Measurement Of Gamma Knife Clinical Accuracy Using Polymer Gel Dosimetry

Karaiskos Pantelis1, Argyris Moutsatsos1, Lukas Petrokokkinos1, Panagiotis Papagiannis1, Chris Constantinou1, Ioannis Seimenis1, Evangelos Georgiou1, Panagiotis Sandilos2, Michael Torrens, FRCS3

1University Medical School 2 Hygeia Hospital 3Department of Neurosurgery, Hygeia Hospital

Keywords: dosimetry, physics, gamma knife, Dose, technique

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     The aim of this work is to demonstrate an experimental procedure which can assess the total clinical accuracy of a Gamma Knife treatment, including inaccuracies stemming from spatial distortion induced by the MR unit and the specific pulse sequences used.
     A custom-made PMMA head phantom, filled with a normoxic polymer gel dosimeter formulation (VIP), was used to accurately reproduce every link in the GK treatment chain; from patient imaging and dosimetry planning to patient positioning and dose delivery using the automated positioning system (APS) of a GK model 4C unit. Multiple 4 mm shots were planned and delivered to the phantom leaving ample space between the shots to preclude any “overlaps”. MR and CT images required for treatment planning were obtained using sequences identical to those employed for patient imaging in daily clinical practice. Planning was performed in CT images which are known to suffer from negligible spatial distortion (less than the mechanical accuracy of the gamma knife method, i.e. 0.5 mm). A custom made algorithm based on standard imaging routines was developed to accurately define the spatial coordinates of the center of each 3D dose distribution registered to the gel substance exploiting the symmetry that the GK single shot dose distributions present in space with respect to this point.
     The magnitude of the spatial distortion induced by the MRI sequences used for patient treatment was quantified via comparison of the planned to the polymer gel measured coordinates of the 4 mm shot centers. The method was found to indentify the MRI distortion with accuracy of the order of the pixel size of the MR pulse sequence used for imaging purposes (< 1mm). The statistical evaluation and examples of images will be demonstrated.
     This is a retrospective study.
     Polymer gel MRI dosimetry can be utilised effectively for assessing the overall clinical accuracy of gamma knife application.
     That includes spatial distortion inherent in MRI by exploiting the 3D dose distribution registered to the gel substance.


Project Roles:

K. Pantelis (), A. Moutsatsos (), L. Petrokokkinos (), P. Papagiannis (), C. Constantinou (), I. Seimenis (), E. Georgiou (), P. Sandilos (), M. Torrens ()