ESTRO 36 Abstract Book

S156 ESTRO 36 _______________________________________________________________________________________________

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

Dosimetric analysis confirmed the ability of the model to approximate the ground truth (mean differences of 0.49Gy, 0.12Gy and 0.38Gy for PTV D98, PTV D2 and spinal cord, respectively). For the patient, variations between the in-room inhale phase and the corresponding planning phase were 3.1mm/4.9mm on the tumor/diaphragm. With respect to the planning inhale CT, the model output CT presented differences mainly on the diaphragm position (Figure A). Dosimetric changes with respect to the planned dose were 3.14Gy, 1.82Gy and 0.42Gy for tumor PTV D98, PTV D2 and spinal cord, respectively (Figure B and C). The delivered dose was higher than planned since less motion was present in the MR images than the planning CT.

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.

Conclusion We provided a dosimetric evaluation based on a global motion model for MRI-guidance. The proposed model built on 4DCT was updated based on interleaved 2D MRI data and validated using a digital phantom. Dosimetric variations on tumor were observed in the patient study, demonstrating the utility and importance of using motion models for dose accumulation. Future work will include improvements in the motion model for MRI-guidance and its application to a larger number of patients. OC-0303 Evaluation of lung anatomy vs. lung volume reproducibility for scanned proton treatments under ABC. L.A. Den Otter 1 , E. Kaza 2 , R.G.J. Kierkels 1 , M.O. Leach 2 , D.J. Collins 2 , J.A. Langendijk 1 , A.C. Knopf 1 1 UMCG University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands 2 The Institute of Cancer Research and The Royal Marsden Hospital, CR-UK Cancer Imaging Centre, London, United Kingdom Purpose or Objective Proton therapy is a highly conformal way to treat cancer. For the treatment of moving targets, scanned proton therapy delivery is a challenge, as it is sensitive to motion. The use of breath hold mitigates motion effects. Due to the treatment delivery over several fractions with delivery times extending the feasible breath hold duration, high reproducibility of breath holds is required. Active Breathing Control (ABC) is used to perform breath holds with controlled volumes. We investigated whether the lung anatomy is as reproducible as lung volumes under ABC, to consider ABC for scanned proton treatments. Material and Methods For five representative volunteers (3 male, 2 female, age: 25-58, BMI: 19 – 29) MR imaging was performed during ABC at two separate fractions. The image voxel size was 0.7x0.7x3.0 mm 3 . Each fraction consisted of four subsequent breath holds, resulting in a total of eight MRIs per volunteer. The interval between fractions was 1-4