Clinical applications of radiochromic gel dosimeter. Three-dimensional Winston-Lutz test using reusable PVA-I gel dosimeter

Radiochromic gel dosimeters are used for three-dimensional (3D) dosimetry by measuring absorbance with optical computed tomography; however, their clinical application is not widespread in Japan. The possibility of clinical application of radiochromic gel dosimetry is being investigated using a general-purpose flatbed scanner as a reading device of optical density. In this paper, the 3D Winston-Lutz (WL) tests have been introduced as a quality assurance (QA) for stereotactic radiosurgery (SRS) for intracranial lesions. PVA-I radiochromic gel dosimeters based on a polyvinyl alcohol-iodide (PVA-I) complex, which can be reused after annealing, were used for the tests. A WL gel phantom was prepared by embedding a small tungsten sphere in the gel as a position detector. A flatbed scanner was used to analyze the isocenter position which was determined from the distance between the center of the irradiation field and the sphere. The 3D isocenter positions obtained by this method agreed with those measured by the electric portal imaging devices, which are commonly used for WL tests, with a deviation of less than 0.1 mm. These results demonstrated that the WL gel phantoms were highly reliable and capable of detecting the isocenter position. In addition, long-tenn SRS isocenter verification using a WL gel phantom exhibited good reusability after repeated irradiation and annealing. The 3D WL test developed in this study provides an easy and rapid process for accurate isocenter position analysis, and the excellent reusability of the PVA-I gel dosimeter suggests its usefulness in SRS routine QA. (author)