. A possible way to bridge the gap between 2D and 3D images is via patient-specific motion models. To date, no dosimetric evaluation of a global motion model based on interleaved MRI images has been reported. In this work, we present the use of a global motion model to compensate for geometric changes during treatment and to evaluate dosimetric variations between the delivered and planned dose distributions. Material and Methods 4DCT and interleaved sagittal/coronal cine-MRI from a diagnostic scanner (1.5T) were acquired for a lung cancer patient. A global motion model was built on the 4DCT dataset using principal component analysis, and updated through the use of surrogates derived from in-room cine- MRI data (tumor, diaphragm and lung vessel motion). An ITV-based IMRT treatment plan (60Gy in 30 fractions) was developed on the 4DCT and applied to the model output for dose evaluation. Validation of the motion model was performed on a CT/MRI XCAT phantom (1mm resolution), in which the ground truth CT output of the in-room scenario was available at the time sample of the simulated cine-MRI. Analysis of different surrogates as well as their sagittal/coronal motion components were performed in terms of both geometric and dosimetric variations. Results Based on the phantom data, the accuracy of the motion model was 1.2mm/1.6mm on tumor/diaphragm.

Results A reduction of 2-6 mm in marker excursion in DIBH compared to FB was observed in the three cardinal directions (anterior-posterior (AP), left-right (LR) and cranio-caudal (CC)). Fourier transformation of the motion trajectories indicated that the lymph node motion during DIBH mainly originated from cardiac motion. The systematic errors during DIBH were 0.5 mm (AP), 0.5 mm (LR) and 0.8 mm (CC). The random errors during DIBH were 0.3 mm (AP), 0.3 mm (LR), and 0.4 mm (CC). The standard deviation of the inter-breath-hold shift was 0.8 mm (AP),