Gel dosimetry

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Gel dosimeters are made of radiation-sensitive chemicals which, after ionizing radiation radiation, undergo fundamental changes in their properties as a function of the absorbed dose of radiation.

For years people have attempted to measure the distribution of radiation doses absorbed by the gel. Since 1950, radiation-induced color changes in dyes were used to investigate the dose of radiation in the gel. Furthermore, in 1957 the doses of photons and electrons in gel agar were investigated using spectrophotometry. Gel dosimetry today, however, was founded primarily in the work of Gore et al which in 1984 showed that changes due to ionizing radiation in the Fricke dosimetry solution, developed in 1920, can be measured using nuclear magnetic resonance (NMR).

Gel dosimeters generally consist of two types; Fricke and gel polymer dosimeters and are usually evaluated or 'read-out' using magnetic resonance imaging (MRI), optical computer tomography (CT), CT x-ray or ultrasound.

Since 1999, the DosGel and IC3DDose Conference Series on gel dosimetry have been held in various international places.

Video Gel dosimetry

Fricke Gel Dosimeter

Gore et al investigated the relaxed nuclear magnetic resonance (NMR) properties of the irradiated Fricke or sulfite sulphide solution indicating that the change caused by radiation, in which iron (Fe ² ) ion is converted to iron (Fe ³ ), can be quantified using NMR relaxation measurements. In 1986 Appleby reported that the Fricke dosimetry solution scattered throughout the gel matrix could be used to obtain three-dimensional spatial dose information (3D) using magnetic resonance imaging (MRI). This then indicates that the irradiated Fricke gel dosimeter does not maintain a spatially stable dose distribution due to ion diffusion in irradiated dosimeters. Fricke solutions with various gelling agents such as gelatine, agarose, sephadex and polyvinyl alcohol (PVA) were investigated together with chelating agents such as xylenol orange (XO) to reduce diffusion. Many authors then published their work to inhibit ion diffusion with limited success and which was summarized by Baldock et al in 2001. In the early 1990s the diffusion problem was considered to be a significant one in the progress of gel dosimetry.

Maps Gel dosimetry

Polymer Gel Dosimeter

The polymer system for the use of radiation dosimetry was first proposed in 1954, in which Alexander discusses the effects of ionizing radiation on polymethylmethacrylate. After this, Hoecker et al in 1958 investigated the induced dosimetry of radiation-induced polymerization in liquids, and in 1961 Boni used polyacrylamide as a gamma dosimeter. Much later in 1991, Audit et al reported changes in transversal measurements of transdermal oxide transrenal oxide polyethelene. In 1992, Kennan et al reported on a longitudinal NMR relaxation study performed on an aqueous solution of N, N'-methylene-bis-acrylamide and agarose irradiation, indicating that the relaxation rate increased with the dose absorbed.

In 1992 a new gel dosimetry formulation was proposed by Maryanski et al , based on the polymerization of acrylamide and N, N'-methylene-bis-acrylamide (bis) monomer infused in aqueous agarose matrix. The system is given BANANA acronyms due to the use of chemical components (bis, acrylamide, nitrous oxide and agarose). This type of gel dosimeter does not have diffusion problems associated with Fricke gel and is shown to have a relatively stable post-irradiation dose distribution. The polymerization reaction takes place by the crosslinking of the monomer induced by the free radical product of water radiolysis. In 1994 the BANANA formulation was enhanced by replacing agarose with gelatin and administered BANG acronyms (bis, acrylamide, nitrogen and aqueous gelatin), the first of a series of new polymer gel formulations. In 1994, this formulation was patented and commercially available through MGS Research Inc. as BANGÃ,® . Furthermore, since the naming of commercial products, PAG becomes the dosimeter dosimeter polymer of choice for most authors. Many authors then publish the work investigating the different compositions and formulations of the gel polymer dosimeters summarized by Lepage et al .

Although polymer type dosimeters do not have diffusion limits from Fricke type dosimeters, there are other significant limits to their use. Due to the chemical nature of their free radicals, gel polymer dosimeters are susceptible to inhibition of atmospheric oxygen from the polymerization process. Consequently, these gel dosimeters should be produced in an oxygen-free environment, as in a glove box pumped with nitrogen gas. Along with the use of potentially toxic chemicals, this is a significant limitation in the introduction of gel dosimetry to the clinic.

