Original PaperA new reference-type ionization chamber with direction-independent response for use in small-field photon-beam dosimetry – An experimental and Monte Carlo study
Introduction
The increased use of intensity modulated fields in radiotherapy has led to the implementation of various detection modalities, all geared towards an improved accuracy in the determination of either the absolute dose or of the relative dose profiles of the treatment beams. A detailed analysis of the physical properties of small and non-standard photon fields has given more insight into the necessary criteria required for proper dosimetric practice [1], [2]. Common characteristics of the frequently used unflattened and small radiation fields are their bell-shaped dose profiles and the lack of secondary charged particle equilibrium, resulting from the prevailing outward transport of Compton electrons. The challenge of performing dosimetry under these conditions has been highlighted in several studies on the use of high-resolution detectors in small, unflattened or fluence-modulated fields [3], [4], [5], [6], [7], [8], [9], [10]. Small-volume air-filled detectors are recommended as the reference detectors for cross-calibration of the high-resolution detectors within a machine-specific small reference field [11], [12].
The aim of the present study is to determine the dosimetric properties of an air-filled detector, the PTW Semiflex 3D 31021, developed to serve as a reference detector in small photon fields. The detector's effective points of measurement, its dose response functions and the correction factors necessary for absolute dose measurements according to the German standard DIN 6800-2 [13], namely polarity correction, saturation correction and the radiation quality corrections, have been investigated. In comparison with the previous version Semiflex 31010, the sensitive volume of the 31021 is designed with equal values of the length and the diameter in order to provide a negligible directional dependence of its response, hence the name Semiflex 3D. In order to confirm the 3D property, dose response functions have been determined for three orientations of the chamber with respect to a collimated slit beam.
The guard electrode of the Semiflex 3D 31021, compared to earlier chamber versions, is manufactured with a new design, expected to improve the shape of the field lines between the outer and the central electrode and thereby resulting in an increase of the ion collection efficiency and polarity effect. This effect would be noticeable on examination of the current–voltage characteristic of the chamber, in that the Jaffé plot would achieve an extended linear region at the higher voltages. The result would be a reduction in the uncertainty of the saturation correction factor ks.
The radiation quality correction factor kQ of the Semiflex 3D 31021 as a function of the tissue-phantom-ratio, TPR20,10, was calculated by Monte Carlo simulation, using published spectra for a wide range of clinical photon beams. To confirm the results, kQ was also determined experimentally for six different megavoltage photon beams at the German Metrology Institute (Physikalisch-Technische Bundesanstalt, PTB). In addition the values of the non-reference condition correction factor kNR of the 31021 were calculated using Monte Carlo simulations. With a small active volume of only about 0.07 cm3, the volume averaging effect of the Semiflex 3D 31021 is negligible at field sizes down to 2.5 × 2.5 cm2, thus rendering this detector a good candidate for dosimetry in either reference or non-reference photon fields.
Section snippets
Materials and methods
The detector under investigation is the Semiflex 3D 31021, a cylindrical ionization chamber manufactured by PTW Freiburg, Germany. The length of the sensitive volume equals its diameter, and the magnitude of each of them is 4.8 mm. The central electrode consisting of aluminium has a diameter of 0.8 mm and a length of 2.8 mm, resulting in a sensitive volume of 0.07 cm3. The inner chamber wall made of graphite has a thickness of 0.09 mm and is protected by 0.57 mm of PMMA. A schematic cross-section of
Effective point of measurement
As explained in Section 2.1, the EPOM of the Semiflex 3D 31021 was determined by comparing depth dose curves with the plane-parallel Roos chamber as the reference. In axial chamber orientation the effective point of measurement was found 1.65 ± 0.1 mm below the chamber tip. This means that the EPOM was at (0.41 ± 0.04) r downstream the tip of the inner surface of the spherical front wall, where r denotes the curvature radius of this surface. In radial chamber orientation the position of the
Discussion
In this work the dosimetric properties of the ionization chamber Semiflex 3D 31021 were evaluated. These investigations show that the detector is well suited for a variety of applications in high energy photon beam dosimetry.
For absolute dose measurements and commissioning purposes, positioning accuracy plays a major role, thereby warranting the investigation of the detector's effective point of measurement. Monte Carlo studies by Kawrakow [24], McEwen et al. [25] and Tessier and Kawrakow [26]
Conclusions
The Semiflex 3D 31021 shows excellent dosimetric characteristics that matches or exceeds the characteristics of its predecessor, with the detector fulfilling the requirements as a reference class detector in compliance with national and international dosimetry standards. The detector's dose response functions, uniform in axial and radial chamber orientation and for longitudinal and lateral scans of a slit beam, demonstrate its three-dimensional measurement behaviour, thus confirming the name
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