ESTRO 36 Abstract Book

S417 ESTRO 36 _______________________________________________________________________________________________

calculation times considerably, making the presented PBA a suitable candidate for integration in a treatment planning system. The current work demonstrates that proton MRI is feasible from a dosimetric point of view. PO-0786 Energy dependence investigation for detectors used in out-of-filed dosimetry L. Shields 1 , L. Leon-Vintro 2 , B. Mc Clean 3 1 St Luke's Hospital, Medical Physics, Dublin, Ireland 2 University College Dublin, Schoool of Physics, Dublin, Ireland 3 St. Luke's Radiation Oncology Network, Medical Physsics, Dublin, Ireland Purpose or Objective Traditionally, energy dependence of a range of detectors used in radiotherapy has been investigated mainly in the Cobalt-60 and 6-15MV photon range. However, when considering detectors for use in out-of-field dosimetry, it is more important that the energy dependence is investigated over a much lower range. This study examined (i) the mean incident energy of radiation out- of-field for a 6MV photon beam and (ii) the energy dependence of a range of clinically available detectors to the typical energies experienced out-of-field and (iii) Monte Carlo (MC) calculated and detector measured out- An Elekta Synergy Linac operating at 6MV and a water phantom at 90cm SSD was defined in BEAMnrc. Phase spaces were scored at 6 different planes in the water phantom - 0.2, 1.4 (dmax), 5, 10, 15 and 20cm. Each phase space file was analysed using the EGSnrc program package BEAMDP to extract energy spectra from each of the phase space files to examine the change in energy spectra with increasing distance from the field edge and depth in the phantom. The energy dependence of each of the detectors was examined using 70, 100, 125 and 200 kV beams on a Gulmay D3225 Orthovoltage Unit and a 6MV Elekta Synergy beam. The kV energies lied within the range of energies which were found to be dominant out-of-field in a 6MV beam. A dose of 1 Gy was delivered to each detector as determined by their respective calibration protocols, and the signal was recorded for all energies. In-plane and cross-plane profiles were measured by each detector and compared to MC calculated. All measurements were performed in an PTW MP3 watertank except for TLDs and Gafchromic EBT3 film which were performed in solid water. Results Figure 1 displays the results of the energy dependence investigation for each detector in the study. The response of each detector was normalised to 1 at 6MV. of-field dose profiles. Material and Methods

Conclusion In general the results of the energy dependence investigation predicted the response of the detectors to out-of-field radiation except for the case of the Pinpoint, TLD and microDiamond detectors. Energy dependence was thought to be the leading source of variation in detector response to out-of-field radiation due to the relative increase in low-energy photons. However, dose-rate and angular dependencies can exist in detector responses but were not investigated as part of this study. Other factors such a charge multiplication and cable effects can contribute to a change in response as observed with the Pinpoint detector. This study highlights the need for careful selection of appropriate detectors when accurate out-of-field dosimetry is required and offers a guide and improved understanding of detector response to out-of- field radiation. The waterproof Farmer chamber showed best agreement with MC calculated out-of-field dose and is recommended for out-of-field dose measurements. PO-0787 A compact and complete model for Bra gg peak degradation in lung tissue R. Dal Bello 1 , C. Möhler 1,2 , S. Greilich 1,2 , O. Jäkel 1,2,3 1 German Cancer Research Center DKFZ, Division of Medical Physics in Radiation Oncology, Heidelberg, Germany 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany 3 Heidelberg Ion Beam Therapy Center HIT, Clinical Research Group Radiotherapy with Heavy Ions, Heidelberg, Germany Purpose or Objective Due to the lack of a reliable model, current analytical treatment planning for proton and heavier ions cannot account for the degradation of the sharp distal fall-off of the Bragg peak caused by microscopic density heterogeneities, which cannot be resolved by clinical CT. Here, we present a systematic study of Bragg peak degradation in stationary lung parenchyma to provide a comprehensive analytical parametrization for implementation in treatment planning systems (TPS) – aiming at the reduction of dose uncertainties in radiotherapy of the lung. Material and Methods We developed a compact model describing the lung parenchyma microscopic geometry based on few geometrical and physical variables allowing for flexible Monte Carlo (MC) simulations of lung specific features (alveolar dimension, lung density) and breathing state parameters (air filling state, water equivalent thickness traversed, WET). To benchmark the accuracy of the simulated model, we performed a MC study to assess the specific contributions of the cumulative physical sources of degradation and a series of transmission experiments

Figure 2 displays a comparison between MC calculated versus detector measured out-of-field dose.