ESTRO 2022

Session Item

Monday

May 09

10:30 - 11:30

Poster Station 1

19: Dosimetry

Chair: Sabrine MEFTAH EP DALI, Tunisia

Session Code: 3270

Session Type: Poster Discussion

Track: Physics

Quantification and correction of volume effect perturbations from detectors in small proton fields

Jana Kretschmer, Germany

Presentation Number: PD-0814

Abstract

Abstract Title:

Quantification and correction of volume effect perturbations from detectors in small proton fields

Authors:

Jana Kretschmer^1,2, Leonie Brodbek^1,3, Carina Behrends^4,5,6, Fabian Kugel^4,5,7, Benjamin Koska^4,5, Christian Bäumer^4,5,6,8, Jörg Wulff^4,5, Beate Timmermann^4,5,8,9, Hui Khee Looe¹, Björn Poppe¹

¹University Clinic for Medical Radiation Physics, Carl-von-Ossietzky University Oldenburg, Medical Campus Pius Hospital, Oldenburg, Germany; ²University Medical Center Groningen, University of Groningen, Department of Radiation Oncology, Groningen, The Netherlands; ³University Medical Center Groningen, University of Groningen, Department of Radiation Oncology , Groningen, The Netherlands; ⁴West German Proton Therapy Centre Essen, (WPE), Essen, Germany; ⁵West German Cancer Center, (WTZ), Essen, Germany; ⁶TU Dortmund University, Department of Physics, Otto-Hahn-Str. 4a, Dortmund, Germany; ⁷Heinrich-Heine University, Department of Physics, Düsseldorf, Germany; ⁸German Cancer Consortium, (DKTK), Heidelberg, Germany; ⁹University Hospital Essen, Clinic for Particle Therapy, Essen, Germany

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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)

Material 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.

Figure 2: Output ratios and the derived output factors after applying the output correction factors k.

Conclusion

The presented lateral beam profiles and output measurements indicate volume effect perturbations in areas of steep dose gradients as well as at the center of small proton fields. The agreement of the independently derived D(x,y=0) profiles and the output factors from measurements using two different detectors confirm the validity of the previously determined K(x,y). Within the framework of the convolution model in dosimetry, these functions can be used to quantify the volume effect perturbation and to assist in detector selection for measurements of small proton fields.