During this period a number of studies were conducted to investigate the clinical application of the PAG-type polymer dosimeter equivalent to tissue radiology using MRI. De Deene et al conducted an investigation of the overall accuracy of the anthropomorphic polymer phantom dosimetry gel for verification of conformal radiotherapy treatments. It was determined that a significant problem related to the accuracy of this dosimetry technique is the result of the inhibition of oxygen in polymeric gel and MRI imaging artifacts.

The authors went on to investigate the clinical aspects of polymer gel dosimetry using MRI including conformal therapy, IMRT and IMAT, stereotactic radiosurgery, brachytherapy, low energy X ray, high LET and proton therapy, neutron capture boron therapy and tissue inhomogeneities.

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Dark Polyether Normal Dosimeter

Significant developments in the field of gel dosimetry occurred when the results of the formulation of alternative polymer gel dosimeters were published by Fong et al. In 2001. The new type of gel dosimeter polymer, known as MAGIC gel, is tied to atmospheric oxygen in the complex metal-organic material thus eliminating the problem of oxygen inhibition and allowing the polymer gel to be produced on the bench-top in the laboratory. This creates what is known as the normoxic gel dosimeter, compared to the previous PAG formulation which came to be known as the hypoxic gel dosimeter. The MAGIC polymer gel formula comprises methacrylic acid, ascorbic acid, gelatin and copper. The main principle behind MAGIC gel is in scavengers of ascorbic acid oxygen. Ascorbic acid binds the free oxygen contained in the aqueous gelatin matrix into the metallic-organic complex and this process is initiated by copper sulfate. This was then demonstrated by De Deene et al. In 2002 that other antioxidants could be used in the manufacture of normoxic gel including tetrakis (hydroxymethyl) phosphonium chloride, after first being recommended to Baldock by Billingham in 1996. Many authors then publishes the work investigating different compositions and formulations of the normoxic polymer gel dosimeter and summarized by Senden. Other work also includes the development of less toxic polymer gels.

The fundamental science underlying polymer gel dosimetry is reviewed along with various 'reading' techniques and the evaluation and application of clinical dosimetry in the publication of the Topical Review 2010 by Baldock et al .

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DOSGel and IC3DDose Conference Series

In June 1995 while attending the annual meeting of the American Association of Physicists in Medicine (AAPM) in Boston, USA, Clive Baldock and L. John Schreiner discussed the suitability of organizing some form of specialist meetings or workshops on gel dosimeters. In September 1996 Clive Baldock and Lars Olsson, while attending the European Society for Radiotherapy & amp; Oncology (ESTRO) annual meeting in Vienna, Austria began organizing a series of international conferences on gel dosimetry that began as DosGel 99 , the First International Workshop on Dosimetry Gel Radiation Therapy held in Lexington, Kentucky in 1999 and held by Geoff Ibbott. Since 1999, the next DosGel conference was held in Brisbane, Australia (2001), Ghent, Belgium (2004), Sherbrooke, Canada (2006) and Crete, Greece (2008). In 2010 the conference was held in Hilton Head, South Carolina, USA and underwent a name change to IC3DDose . The next IC3DDose conferences are held in Sydney, Australia (2012) and Ystad, Sweden (2014). In November 2016, IC3DDose was held in Galveston, Texas, USA.

The purpose of the first workshop was to bring together individuals, both researchers and users, with an interest in the application of 3-dimensional radiation dosimetry techniques in cancer treatment, with a mixture of presentations from basic science to clinical applications. This remains the destination for all conferences. One of the reasons of Dosgel 99 is stated as supporting the increased clinical application of gel dosimetry, as it appears, at the time, to leave the dosimeter gel dosimeter's laboratory and enter clinical practice. Clearly by labeling the first workshop as the first, there is a sustainable series vision, which has been fulfilled. On the other hand, expectations of widespread clinical use of gel dosimeters may not have been what was expected and anticipated. Nevertheless, the rapidly increasing demand for sophisticated high-precision technology and advanced radiotherapy technology continues. The need for practical and accurate 3D dosimetry methods for development and quality assurance has only increased. At the 6th meeting, held in South Carolina in 2010, the Conference Scientific Committee recognized a broader development in 3D systems and methods and decided to expand its scope, while maintaining the same range of basic science for applications. This is marked by a name change from DosGel to IC3DDose , a name that continues to the last conference held in Houston in 2016.

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References

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