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Title: SU-G-BRB-12: Polarity Effects in Small Volume Ionization Chambers in Small Fields

Abstract

Purpose: Dosimetric quantities such as the polarity correction factor (Ppol) are important parameters for determining the absorbed dose and can influence the choice of dosimeter. Ppol has been shown to depend on beam energy, chamber design, and field size. This study is to investigate the field size and detector orientation dependence of Ppol in small fields for several commercially available micro-chambers. Methods: We evaluate the Exradin A26, Exradin A16, PTW 31014, PTW 31016, and two prototype IBA CC-01 micro-chambers in both horizontal and vertical orientations. Measurements were taken at 10cm depth and 100cm SSD in a Wellhofer BluePhantom2. Measurements were made at square fields of 0.6, 0.8, 1.0, 1.2, 1.4, 2.0, 2.4, 3.0, and 5.0 cm on each side using 6MV with both ± 300VDC biases. PPol was evaluated as described in TG-51, reported using −300VDC bias for Mraw. Ratios of PPol measured in the clinical field to the reference field are presented. Results: A field size dependence of Ppol was observed for all chambers, with increased variations when mounted vertically. The maximum variation observed in PPol over all chambers mounted horizontally was <1%, and occurred at different field sizes for different chambers. Vertically mounted chambers demonstrated variations as largemore » as 3.2%, always at the smallest field sizes. Conclusion: Large variations in Ppol were observed for vertically mounted chambers compared to horizontal mountings. Horizontal mountings demonstrated a complicated relationship between polarity variation and field size, probably relating to differing details in each chambers construction. Vertically mounted chambers consistently demonstrated the largest PPol variations for the smallest field sizes. Measurements obtained with a horizontal mounting appear to not need significant polarity corrections for relative measurements, while those obtained using a vertical mounting should be corrected for variations in PPol.« less

Authors:
;  [1];  [2];  [3];  [4]
  1. University of Toledo Medical Center, Toledo, OH (United States)
  2. University of Minnesota, Minneapolis, MN (United States)
  3. SUNY Upstate Medical University, Syracuse NY (United States)
  4. University of Toledo Medical Center, Sylvania, OH (United States)
Publication Date:
OSTI Identifier:
22649284
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
07 ISOTOPES AND RADIATION SOURCES; ABSORBED RADIATION DOSES; CORRECTIONS; IONIZATION CHAMBERS; VARIATIONS

