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Two-dimensional and Three-dimensional Dosimetry
Published in W. P. M. Mayles, A. E. Nahum, J.-C. Rosenwald, Handbook of Radiotherapy Physics, 2021
Mark Oldham, Devon Godfrey, Titania Juang, Andrew Thomas
Radiochromic plastic dosimeters are the most recent development in the selection of 3D dosimeters and differ significantly from Fricke and polymer gel dosimeters in that they are made with an optically transparent plastic (e.g. polyurethane) matrix rather than a water-based gel. The mechanism of dose–response is irradiation-induced oxidation of a colourless leuco dye, which converts the compound into a coloured, light-absorbing form (Adamovics and Maryanski 2006). One example of this chemical reporting system is leucomalachite green (LMG) in combination with a trihalomethane or tetrahalomethane free radical initiator. Exposure to radiation creates halogen radicals, which subsequently oxidise LMG to a malachite green radical and then to malachite green, both of which are coloured (Figure 18.3). This creates a light-absorbing distribution proportional to the dose delivered and can be read out with optical CT imaging.
Detectors, Relative Dosimetry, and Microdosimetry
Published in Harald Paganetti, Proton Therapy Physics, 2018
Radiochromic dosimeters are based on a change of color as a result of radiation-induced chemical reactions. Various chemicals have been used as radiochromic dosimeters, such as aminotriphenyl-methane dyes and leuco dyes. Commercial radiochromic films, such as MD-55, EBT, EBT2, EBT3, and EBT-XD (Ashland, NJ), are 2D detectors that are widely used in radiotherapy in recent years. These all contain pentacosa-10,12-diyonic acid (PCDA) or its lithium salt LiPCDA in the sensitive layer. The crystalline PCDA or LiPCDA polymerize after irradiation, forming an intense blue darkening [47] without a requirement for any postirradiation development process. One or more thin layers of the radiochromic material are embedded in polyester films, and the overall composition of the films is nearly tissue equivalent. This dosimeter exhibits energy dependence due to the LET dependence of the chemical yields (number of molecules formed per 100 eV of energy imparted) of initial reactive species formed and, in addition, due to recombination or termination reactions that depend on the ionization density. For very low-energy protons, response quenching can be up to 50% [18], while in the Bragg peak of clinical low-energy proton beams, the under response is typically between 10% and 20%.
Film Dosimetry
Published in Gad Shani, Radiation Dosimetry, 2017
Most radiochromic systems are chemical radiation sensors consisting of solid or liquid solutions of colorless leuco dyes; these become colored without the need for development when exposed to ionizing radiation. Various radiochromic forms, such as thin films, thick films and gels, liquid solutions, and liquid-core waveguides, have been in routine use for dosimetry of ionizing radiation over a wide range of absorbed doses (10-2 to 106 Gy). Radiochromic film is used for general dosimetry of ionizing radiation in high-gradient areas of electron and photon beams in a wide energy range. The film allowing approximately tissue-equivalent dosimetry has been applied to mapping of dose distribution in brachytherapy.
Tissue-mimicking thermochromic phantom for characterization of HIFU devices and applications
Published in International Journal of Hyperthermia, 2019
Avinash Eranki, Andrew S. Mikhail, Ayele H. Negussie, Prateek S. Katti, Bradford J. Wood, Ari Partanen
Thermochromic materials possess the ability to change color based on temperature. The change in color can be permanent or reversible, and with or without intermediate stages (colors). Several different materials may be used to achieve color changes over a range of temperatures including liquid crystals, leuco dyes or inks [27–29]. For example, a heat sensitive phantom utilizing thermochromic liquid crystals has been recently developed for HIFU QA [30]. This phantom consists of a layer of thermochromic material on top of an absorbing disc, but is not tissue-mimicking and does not inform on volumetric spatial heating patterns. In addition, a tissue-mimicking thermochromic phantom (TMTCP) material that reports on HIFU heating patterns has been described [31]. This TMTCP incorporates a thermochromic material to produce a reversible change upon heating. However, the reversible nature of the color change requires rapid analysis in order to inform on temperatures before color reversal. Furthermore, the color change threshold (50 °C) of this phantom is low for thermal ablation and the color change process is affected by a hysteresis effect.