ESTRO 2022 - Abstract Book

S734

Abstract book

ESTRO 2022

J. Kretschmer 1,2 , L. Brodbek 1,3 , C. Behrends 4,5,6 , F. Kugel 4,5,7 , B. Koska 4,5 , C. Bäumer 4,5,6,8 , J. Wulff 4,5 , B. Timmermann 4,5,8,9 , H.K. Looe 1 , B. Poppe 1 1 University Clinic for Medical Radiation Physics, Carl-von-Ossietzky University Oldenburg, Medical Campus Pius Hospital, Oldenburg, Germany; 2 University Medical Center Groningen, University of Groningen, Department of Radiation Oncology, Groningen, The Netherlands; 3 University Medical Center Groningen, University of Groningen, Department of Radiation Oncology , Groningen, The Netherlands; 4 West German Proton Therapy Centre Essen, (WPE), Essen, Germany; 5 West German Cancer Center, (WTZ), Essen, Germany; 6 TU Dortmund University, Department of Physics, Otto-Hahn-Str. 4a, Dortmund, Germany; 7 Heinrich-Heine University, Department of Physics, Düsseldorf, Germany; 8 German Cancer Consortium, (DKTK), Heidelberg, Germany; 9 University Hospital Essen, Clinic for Particle Therapy, Essen, Germany Purpose or Objective Lateral dose profile or output factor measurements in small proton fields may be perturbed by the detector’s volume effect. This volume effect can be characterized by the detector-specific lateral dose response function K(x,y) that acts as the convolution kernel transforming the dose profile D(x,y) into the measured signal profile M(x,y) (Looe et al. , PMB 60 (2015) 6585) according to Equation 1. Recently, K(x,y) were determined for various point detectors in a proton field (Kretschmer et al ., ESTRO 2021, PD-0898). By application of these K(x,y) , the aim of this work is to quantify and to correct for the volume effect in measured lateral beam profiles and output ratios of narrow proton fields created with a passive scattered beam and apertures. Equation 1: M(x,y) = D(x,y) * K(x,y) Materials and Methods Experiments were performed at the eye beam line of the West German Proton Therapy Centre Essen. Lateral beam profiles M(x,y=0) and M(x=0,y) and output M(x=0,y=0) measurements were performed with a PTW microSilicon diode 60023 and a PTW PinPoint 3D ionization chamber 31022. The measurements were performed with detectors positioned axially at 15 mm depth in a water phantom using proton fields with 25 mm residual range and 20 mm spread out Bragg peak modulation width. Proton fields created with circular brass apertures with opening diameters between 3 mm and 20 mm were used. By applying the K(x,y) of the detectors, the dose distributions, D microSilicon (x,y) and D PinPoint3D (x,y) , were derived via 2D deconvolution according to Equation 1. The output correction factors k were calculated as the ratio D(x=0,y=0)/M(x=0,y=0) for each detector and field size. Results Figure 1 shows exemplarily the measured lateral beam profiles M(x,y=0) of both detectors in comparison to the corresponding deconvolved dose profiles D(x,y=0) for the smallest 3 mm aperture. The M(x,y=0) and D(x,y=0) for each detector were normalized to the maximum of the corresponding D(x,y=0) distribution. The output ratios M(x=0,y=0)/M 20mm (x=0,y=0) measured with the microSilicon and PinPoint 3D as well as the corrected output factors D(x=0,y=0)/D 20mm (x=0,y=0) are shown in Figure 2.

Figure 1: M ( x,y=0 ) profiles and the corresponding D(x,y=0) profiles for the 3 mm circular aperture field. The difference between the D(x,y=0) is shown in the lower panel.