Brief History of Gel Dosimetry: History
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Contributor:
Micaela A. Macchione
,
Sofía Lechón Páez
,
Miriam C. Strumia
,
Mauro Valente
,
Facundo Mattea

Advances in radiotherapy technology have significantly improved both dose conformation to tumors and the preservation of healthy tissues, achieving almost real-time feedback by means of high-precision treatments and theranostics. Therefore, developing high-performance systems capable of coping with the challenging requirements of modern ionizing radiation is a key issue to overcome the limitations of traditional dosimeters. In this regard, a deep understanding of the physicochemical basis of gel dosimetry, as one of the most promising tools for the evaluation of 3D high-spatial-resolution dose distributions, represents the starting point for developing new and innovative systems. 

  • three-dimensional dosimetry
  • Fricke
  • polymer gel dosimetry
  • nanoparticles

1. Introduction

Radiation therapy has been used to treat tumors and cancer diseases for more than one hundred years, only being surpassed by surgery in adult cancer treatment [1]. This treatment modality uses ionizing radiation, mainly high-energy particles, such as X- and γ-rays, electrons, and, to a lesser degree (but with an increasing trend), protons and carbon ions, to selectively kill tumor cells in patients with cancer. Routine radiotherapy treatments need dose-verification protocols and dedicated quality assurance programs to be implemented [2].
Technological advances during the last 25 years have had a huge impact on the delivery of accurate and complex three-dimensional radiation dose distributions in radiotherapy. Technologies such as multi-leaf collimators, together with intensity-modulated beams, have given rise to treatments such as intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT), which, in combination with optimized treatment planning systems (TPSs), are currently used as the standard planning of the optimal configuration for a specific patient or case [3]. Additionally, image-guided technology, either computed tomography (CT) or magnetic resonance imaging (MRI) [4] combined with IMRT (Image-Guided RadioTherapy (IGRT), MRI-based IGRT, or Magnetic Resonance-guided Radiotherapy (MRgRT)), have been significantly improved and can be used to enhance the precision of treated volumes, reducing healthy tissue irradiation, and have the possibility of correcting the beam position in near-real-time if the tumor moves. 
Modern radiotherapy treatments require novel quality assurance methods, must be able to determine highly complex dose distributions with sharp dose gradients, and must have high resolution. Gel dosimetry represents one of the few—if not the only—“true” three-dimensional dosimetry system with an inherent resolution only limited by the analytical method used to read out the response [5]. Moreover, most gel dosimetry formulations are radiological tissue-equivalent, and their response is independent of the radiation orientation, dose rate, and quality [2][6]. In addition, they can be modified and designed for specific purposes or treatments, such as those including the use of nanoparticles, specific drugs, or chemicals. The optimal traditional properties for a dosimetry system, such as reliability, reproducibility, accuracy, and traceability, have not drastically changed during the last few years and have been thoroughly reviewed and discussed elsewhere [2][7]. However, new specific requirements and demands, such as real-time (4D) resolution and improved Linear Energy Transfer (LET) independence, emerged during the development of technologies such as MRgRT, hadron therapy, and Boron Neutron-Capture Therapy (BNCT). For instance, the challenges derived from modern techniques, such as MRgRT, were preliminarily summarized by De Deene [2], while the performance and drawbacks of gel dosimetry for hadron beams, mainly their sensitivity quenching, have been reported by other authors [8][9][10]. Although the traditional characteristics of most used gel dosimetry systems have been properly defined and optimized during the last few decades, it is worth mentioning that the formulation and design of specific gel dosimeters involve organic and inorganic chemistry principles, physics interactions, and analytical techniques that generally exceed the capacity of most of the personnel in typical clinical radiation facilities.
Several review articles have been published in the last few years; for example, Marrale and d’Errico [11] reported thorough and complete analyses of the use of ferrous sulfate-based dosimetry systems, including not only key information on the formulation, synthesis, and read-out of the dosimeters, but also the effect of the gelation temperature and purity of reagents on the reproducibility, response, and dose stability due to ion diffusion or spontaneous oxidation. The authors briefly described the most recent studies on these types of dosimeters, such as reusable Fricke gel dosimetry, for example. Moreover, the use of polyacrylamide gels for ionizing radiation dosimetry was also reviewed by Marrale and d’Errico, with a special focus on clinical applications of the different polymer formulations. In addition, the main scanning techniques were described and compared for Fricke and polymer gels. De Deene [2] recently reported a full review of the historical advances in gel dosimetry, highlighting three main groups: (i) Fricke, (ii) radiochromic, and (iii) polymer gel dosimeters. The state-of-the-art of these three groups was carefully reported, and personal perspectives were presented, focusing on the ongoing challenges for high-performance dosimetry systems. Furthermore, a brief revision of hydrogel-based dosimeters for measuring ionizing radiation doses was recently published by Zhang et al. [12]. The authors classified the dosimeters into the following categories: (i) polymer hydrogel, (ii) Fricke hydrogel, (iii) radio-chromic, (iv) radio-fluorescence, and (v) NPs-embedded dosimeters, focusing on the main features and drawbacks of their use for routine clinical applications. The sensitivity, accuracy, and dose resolution were specifically considered for each dosimeter.

