Mechanical and dose delivery accuracy evaluation in radiosurgery using polymer gels

The polymer gel-Magnetic Resonance Imaging (MRI) dosimetry technique was employed to evaluate the mechanical and dose delivery accuracy in Leksell gamma knife stereotactic radiosurgery for the treatment of multiple targets. Two different polymer gel dosimeter formulations that have been reported in the literature were prepared in-house. A plan for the treatment of four brain metastases (targets) was generated. The plan involved the delivery of four, 8 mm collimator shots using different prescription isodose lines and different prescription doses for each target, keeping the maximum dose constant for all the targets. A sample of each gel formulation was irradiated using a custom made phantom with an experimental procedure capable of testing the increased nominal mechanical accuracy of stereotactic radiosurgery. The irradiated dosimeters were evaluated using a clinical 1.5 T Philips ACS NT MR imager. Result manipulation in 3-D allowed for the determination of the mechanical accuracy in the delivery of each shot through the comparison of measured versus planned shot center coordinates. Dose delivery accuracy was also evaluated by comparison of maximum dose values measured at the center of each shot as well as dose distribution measurements, with corresponding treatment planning calculations. Polymer gel dosimetry was found capable of verifying the complete chain of radiosurgery treatment in gamma knife applications involving the irradiation of multiple targets.