ESTRO 36 Abstract Book

S221 ESTRO 36 2017 _______________________________________________________________________________________________

obtained are instrumental for building and validating beam models for MRiPT, as reported in a separate abstract. PV-0422 Direct determination of kQ in a clinical carbon ion beam using water calorimetry J.M. Osinga-Blättermann 1,2 , U. Ankerhold 1 , S. Brons 3 , S. Greilich 2 , O. Jäkel 2,3 , A. Krauss 1 1 Phys. Techn. Bundesanstalt PTB, Department of Dosimetry for Radiation Therapy and Diagnostic Radiology, Braunschweig, Germany 2 German Cancer Research Center DKFZ, Division of Medical Physics in Radiation Oncology, Heidleberg, Germany 3 Heidelberg Ion-Beam Therapy Center, HIT, Heidelberg, Germany Purpose or Objective Until now, the dosimetry of carbon ions with ionization chambers has not reached the same level of accuracy as that of high-energy photons: the associated standard uncertainties differ by about a factor of three [TRS-398, IAEA, 2000]. This is mainly caused by the limited knowledge of the so-called k Q factor, which corrects for the different response of the ionization chamber to the actual user beam quality Q (here: 12 C) compared to the reference beam quality Q 0 (here: 60 Co). The aim of this work is to experimentally determine the k Q factor in order to exploit the possibility of significantly improving the accuracy of ionization chamber-based dosimetry of clinical carbon ion beams. Material and Methods Water calorimetry by means of the transportable water calorimeter of the National Metrology Institute of Germany (PTB - Physikalisch-Technische Bundesanstalt) is implemented in the entrance channel of a scanned 6 cm x 6 cm radiation field of 429 MeV/u carbon ions at the Heidelberg Ion-Beam Therapy Center (HIT). This enables the direct calibration of ionization chambers and thus the experimental determination of k Q . In order to achieve an overall low measurement uncertainty, the irradiation parameters and the resulting radiation field have been characterized in detail as they strongly influence several calorimetric and ionometric correction factors. In total, three separate series of measurements were performed to determine the values for k Q for the two Farmer-type ionization chambers FC65-G (IBA) and TM30013 (PTW). Results By means of water calorimetry, a standard measurement uncertainty of 0.8% could be achieved for the experimental k Q values corresponding to about a threefold reduction of the uncertainty compared to calculated values. For both ionization chambers, a comparison of the experimental k Q factors with corresponding literature values (TRS-398, German DIN 6801-1) will be presented and discussed. Conclusion This study showed for the first time that the experimental determination of the k Q factor for carbon ion beams by means of water calorimetry is achievable with unprecedented accuracy. This result enables the significant reduction of the overall uncertainty related to ionization-based dosimetry of clinical carbon ion beams. PV-0423 AAPM TG-158 recommendations for neutron dosimetry for photon, electron, and light-ion therapy. R. Howell 1 , B. Bednarez 2 , S. Kry 1 1 UT MD Anderson Cancer Center Radiation Physics, Radiation Physics, Houston- TX, USA

outside the treatment volume from external-beam radiation therapy (EBRT), was created to provide guidance for physicists in assessing and managing non-target doses. Neutron detection in particular, presents many unique challenges and is infrequently performed by medical physicists. Neutron data in the literature span many orders of magnitude and are difficult to interpret and compare. The primary objectives of this presentation are convey the neutron relevant information from TG-158 for photon, electron, and light-ion EBRT: (1) to provide an overview of neutron data reported in the literature (2) to summarize various detectors that can be used to measure secondary neutrons and specifically address limitations in different measurement environments and (3) summarize recommendations from AAPM TG-158 for neutron dosimetry for clinical care and research applications. Material and Methods The TG-158 was completed in 2016 and is expected to be published in 2017. The committee reviewed approximately 320 publications in the litera ture, a large fraction of which focused on secondary neutrons and neutron measurement techniques for photon, electron, and light-ion EBRT. Results This presentation will provide an overview of neutron data reported in the literature, which span m any orders of magnitude. Figure 1 is an example of neut ron data from proton and carbon therapy. Similar examples will be discussed for photon and electron beam EBRT. This presentation will also summarize various detectors that can be used to measure secondary neutr ons. Neutron detectors are highly energy dependen t and thus, knowledge of the energy spectrum being measured is essential. The secondary neutrons from electron, photon, and light-ion therapy have a wide energy range, i.e., from thermal up to about 10 MV for photon/electron therapy and thermal up to 250 MeV for light ion therapy. Moreover, many neutron detectors cannot be used in or near the primary field because of issues such as pulse-pile-up and interactions of particles within the detector, among others. Thus, each neutron detector will be presented in the context of its energy sensitivity and its suitability for measurements in-or near the primary field. Finally, this presentation will summarize the recommendations of TG-158 for neutron dosimetry for clinical care and research applications.

Figure 1: Summary of published data from several studies of neutron dose equivalent from proton and carbon therapy. The upper and lower bounds of neutron dose equivalent from photon IMRT data are also included for reference (solid grey lines). Conclusion This presentation will highlight the unique challenges of measuring neutrons and will provide guidance on how to select the most appropriate for these measurements for photon, electron, and light-ion therapy.

2 University of Wisconsin, Medical Physics, Madisson, USA

Purpose or Objective The American Association of Physicists in Medicine Task Group (TG) 158, measurement and calculation of doses