Gel dosimetry: An overview of dosimetry systems and read out methods

Gel dosimetry has emerged over the past three decades in response to a growing need in high-precision radiotherapy to assess, in three dimensions, the absorbed radiation dose, as would be administered in cancer patients. Radiation-induced reaction mechanisms are dependent on the class of gel dosimeter, with four classes emerging as primary dosimeters for use in radiation therapy dose verification: (i) Fricke gel dosimeters contain a Fricke solution consisting of ammonium iron (II) sulfate in an acidic solution of sulfuric acid. In Fricke systems an oxidation of ferrous ions results in a change in the nuclear magnetic resonance (NMR) relaxation rate, which enables reading out Fricke gel dosimeters by use of MRI. The radiation-induced oxidation in Fricke gel dosimeters can also be visualized by adding a redox indicator. (ii) Polymer gel dosimeters exploit the radiation induced polymerization reaction of vinyl monomers and are predominantly read out by quantitative MRI or X-ray CT. (iii) Radiochromic dosimeters do not demonstrate a significant radiation-induced change in NMR properties but can be scanned by use of optical scanners (optical CT). In contrast to Fricke gel dosimeters, radiochromic gel dosimeters do not rely on the oxidation of a metal ion but exhibit a color change upon radiation. (iv) Radiofluorogenic dosimeters become fluorescent when exposed to ionizing radiation and can be read out with a planar scanning light beam. Likewise, the imaging modality used to extract quantitative dose information depends on the class of dosimeter being used, and three primary imaging modalities have emerged in this context: quantitative MRI, x-ray CT, and optical CT imaging. The accuracy and precision of the dose information extracted from gel dosimetry systems depends on both the dosimetric properties of the gel dosimeters and the readout technique, and the optimal readout method depends on the gel dosimeter response. Despite remaining an active field of research and illustrations of the application of gel dosimetry for the validation of clinical dose distributions, the utilization of gel dosimetry as a routine clinical dosimeter has been rather limited. However, with the introduction of new radiotherapy techniques that focus on organ motion compensation, new fractionation schemes, and extreme dose rates, the need for 3D radiation dosimetry is apparent. Even with the need for 3D dosimetry being apparent, gel dosimetry faces continued challenges in areas regarding the extraction of reproducible, accurate, and precise dose information. This review paper focuses on an introduction to gel dosimeter classes; a detailed examination of the three readout techniques with emphasis on the achievable accuracy, precision, and optimization of readout parameters; an outlook on future applications in emerging new radiotherapy techniques. We note that the introduction of theragnostic hybrid MRI-Linacs that combine an MRI-scanner and a clinical linear accelerator create new opportunities for inline polymer gel dosimetry. Likewise, the use of cone beam CT on linear accelerators opens up the possibility to read out gel dosimeters on the linac. Multiple optical CT designs have shown that optical CT gel dosimetry is eminently capable of providing high quality dosimetric information from clinically relevant treatment regimes. As a result, gel dosimetry provides exciting opportunities for 3D radiation dosimetry that were not available even a few years ago. The unique feature set of a properly executed gel dosimetry workflow allows for the extraction of dosimetric information, in 3D, that is not possible with any other dosimetry system and hence gel dosimetry provides exciting opportunities for clinical and research work in the area of radiation dose measurement.