ESTRO 36 Abstract Book

S66 ESTRO 36 _______________________________________________________________________________________________

PV-0137 Validation of fast motion-including dose reconstruction for proton scanning therapy in the liver E. Colvill 1 , J. Petersen 1 , M. Høyer 1 , E. Worm 1 , R. Hansen 1 , P . Poulsen 1 1 Aarhus University Hospital, Radiation Oncology, Aarhus C , Denmark Purpose or Objective Th e large respiratory motion present during liver treatments means that the interplay effect could be of grea t concern for hypofractionated proton scanning therapy. A method has been developed to use recorded 3D motion to calculate the dose delivered to moving targets in scanning proton therapy using a commercial treatment planning system (TPS). In this study, we validate the method for the application in liver where tumor motion and other anatomical changes are large. Material and Methods The fast dose reconstruction utilizes an in-house computer program to manipulate dicom plans exported from the TPS and incorporates the motion by shifting each proton spot to its position as seen in tumor’s eye view. Depth motion and SSD variations are modeled by modifying the beam energy. The motion-included plans are imported and recalculated in the TPS in the original CT scan. To validate the accuracy of the dose reconstruction the 4DCTs of 13 liver patients treated with photon SBRT were used. For each patient, an IGTV was created from exhale and inhale GTVs. A plan was created on both inhale and exhale phases using three-field single-field-optimization with ~6mm spot spacing and mean IGTV coverage of 56.25Gy in three fractions. Four dose reconstruction plans were created for each patient to represent potential interplay effects, two from each of the original inhale and exhale plans; One plan with all odd layers shifted to the opposite phase and one with all even layers shifted. These were imported and calculated on the original CT phase. Corresponding ground truth doses were created by splitting the original plans into sub-plans with odd and even layers, calculating one sub-plan in the original CT phase and the other in the opposite phase, and then summing the two dose maps. The reconstructed plans were compared to the ground truth plans to assess the accuracy of the fast dose reconstruction and the original to the ground truth plans to assess the magnitude of interplay effects. The comparisons included D5, D95 and the homogeneity index (HI=(D2-D98)/D50) for the GTV and the root-mean-square (RMS) dose errors for dose points above 70% and 90% cutoff levels. Results To date validation on two patients (8 plans) has been completed. Figure1 shows the results for one plan. The mean and range of errors of the fast dose reconstruction GTV D5, D95 and RMS errors above 70% dose and above 90% dose are shown in Table1 along with the interplay effect results for comparison. The mean and range of the HI for the original, ground truth and reconstructed plans were 0.07(0.05-0.08), 0.19(0.13-0.28) and 0.19 (0.13-0.28) respectively. Conclusion Validation of a method of fast dose reconstruction to assess the dosimetric impact of tumor motion on scanning proton therapy for liver SBRT treatments was performed. The results show that the dose reconstruction can utilize a single phase of a 4DCT with dose calculation errors that are much smaller than the dose distortion created by the interplay effect itself.

PV-0138 Pencil beam scanning treatments in free- breathing lung cancer patients – is 5 mm motion a limit? A. Jakobi 1 , R. Perrin 2 , A. Knopf 2,3 , C. Richter 1,4,5,6 1 OncoRay - Center for Radiation Research in Oncology, High Precision Radiotherapy, Dresden, Germany 2 Paul Scherrer Institute, Center for Proton Therapy, Villigen, Switzerland 3 University of Groningen - University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands 4 German Cancer Consortium Dresden and German Cancer Research Center Heidelberg, DKTK, Dresden, Germany 5 Faculty of Medicine and University Hospital Carl Gustav Carus - Technische Universität Dresden, Department of Radiation Oncology, Dresden, Germany 6 Helmholtz-Zentrum Dresden – Rossendorf, Institute of Radiooncology, Dresden, Germany Purpose or Objective To evaluate the dose degradation when treating lung cancer patie nts with proton pencil beam scanning during free-breathing. We assess if treatments without rescanning are feasible in order to avoid prolonged treatment time, especially for slow scanning facilities. Material and Methods For 40 lung cancer patients, 4DCT imaging was used to generate 4D dynamic dose distributions of 3D treatment plans with 3 pencil beam scanning fields optimised with the single field uniform dose technique. Simulations included the use of random breathing states of the patient at start of irradiation resulting in multiple possible 4D dynamic dose distributions per fraction. Complete treatment was assumed to consist of 33 fractions of probabilistically chosen single fractions. Treatments were assumed to be delivered with an IBA universal nozzle without rescanning (1.5ms between spots, 2s between energy layers, spot sigma 4mm at highest energy). Tumour motion amplitude was the maximum displacement in tumour centre-of-mass assessed by the 4DCT. Evaluation was done by looking at under- and overdosage in the target structure. In addition, changes in the dose distribution due to changes in motion and anatomy during treatment were analysed using a repeated 4DCT for 4D dynamic dose calculation in one patient case.