Radiation dosimetry using polymer gels: methods and applications.

New, complex radiotherapy delivery techniques require dosimeters that are able to measure complex three-dimensional dose distributions accurately and with good spatial resolution. Polymer gel is an emerging new dosimeter being applied to these challenges. The aim of this review is to present a practical overview of polymer gel dosimetry, including gel manufacture, imaging, calibration and application to radiotherapy verification. The dosimeters consist of a gel matrix within which is suspended a solution of acrylic molecules. These molecules polymerize upon exposure to radiation, with the degree of polymerization being proportional to absorbed dose. The polymer distribution can be measured in two or three dimensions using MRI or optical tomography and, after calibration, the images can be converted into radiation dose distributions. Manufacture of the gel is reported to be reproducible, and measured dose in the range 0-10 Gy is accurate to within 3-5%. In-plane image resolution of 1 mm x 1 mm, with image slice thicknesses of between 2-5 mm, is typically achievable using clinical 1.5 T MR scanners and standard T2 weighted imaging sequences. The gels have been used to verify a number of conventional and novel radiotherapy modalities, including brachytherapy, intensity modulated radiotherapy and stereotactic radiosurgery. All the studies have confirmed the value and versatility of the dosimetry technique.