ESTRO 36 Abstract Book

S72 ESTRO 36 2017 _______________________________________________________________________________________________

and range deviations are quantified for two different treatment sites. Material and Methods Based on a database of more than 1000 clinical DECT scans acquired with a single-source DECT scanner (Siemens Somatom Definition AS), 10 prostate cancer and 54 head tumor patients were selected to assess intra- and interpatient tissue diversity and its impact on SPR prediction. To evaluate age- and sex-dependent variability, the head tumor cohort was divided in children, women and men. DECT scans were converted in 79 keV pseudo-monoenergetic CT scans (MonoCTs) and SPR datasets derived by voxelwise calculations of electron density and effective atomic number using syngo.via (Siemens Healthineers). In XiO (Elekta) clinical proton treatment plans were recalculated (a) on MonoCTs using the clinical HLUT and (b) on SPR datasets to quantify range and dose differences. Results The voxelwise correlation of SPR and CT number is similar for men and women, but differs considerably between adults and children in bony tissue, likely due to the amount of calcium embedded in bones, which increases with age. Based on voxelwise SPR comparisons, the clinical HLUT predicts on average (2.2 ± 0.6) % larger SPRs for head tumor patients and (1.7 ± 0.3) % larger SPRs for prostate cases. The impact of both approaches on dose distributions is shown in Fig. 1 and 2 for an exemplary head tumor and prostate cancer patient. In the head case, the HLUT predicts a 1.7 % shorter range (2.4 mm) resulting from a 0.7 mm range underestimation in water-filled ventricles (not precisely predicted by the HLUT) and different SPR predictions for brain. A range deviation of up to 3.0 % (7.1 mm) is obtained in the prostate case, which is mainly caused by different SPR predictions for bone marrow and muscle. These range differences in single beams are not compensated in the overall treatment plan.

Conclusion This study highlights the need for chamber-depended response maps when using LAICs for absolute and relative dosimetry with proton pencil beams or small photon beams. OC-0150 Dual-energy CT-based proton treatment planning to assess patient-specific range uncertainties P. Wohlfahrt 1,2 , C. Möhler 3,4 , W. Enghardt 1,2,5,6 , S. Greilich 3,4 , C. Richter 1,2,5,6 1 OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden- Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany 2 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology, Dresden, Germany 3 German Cancer Research Center DKFZ, Division of Medical Physics in Radiation Oncology, Heidelberg, Germany 4 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany 5 Department of Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Dresden, Germany 6 German Cancer Consortium DKTK, Dresden, Germany Purpose or Objective To reduce range uncertainties in particle therapy arising from a generic heuristic conversion (HLUT) of CT numbers in stopping-power ratios (SPRs), an accurate patient- specific SPR prediction is desirable. Treatment planning based on dual-energy CT (DECT) can account for tissue diversity and potentially contribute to shrink clinical safety margins. Consequently, in this study dose distributions derived from both a clinical HLUT and a patient-specific DECT-based SPR prediction are compared