Citation Formats

Arora, V, Parsai, E, Mathew, D, Tanny, S, and Sperling, N. SU-G-BRB-12: Polarity Effects in Small Volume Ionization Chambers in Small Fields. United States: N. p., 2016. Web. doi:10.1118/1.4956919.
Arora, V, Parsai, E, Mathew, D, Tanny, S, & Sperling, N. SU-G-BRB-12: Polarity Effects in Small Volume Ionization Chambers in Small Fields. United States. doi:10.1118/1.4956919.
Arora, V, Parsai, E, Mathew, D, Tanny, S, and Sperling, N. 2016. "SU-G-BRB-12: Polarity Effects in Small Volume Ionization Chambers in Small Fields". United States. doi:10.1118/1.4956919.
@article{osti_22649284,
title = {SU-G-BRB-12: Polarity Effects in Small Volume Ionization Chambers in Small Fields},
author = {Arora, V and Parsai, E and Mathew, D and Tanny, S and Sperling, N},
abstractNote = {Purpose: Dosimetric quantities such as the polarity correction factor (Ppol) are important parameters for determining the absorbed dose and can influence the choice of dosimeter. Ppol has been shown to depend on beam energy, chamber design, and field size. This study is to investigate the field size and detector orientation dependence of Ppol in small fields for several commercially available micro-chambers. Methods: We evaluate the Exradin A26, Exradin A16, PTW 31014, PTW 31016, and two prototype IBA CC-01 micro-chambers in both horizontal and vertical orientations. Measurements were taken at 10cm depth and 100cm SSD in a Wellhofer BluePhantom2. Measurements were made at square fields of 0.6, 0.8, 1.0, 1.2, 1.4, 2.0, 2.4, 3.0, and 5.0 cm on each side using 6MV with both ± 300VDC biases. PPol was evaluated as described in TG-51, reported using −300VDC bias for Mraw. Ratios of PPol measured in the clinical field to the reference field are presented. Results: A field size dependence of Ppol was observed for all chambers, with increased variations when mounted vertically. The maximum variation observed in PPol over all chambers mounted horizontally was <1%, and occurred at different field sizes for different chambers. Vertically mounted chambers demonstrated variations as large as 3.2%, always at the smallest field sizes. Conclusion: Large variations in Ppol were observed for vertically mounted chambers compared to horizontal mountings. Horizontal mountings demonstrated a complicated relationship between polarity variation and field size, probably relating to differing details in each chambers construction. Vertically mounted chambers consistently demonstrated the largest PPol variations for the smallest field sizes. Measurements obtained with a horizontal mounting appear to not need significant polarity corrections for relative measurements, while those obtained using a vertical mounting should be corrected for variations in PPol.},
doi = {10.1118/1.4956919},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • Purpose: To investigate the polarity effects for small volume ionization chambers in {sup 60}Co gamma-ray beams using the Leksell Gamma Knife Perfexion. Methods: Measurements were made for 7 small volume ionization chambers (a PTW 31016, an Exradin A14, 2 Capintec PR0-5P, and 3 Exradin A16) using a PTW UNIDOSwebline Universal Dosemeter and an ELEKTA solid water phantom with proper inserts. For each ion chamber, the temperature/pressure corrected electric charge readings were obtained for 16 voltage values (±50V, ±100V, ±200V, ±300V, ±400V, ±500V, ±600V, ±700V). For each voltage, a five-minute leakage charge reading and a series of 2-minute readings were continuouslymore » taken during irradiation until 5 stable signals (less than 0.05% variation) were obtained. The average of the 5 reading was then used for the calculation of the polarity corrections at the voltage and for generating the saturation curves. Results: The polarity effects are more pronounced at high or low voltages than at the medium voltages for all chambers studied. The voltage dependence of the 3 Exradin A16 chambers is similar in shape. The polarity corrections for the Exradin A16 chambers changes rapidly from about 1 at 500V to about 0.98 at 700V. The polarity corrections for the 7 ion chambers at 300V are in the range from 0.9925 (for the PTW31016) to 1.0035 (for an Exradin A16). Conclusion: The polarity corrections for certain micro-chambers are large even at normal operating voltage.« less
  • Accurate small-field dosimetry has become important with the use of multiple small fields in modern radiotherapy treatments such as IMRT and stereotactic radiosurgery. In this study, we investigate the response of a set of prototype plane-parallel ionization chambers, based upon the Exradin T11 chamber, with active volume diameters of 2, 4, 10, and 20 mm, exposed to 6 MV stereotactic radiotherapy x-ray fields. Our goal was to assess their usefulness for accurate small x-ray field dose measurements. The relative ionization response was measured in circular fields (0.5 to 4 cm diameter) as compared to a 10x10 cm{sup 2} reference field.more » A large discrepancy ({approx}40%) was found between the relative response in the smallest plane-parallel chamber and other small volume dosimeters (radiochromic film, micro-metal-oxide-semiconductor field-effect transistor and diode) used for comparison. Monte Carlo BEAMnrc simulations were used to simulate the experimental setup in order to investigate the cause of the under-response and to calculate appropriate correction factors that could be applied to experimental measurements. It was found that in small fields, the air cavity of these custom-made research chambers perturbed the secondary electron fluence profile significantly, resulting in decreased fluence within the active volume, which in turn produces a chamber under-response. It is demonstrated that a large correction to the p{sub fl} correction factor would be required to improve dosimetric accuracy in small fields, and that these factors could be derived using Monte Carlo simulations.« less
  • A plane-parallel ionization chamber having a sensitive volume of 2 mm{sup 3} and using the dielectric liquid tetramethylsilane as the sensitive medium instead of air is described. In the design of the chamber special attention was given to the factors that can cause unwanted currents in the cable, stem, or the chamber dielectric material. The chamber has been tested with respect to the polarity effect in regions of radiation fields where ordinary plane-parallel ionization chambers will often fail. These regions are the build-up region in photon fields, and the region close to the practical range for electrons where nonelectronic equilibriummore » is significant. Experimental results show that, despite the extremely small ionization volume in the liquid ionization chamber, the polarity effect never exceeds a few tenths of a percent in field positions where well-known commercially available chambers with much less spatial resolution designed for measurements in radiation therapy fields can show polarity effects of 5% to 30%. The origin of spurious currents and how they must be minimized in the design of either a liquid- or gas-filled ionization chamber is discussed.« less
  • The zero-current and polarity-effect of ionization chambers were studied experimentally. The former turns out to be mainly from the contact potential difference between two electrodes. For the latter, it was found that the zerocurrent of the output electrical connector makes it most dominant under ordinary conditions. When it comes to lower pressure (< approximately 1 mmHg), the Taylor-Greening Effect becomes effective. An effective range is introduced to understand the characteristics of ionization current at the recombination region.
  • The effect of reversing the voltage polarity applied to an ionization chamber has been investigated in electron beams for several types of chambers and several irradiation conditions. It has been found that differences in readings can be significant for cylindrical chambers (about 10%) as well as for plane parallel chambers (20%). The effect is larger for large field sizes than small ones. It generally includes an appreciable stem and cable effect. Differences in readings with both polarities are related to the energy distribution of the electron beam and are greater for lower electron energies than higher. Polarity effect and chargemore » deposit within the chamber wall material appear to be closely connected. This charge deposit, expressed as a proportion of the total collected charge, can be directly derived from double polarities measurements. Careful investigation of the effect should be made to avoid significant error (over 5%) in the determination of the absorbed dose.« less