ESTRO 2021 Abstract Book

S738

ESTRO 2021

Experiments were performed at the PARTREC Accelerator Facility, University Medical Center Groningen, University of Groningen. Following an approach applied for photon fields (Poppinga et al. PMB 60 (2015) 9421), a 0.5 mm slit beam of 150 MeV protons was created with a brass collimator and used to derive one- dimensional K ( x ) for three ionization chambers (PTW Semiflex 3D 31021, PTW PinPoint 3D 31022, IBA Razor camber CC01-G), a PTW microDiamond detector 60019 and a PTW mircoSilicon diode 60023. The high- resolution D ( x ) was acquired using EBT3 film in a water phantom at 2 cm depth. M ( x ) were measured by scanning the detectors across the narrow side of the slit beam. K ( x ) for each detector was computed according to equation 1 using an iterative deconvolution approach. To examine the validity of the derived K ( x ), additional measurements have been performed at 8 cm and 13 cm water depth. Results Figure 1a shows the normalized D ( x ) profile in comparison to the M ( x ) profiles measured using the investigated detectors. While the D ( x ) profile measured by film has a full-width-at-half-maximum (FWHM) of 0.7 mm, the M ( x ) measured with the detectors have FWHM between 1.3 mm (microSilicon) and 3.9 mm (Semiflex 3D) revealing a detector dependent volume effect. Figure 1b shows the derived K ( x ) for all investigated detectors. Good agreement has been achieved by applying the determined K ( x ) to predict the measured M ( x ) from D ( x ) in larger depths.

Figure 1: a) D ( x ) and M ( x ) profiles. b) Detector specific lateral dose response functions K ( x ).

Conclusion Lateral dose response functions K ( x ) of 150 MeV protons were determined experimentally for three ionization chambers, a microDiamond and a microSilicon diode. Their applicability in lower proton beam energies and larger measurement depths has been validated. The functions can be used to correct the perturbed measured signal profiles caused by the detectors volume effect, which is especially important for dose measurements in small fields and in areas of steep dose gradients. PD-0899 Report dose-to-medium in clinical trials; a consensus from the Global Harmonisation Group S. Kry 1 , J. Lye 2 , C. Clark 3 , N. Andratschke 4 , A. Dimitriadis 5 , D. Followill 1 , R. Howell 6 , M. Hussein 7 , M. Ishikawa 8 , S. Kito 9 , T. Kron 10 , J. Lee 11 , J. Michalski 12 , A. Monti 13 , N. Reynaert 14 , P. Taylor 1 , K. Venables 15 , Y. Xiao 16 , J. Lehmann 17 1 Imaging and Radiation Oncology Core / MD Anderson Cancer Center, Radiation Physics, Houston, USA; 2 Australian Clinical Dosimetry Service, Australian Radiation Protection and Nuclear Safety Agency; Olivia Newton-John Cancer Research and Wellness Centre, Radiation physics, Melbourne, Australia; 3 NIHR Radiotherapy Trials Quality Assurance Group; University College London Hospital; University College London; National Physical Laboratory, Radiation Physics, London, United Kingdom; 4 European Organisation for Research and Treatment of Cancer Radiotherapy Quality Assurance Group; University Hospital of Zurich, University of Zurich, Radiation Oncology, Zurich, Switzerland; 5 International Atomic Energy Agency, Dosimetry and Medical Radiation Physics Section, Vienna, Austria; 6 Radiation Dosimetry Services / MD Anderson Cancer Center, Radiation Physics, Houston, USA; 7 National Physical Laboratory, Physics, Teddington, United Kingdom; 8 Japan Clinical Oncology Group / Hokkaido University, Faculty of Health Sciences, Hokkaido, Japan; 9 Japan Clinical Oncology Group / Tokyo Metropolitan Cancer and Infectious Disease Center Komagome Hospital, Radiation Oncology, Tokyo, Japan; 10 Trans Tasman Radiation Oncology Group (TROG) / Peter MacCallum Cancer Centre, Radiation Oncology, Melbourne, Australia; 11 NIHR Radiotherapy Trials Quality Assurance Group / Mount Vernon Cancer Centre, Radiation Oncology, Northwood, United Kingdom; 12 Imaging and Radiation Oncology Core / Washington University School of Medicine, Radiation Oncology, St. Louis, USA; 13 European Organisation for Research and Treatment of Cancer (EORTC) / ASST GOM Niguarda Hospital, Medical Physics, Milan, Italy; 14 European Organisation for Research and Treatment of Cancer / Jules Bordet Institute, Université Libre de Bruxelles, Medical physics, Brussels, Belgium; 15 NIHR Radiotherapy Trials Quality Assurance Group / Mount Vernon Cancer Centre, Radiation oncology, Northwood, United Kingdom; 16 Imaging and Radiation Oncology Core / University of Pennsylvania, Radiation Oncology, Philadelphia, USA;