2. Brief History of Gel Dosimetry

Two different types of gel dosimetry are typically considered—those based on the oxidation of ferrous sulfate into ferric ions in an acid solution within a gelatin matrix or, more recently, in a polyvinyl alcohol-based matrix, known as Fricke and PVA-GTA Fricke gels, respectively, and those based on polymerization reactions induced by the radicals formed during the radiolysis of water, commonly known as polymer gel dosimetry (PGD).
Fricke gel dosimetry was derived from the initial studies by Fricke and Morse in 1927 [13] and emerged with the use of nuclear magnetic resonance (NMR) to measure the radiation-induced chemical changes while stabilizing the dose information with the aid of a gel matrix [14]. In these dosimeters, ferrous sulfate is incorporated into a gel matrix and the physicochemical change upon irradiation is the oxidation of Fe2+ ions to Fe3+ aided by the radicals generated in the radiolysis of water by the ionizing radiation. Therefore, the final concentration of Fe3+ depends on the absorbed dose and the chemical yield of ferric ions G(Fe3+). The Fe3+ concentration can be measured by spectrophotometry in the ultraviolet region, mainly at 304 nm and 224 nm [14]. The main limitation of these dosimetry systems is the loss of spatial information due to the diffusion of ferric ions within the gel matrix [15]. Many improvements and enhancements have been proposed over the decades, with a special focus on reducing autoxidation processes and the diffusion of ferric ions, and on improving or simplifying the response read-out of the dosimeters. For example, adding an indicator, such as xylenol orange, to an acid solution of ferric ions enhances the optical absorbance in the visible range and reduces the diffusion of ferric ions by forming a metal coordination complex [16][17]. Another example is the use of sodium chloride to increase the reproducibility of MRI readouts in Fricke gels containing organic impurities [18]. A completely different approach was proposed by Chu et al. [19] aiming to reduce the diffusion of ferric ions in Fricke dosimetry by using a different gel matrix. In their study, the authors used polyvinyl alcohol (PVA) cryogels or PVA hydrogels prepared with PVA concentrations of up to 20 wt%, which exhibited ten times lower auto-oxidation rates than gelatin-based Fricke gels, and diffusion coefficients of less than half of the lowest reported value. However, the obtained cryogels had a rubbery consistency, opaque appearance, and expanded upon freezing, limiting their application and readouts for MRI. Then, Marini et al. [20] suggested preparing the gel matrix by the chemical crosslinking of PVA with glutaraldehyde (PVA-GTA). The obtained dosimetry system overcame the limitations of PVA cryogels, exhibiting high sensitivity, a low minimum detection dose, and low diffusion rates. Moreover, great advances have been made by using PVA-GTA formulations [21][22][23][24][25].
The main chemical reactions in these types of dosimetry systems can be summarized in the reaction of ferrous ions and radicals formed upon irradiation or their subproducts in an acid solution.
F e 2 + + O H •   → F e 3 + + O H −
F e 2 + + H O 2 •   → F e 3 + + H O 2 −  
H O 2 − + H 3 O + → H 2 O 2 + H 2 O  
F e 2 + + H 2 O 2 → F e 3 + + O H − + O H •  
The use of polymers in radiation dosimetry was originally focused on the degradation of the macromolecules upon radiation and the subsequent viscosity decrease [26][27]. In 1958, Hoecker and Watkins proposed a polymerization reaction induced by radiation and a dosimetry system relying on adjusting the sensitivity of the polymerization with dissolved iodine or oxygen [28]. Several decades later, the use of NMR to assess changes during the polymerization of N,N′-methylene-bis-acrylamide (MBA) [29] and acrylamide (AAm) with MBA [30] within a gel matrix set the basis of modern PGDs. In the early stages of polymer gel dosimetry, several issues related to their accuracy and reproducibility for clinical applications appeared because of the inhibitory effects of oxygen in free radical polymerizations, which forced their manufacture, storage, and irradiation under hypoxic conditions [7]. It was only in 2001 that Fong et al. [31] included, in a PGD, a complex formed with ascorbic acid, copper sulfate, and oxygen that could produce free radicals and be used in polymer gel dosimetry under a normal atmosphere, but it required the use of a radical inhibitor, such as hydroquinone, to avoid any polymerization prior to their use. After that, complexation with oxygen was replaced by just