ESTRO 36 Abstract Book

S73 ESTRO 36 _______________________________________________________________________________________________

1 Medizinische Universität Wien Medical University of Vienna, Department of Radiation Oncology and Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria 2 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria Purpose or Objective A large area ionization chamber (LAIC) can be used to measure output factors of narrow beams. In principle, dose area product measurements are an alternative to central-axis point dose measurements. Using an LAIC requires detailed information on the uniformity of the signal response across its sensitive area. Material and Methods 8 LAICs (sensitive area with nominal diameter of 81.6mm) were investigated in this study, 4 of type PTW-34070 (LAIC Thick ) and 4 of type PTW-34080 (LAIC Thin ) with water- equivalent entrance window thicknesses of 4mm and 0.7mm, respectively. Measurements were performed in an X-ray unit (YXLON) using peak voltages of 100-200kVp and a collimated beam of 3.1mm FWHM. The LAICs were mounted on the moving mechanism of an MP3-P (PTW) and moved with a step size of 5mm to measure the chamber’s response at lateral positions. To account for beam positions where only a fraction of the beam overlapped with the sensitive area of the LAIC, a corrected response was calculated as the basis for determining relative response as a function of radial distance from the centre. The impact of a heterogeneous LAIC response, based on the obtained response maps was henceforth investigated for small field photon beams (as small as 6x6mm²) and proton pencil beams (FWHM=8mm). Results A pronounced heterogeneity of the spatial responses was observed in both the thick and thin window LAICs. These heterogeneities could be calculated as a function of the radial coordinate as there was no pronounced angular dependency. All 4 LAIC Thick followed a monotonously increasing response towards the chamber centre, while the absolute response values varied up to 1.5%, excluding the 2mm borders of the LAICs. In contrast the LAIC Thin trends were not uniform and responses varied by up to 10% (Fig 1). Investigating absolute dosimetry for a proton pencil beam the signal varies with a systematic offset between 2.4% and 4.1% for LAIC Thick and between -9.5% and 9.4% for LAIC Thin . For relative dosimetry (e.g. depth-dose profiles) the increase of beam size with increasing depth was investigated as the influencing factor. Systematic response variation by 0.4% and 1% at the most were found for the investigated LAICs. The systematic offset for absolute dose measurements for decreasing photon field size showed that for 6x6mm² field sizes the response was systematically 2.5-4.5% higher for LAIC Thick . For LAIC Thin the response varies even over a range of 20%. The entrance window thickness was evaluated to be constant within measurement uncertainty by performing measurement at multiple peak voltages.

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