Purpose: To develop a three-dimensional (3D) Winston-Lutz (WL) test for stereotactic radiosurgery (SRS) using a novel radiochromic gel dosimeter based on a polyvinyl alcohol-iodide (PVA-I) complex that can be reused after annealing.
Methods: The prepared PVA-I gels were poured into acrylic containers (2 Ã— 2 Ã— 12 cmÂ³) and a small tungsten sphere was embedded in the gels. The PVA-I gel dosimeter was used as a position detector in the present study. The sphere in the dosimeter was aligned with the laser-defined isocenter. The eight dosimeters were irradiated with multiple circular fields (Î¦10 mm) with various gantry and table combinations using a Novalis Tx linac. Electric portal imaging device (EPID) images were recorded during the irradiation, and the 3D isocenter displacement between the sphere and the radiation isocenter was calculated mathematically from all images. Following irradiation, the dosimeters were scanned with a flatbed scanner to analyze the direct 3D isocenter displacement. In addition, the SRS isocenter verification, in which the dosimeter was irradiated with multiple non-coplanar arcs that encompassed the clinical SRS cases, was repeated 12 times every week by repeated annealing. All isocenter analyses were conducted by an in-house developed MATLAB program.
Results: An excellent agreement of the 3D isocenter displacement between the WL test from EPID and PVA-I gel dosimeters was observed with average differences of 0.04 Â± 0.03 mm, 0.06 Â± 0.02 mm, and 0.07 Â± 0.06 mm in the left/right, superior/inferior, and anterior/posterior directions, respectively. The average 3D isocenter displacement by the repeated weekly SRS isocenter verification using the dosimeter was 0.18 Â± 0.06 mm, 0.10 Â± 0.05 mm, and 0.12 Â± 0.07 mm in the three directions, respectively.
Conclusion: The accuracy of the 3D WL test using the PVA-I gel dosimeter was excellent. Furthermore, repeatability of the dosimeter was reliable for the SRS isocenter verification.