ascorbic acid as an antioxidant, and then with a better antioxidant, such as tetrakis (hydroxymethly) phosphonium chloride (THPC), that may also act as a promoter of the polymerization reaction [32]. These new types of material called normoxic polymer gel dosimeters have been widely used for clinical applications, as reviewed by Farhood et al., up to late 2017 [33]. A similar systematic approach to that used by Farhood et al. was followed in the present contents to include some recent studies intimately related to clinical applications. The adopted methodology consisted of a meta-search of the titles and abstracts of articles in three different electronic databases, namely Scopus, PubMed, and Web of Science, from the last five years up to 9 October 2022. The search terms included three groups related to “polymer gel dosimetry”, “type of radiation therapy”, and “type of polymer gel dosimetry”. Further steps consisted of the exclusion of duplicates, manual screening based on the title and abstract, and final selection by full-text analysis. The extracted data from the selected articles are summarized in Table 1.
Table 1. Included clinical studies with polymer gel dosimetry published since 2017.
Author and Year Gel Type Beam Type Treatment Type Technique Type Readout System
Yao et al. 2019 [34] NIPAM Photon External IMRT and VMAT Optical CT
Elter et al., 2019 [35] PAGAT Photon External MRgRT MRI
Hillbrand et al., 2019 [36] VIP Proton External Pencil beam MRI
Watanabe et al., 2019 [37] VIPET 192Ir Brachytherapy - MRI
Abtahi et al., 2020 [38] NIPAM Photon External Intraoperative radiotherapy MRI
Chou et al., 2020 [39] NIPAM Photon External IGRT Optical CT
Kozicki et al., 2020 [40] VIC and VIC-T Photon External Stereotactic radiosurgery MRI
Pant et al., 2020 [41] NIPAM Photon External Stereotactic radiosurgery CBCT
Mann et al., 2020 [42] PAGAT Photon External IGRT MRI
Schwahofer et al., 2020 [43] PAGAT Photon External MRgRT MRI
Elter et al., 2021 [44] PAGAT Photon External MRgRT MRI
Nezhad et al., 2021 [45] MAGAT Photon External Intraoperative radiotherapy MRI
Azadeh et al., 2022 [46] MAGIC-f Photons External Stereotactic radiosurgery MRI
Fuse et al., 2022 [47] PAGAT-MgCl2 Photon External IMRT MRI
Kim et al., 2022 [48] MAGAT Photon External MR-Linac (isocenter verification) MRI
Kudrevicius et al., 2022 [49] nPAG Photon External Gamma knife MRI
Watanabe et al., 2022 [50] VIPET 192 Ir and photon Brachytherapy and external -
IMRT		 MRI
New trends, compositions, and variations in most gel dosimetry systems used have been presented over the last twenty years, aiming to provide dosimetry systems for specific readout techniques, and mainly those implemented in standard-equipped linacs, such as the case of the massive incorporation of conventional CT, kilo- and mega-voltage cone-beam CT into treatment rooms for near real-time monitoring. This approach was pioneered in 1999 [51], extended to real-time 2D IGRT to detect gross motion and correct the treatment, and then evolved into 3D real-time IGRT [52]. In this framework, gel dosimetry specially designed to exhibit higher sensitivity for X-ray CT and the optimization of CT imaging protocols were proposed in 2000 [53] and improved over the years. For example, Jirasek et al. [54] used a highly concentrated NIPAM-based dosimetry system containing 30% isopropanol as a cosolvent to increase the solubility of the reactive system from the typical 6% used in PGDs up to 20 wt% with 50% crosslinkers. Chain et al. [55] proved that the solubility of the commonly used crosslinker MBA can be increased from 3% in water to 5.5% in NIPAM solutions, thereby achieving high total concentrations (~19.5 wt%) suitable for X-ray CT readout with only aqueous solutions. Recently, Javaheri et al. [56] reported an optimized CT protocol that was able to measure CT number changes in NIPAM dosimeters with typical mass concentrations (3% NIPAM and 3% MBA) for irradiations with doses from 2 to 8 Gy. Jirasek et al. [57] studied gels with 14.5% NIPAM and 4.5% MBA for near-real-time readouts in Linac-integrated kV cone-beam CT. The obtained results proved that, with a post-irradiation time of 20 min, 90–93% of the radiation-induced CT number change takes place if a proper number of acquisitions, image averaging, and filtering are used. The authors also demonstrated that the accuracy and precision can be improved to 2–4%, and 3D gamma tests against treatment plans yielded values above 90% with the proposed setup.

This entry is adapted from the peer-reviewed paper 10.3390/gels8100663

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