Original Paper
A new reference-type ionization chamber with direction-independent response for use in small-field photon-beam dosimetry – An experimental and Monte Carlo study

https://doi.org/10.1016/j.zemedi.2018.05.001Get rights and content

Abstract

The frequently applied narrow and non-standard transverse dose profiles of intensity modulated photon-beam radiotherapy, lacking lateral secondary electron equilibrium, require the use of high-resolution dosimetry detectors, and small air-filled detectors are recommended as the reference detectors for cross-calibration of the high-resolution detectors. The present study focuses on the dosimetric properties of a novel cylindrical ionization chamber, the PTW Semiflex 3D 31021. The chamber's effective point of measurement was found to lie at (0.41 ± 0.04) r downstream the tip of the inner surface of the spherical front wall in the axial orientation and (0.46 ± 0.04) r upstream the chamber axis in the radial orientation. Due to its symmetrical design, the sigma values of its lateral dose response functions for all chamber's orientations are the same (2.10 ± 0.05 mm). The polarity correction factors obtained in this work do not exceed 0.1% and the saturation correction factor was below 1% up to a dose-per-pulse value of 0.956 mGy. The radiation quality correction factor kQ of the chamber as a function of the tissue-phantom-ratio, TPR20,10, has been calculated by Monte Carlo simulation and has been determined experimentally at the German Metrology Institute (Physikalisch-Technische Bundesanstalt, PTB). The values of the non-reference condition correction factor kNR have been Monte-Carlo-calculated for use of the chamber at various depths and field sizes.

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

References (29)

  • F. Araki et al.

    The response of a radiophotoluminescent glass dosimeter in megavoltage photon and electron beams

    Med Phys

    (2014)
  • P. Carrasco et al.

    Characterization of the Exradin W1 scintillator for use in radiotherapy

    Med Phys

    (2015)
  • A. Gonzalez-Lopez et al.

    Small fields measurements with radiochromic films

    J Med Phys

    (2015)
  • German Institute of Standards, DIN 6809-8

    Clinical dosimetry – Part 8: dosimetry of small photon fields

    (2014)
  • Cited by (10)

    • On-line beam monitoring and dose profile measurements of a <sup>208</sup>Pb beam of 150 GeV/n with a liquid-filled ionization chamber array

      2021, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
      Citation Excerpt :

      The characteristics of these clinical implemented 2D arrays differ in terms of detector type (semiconductor, air- or liquid-filled ionization chamber), the single chamber dimensions, as well as the number and arrangement of the single detectors [16]. A 2D liquid-filled ionization chamber array has been used in this work, which represents the device with the highest spatial resolution and sampling frequency available for clinical use [16–18] up to date. The results obtained with the liquid-filled ionization chamber array will be compared in this work to those measured using the standard monitors further upstream of the experimental setup and films at the same position as the array.

    • Experimental determination of the recombination correction factor k<inf>S</inf> for SNC 125c, SNC 350p and SNC 600c ionization chambers in pulsed photon beams

      2020, Zeitschrift fur Medizinische Physik
      Citation Excerpt :

      The relative expanded uncertainty of the determination of the absorbed dose to water is below 2%. The absorbed dose per pulse was calculated from the absorbed dose rate and the pulse frequency, then scaled to different SSD values using the inverse square law [15–17]. Figures 1–3 show the Jaffé plots of the SNC 125c thimble ionization chamber, the 600c Farmer chamber and the 350p parallel-plate chamber.

    • Experimental and Monte-Carlo characterization of the novel compact ionization chamber PTW 31023 for reference and relative dosimetry in high energy photon beams

      2019, Zeitschrift fur Medizinische Physik
      Citation Excerpt :

      For measurements under non-reference conditions, the correction factor kNR is applied according to the DIN 6800-2 to account for the influence of spectral changes from the reference condition due to the energy dependence of the detector response. The determination of the correction factor kNR for the PTW 31023 was performed using the approach described in Delfs et al. [11]. The response function r(E) of the PinPoint 31023 as a function of photon energy was simulated using the EGSnrc package and the egs_chamber user-code for monoenergetic photon beams with energies of 20 keV to 15 MeV under the conditions of secondary electrons equilibrium.

    View all citing articles on Scopus
    View full text