A feasibility study of Fricke dosimetry as an absorbed dose to water standard for 192Ir HDR sources.

Carlos Eduardo Dealmeida,Mario Antônio Bernal,José Guilherme Peixoto,Mariano Gazineu David,Evandro Jesus Pires,Camila Salata,Marilene Coelho De Lima,Ricardo Ochoa,Qinghui Zhang

doi:10.1371/journal.pone.0115155

Carlos Eduardo Dealmeida, Mario Antônio Bernal + Show 7 more

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https://doi.org/10.1371/journal.pone.0115155

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Abstract

High dose rate brachytherapy (HDR) using 192Ir sources is well accepted as an important treatment option and thus requires an accurate dosimetry standard. However, a dosimetry standard for the direct measurement of the absolute dose to water for this particular source type is currently not available. An improved standard for the absorbed dose to water based on Fricke dosimetry of HDR 192Ir brachytherapy sources is presented in this study. The main goal of this paper is to demonstrate the potential usefulness of the Fricke dosimetry technique for the standardization of the quantity absorbed dose to water for 192Ir sources. A molded, double-walled, spherical vessel for water containing the Fricke solution was constructed based on the Fricke system. The authors measured the absorbed dose to water and compared it with the doses calculated using the AAPM TG-43 report. The overall combined uncertainty associated with the measurements using Fricke dosimetry was 1.4% for k = 1, which is better than the uncertainties reported in previous studies. These results are promising; hence, the use of Fricke dosimetry to measure the absorbed dose to water as a standard for HDR 192Ir may be possible in the future.

Highlights

High dose rate brachytherapy (HDR) using 192Ir is well accepted as an important treatment option for cancer patients and requires an accurate dosimetry standard
This paper presents important improvements compared with previous studies using this type of dosimetry [6], including a newly designed irradiation vessel, a new reading device, careful temperature control during irradiation and reading, and a more accurate calculation of the correction factors and uncertainties, resulting in a significant reduction of the overall uncertainty
The results presented here are consequences of a careful improvement of several aspects of our methodology compared with the previous work of Austerlitz et al [6], such as the following: N The overall dimension of the irradiating vessel; N The use of a calibrated thermistor in the spectrophotometer; and N The use of polymethyl methacrylate (PMMA), which made the construction of the vessel easier and, as discussed earlier, has no measurable effect on the solutions

Summary

Introduction

High dose rate brachytherapy (HDR) using 192Ir is well accepted as an important treatment option for cancer patients and requires an accurate dosimetry standard. A dosimetry standard for the direct measurement of the absolute. The AAPM TG-43 Report [1] and its update [2] outline the accepted protocol for determining the dose to water based on an air kerma strength (Sk) measurement. The dose to water conversion is performed via the dose rate constant L, which converts the air-kerma strength to the dose to water, and several relative correction factors, which account for scatter, attenuation, and anisotropy of the dose distribution, among other effects [2]. The main concern regarding this method is that it is not a direct measure of the dose to water and may induce high uncertainties

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