ESTRO 36 Abstract Book

S188 ESTRO 36 _______________________________________________________________________________________________

plans needs to be built to achieve the full potential of ART. OC-0354 Dosimetric impact of anatomical changes in photon and particle therapy for pancreatic cancer K. Crama 1 , A.C. Houweling 1 , J. Visser 1 , K. Fukata 2 , C.R.N. Rasch 1 , T. Ohno 2 , A. Bel 1 , A. Van der Horst 1 1 Academic Medical Center, Department of Radiation Oncology, Amsterdam, The Netherlands 2 Gunma University, Gunma University Heavy Ion Medical Center, Maebashi, Japan Purpose or Objective Radiotherapy of pancreatic tumors is toxic due to the high dose to surrounding organs-at-risk (OARs). Irradiation with charged particles is characterized by a sharp dose fall-off around the target area. Compared to photon therapy, OARs can be further spared while delivering a high dose to the tumor. Treatment planning studies in pancreatic cancer patients have shown this benefit of charged particle therapy over photon therapy. However, intra- and interfractional changes may greatly affect the robustness of particle therapy. Past studies only investigated differences in planned dose; studies comparing the robustness of different modalities have not been published yet. We compared the dosimetric impact of interfractional anatomical changes (i.e. body contour differences, gastrointestinal gas volume changes and setup errors) in photon, proton and carbon ion therapy for pancreatic cancer patients. Intrafractional changes were not taken into account in this study. Material and Methods Photon, proton and carbon ion treatment plans (36 Gy, 12 fractions) were created for 9 patients. For the particle therapy plans, the relative radiobiological effectiveness was taken into account. To simulate daily online setup correction, the CBCTs were rigidly registered (only translations) to the planning CT using fiducial markers. Fraction dose calculation was then made possible by deformable registering the planning CT to each of the 12 CBCTs. Gastrointestinal gas was delineated on each CBCT and copied to the deformed CT, a relative density override was applied for dose calculation (0.01). Fraction doses were accumulated rigidly. To compare planned and accumulated dose, for each radiotherapy modality, dose volume histogram (DVH) parameters of the planned and accumulated dose were determined for the internal gross tumor volume (iGTV), internal clinical target volume (iCTV) and OARS (duodenum, stomach, kidneys, liver and spinal cord).

Results Photon plans were highly robust against interfractional anatomical changes. The difference between planned and accumulated DVH parameters for the photon plans was ≤0.5% for the target and OARs. For proton therapy, coverage of the iCTV was considerably reduced for the accumulated compared to the planned dose: the mean near-minimum dose (D98%) of the iCTV reduced from 98.1% to 90.3% [79.4%–95.3%](Figure). For carbon ion therapy it was even worse; D98% was reduced with 10%, from 98.6% to 88.6% [80.7%–92.7%]. The DVH parameters of the OARs differed ≤3% between both particle modalities. For all modalities the near-maximum dose (D2%) did not differ. Conclusion Photon therapy is highly robust against interfractional anatomical changes and setup errors in pancreatic cancer patients. However, in particle therapy with either protons or carbon ions, severe reductions in target dose coverage were observed. Implementation of particle therapy for pancreaticcancer patients should be done with great care and interfractional anatomical changes must be accounted for. OC-0355 Which anatomical changes in Head&Neck cancer lead to Repeat CT/planning? S. Van Beek 1 , O. Hamming-Vrieze 1 , A. Al Mamgani 1 , A. Navran 1 , J. Van de Kamer 1 , P. Remeijer 1 1 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands

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