Verification of beam placement accuracy in Gamma Knife radiosurgery by a BANG3 polymer gel dosimeter

Polymer gel dosimeter BANG3® was used to measure beam placement accuracy of a Leksell Gamma Knife radiosurgery unit. We did two experiments over a five-month period. Both show a 1.8-mm (corresponding to the size of two pixels) shift of measured dose distributions relative to the isodose lines calculated by the treatment planning software in the left-right direction. There is no observable error in the anterior-posterior direction. We have considered several sources for the observed shift, which include phantom setup error, MRI image distortion due to field inhomogeneity and non-linearity, treatment planning software error, and design/manufacturing error of the radiosurgery unit. The observed error has not been identified using the quality assurance procedure recommended by the manufacturer. We have demonstrated, therefore, that the polymer gel dosimeter is useful to verify the beam placement accuracy of a Leksell Gamma Knife unit as a part of acceptance test of hardware and treatment planning software. We recommend that the treatment prescription should be made considering the accuracy limit presented in this study.