Abstract

Title

Experimental determination of detector specific lateral dose response functions in proton beams

Authors

Jana Kretschmer1,2, Leonie Brodbek1,2, Hui Khee Looe1, Emiel van der Graaf2, Marc Jan van Goethem2, Harry Kiewiet2, Christoph Meyer2, Björn Poppe1, Sytze Brandenburg2

Authors Affiliations

1Carl-von-Ossietzky University Oldenburg, University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Oldenburg, Germany; 2University of Groningen, Department of Radiation Oncology, University Medical Center Groningen, Groningen, The Netherlands

Purpose or Objective

Dose measurements using an extended point detector may be perturbed by the volume effect, which inherits on the one hand the volume-averaging effect caused by the extended sensitive volume of the detector; and on the other hand the disturbance of the charged particle fluence caused by the non-water-equivalent detector components. The volume effect can be characterized by lateral dose response functions K(x,y) acting as the convolution kernel transforming the dose profile D(x,y) into the measured signal profile M(x,y) according to equation 1 (Looe et al. PMB 60 (2015) 6585). As a result of the volume effect, M(x,y) is perturbed in the penumbra regions in relative profile measurements, as well as along the central axis in output measurements of small fields. The aim of this work is to experimentally determine K(x,y) for five point detectors in proton beams as they can be used to derive correction strategies for the above-mentioned perturbation effects.

Materials and Methods

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.