ESTRO 36 Abstract Book

S155 ESTRO 36 2017 _______________________________________________________________________________________________

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.

weeks, keeping the same positioning. The intra-fraction reproducibility of the lung anatomy during breath hold was investigated, by comparing the MRI of the first breath hold with the three other MRIs of the same session. The inter- fraction anatomical reproducibility was investigated by comparing the first breath hold MRI of the first session with the four MRIs during the second session. To avoid any influence of setup variation, first a global rigid image registration was performed. Then the lung volume was semi-automatically segmented to define a region of interest for the deformable image registration (DIR). DIR was performed using Mirada RTx v1.2 (Mirada Medical, Ltd.), with a DIR grid resolution of 3.5x2x3 mm 3 . The deformation vector fields were analyzed using MATLAB v2014b. Magnitudes of the deformation vectors were calculated and combined for all five volunteers. The lung volumes were divided into six segments, to analyze the anatomical displacements on a local level. A boxplot showing the intra- and inter-fraction displacements with a schematic view of the six segments can be seen in figure 1. Results The lung volumes for all breath holds varied by 2% within and 7% between fractions. Looking at all five volunteers, up to 2 mm median intra- and inter-fraction displacements were found for all lung segments. The anatomical reproducibility decreased towards the caudal regions. Inter-fraction displacements were larger than intra- fractional displacements. Maximum displacements (99.3% of the magnitude vectors) reached 6 mm intra-fractionally and did not exceed 8 mm inter-fractionally.

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

Conclusion While the lung volume differences were insignificant, relevant anatomical displacements were found. Moreover, a trend of increased displacements over time could be seen. ABC mitigates motion to some extent. Nevertheless, the remaining reproducibility uncertainties need to be considered during scanned proton therapy treatments. As next step, we aim to include this knowledge in a model to estimate their dosimetric influence for scanning proton therapy.