Verifying the accuracy of dose distribution in gamma knife unit in presence of inhomogeneities using PAGAT polymer gel dosimeter and Monte Carlo simulation

Polymer gel dosimetry is still the only dosimetry method for direct measuring of three-dimensional dose distributions. MRI Polymer gel dosimeters are tissue equivalent and can act as a phantom material. In this study the obtained isodose maps with PAGAT polymer gel dosimeter were compared to those calculated with EGSnrc for single shot irradiations of 8 and 18 mm collimators of Gamma Knife unit in homogeneous and inhomogeneous phantoms. Materials and Methods: A custom-built, 16 cm diameter spherical Plexiglas head phantom was. Inside the phantom, there was one cubic cutout for insertion of gel phantoms, and another cutout for inserting the inhomogeneities. The phantoms were scanned with a Siemens clinical 1.5 T MRI scanner. The multiple spin-echo sequence with 32 echoes was used for the MRI scans. Results: The results of measurement and simulation in homogeneous and inhomogeneous phantoms showed that the presence of inhomogeneities in head phantom could cause spatial uncertainty higher than ±2 mm and dose uncertainty higher than 7%. Conclusion: the presence of inhomogeneities could cause dose differences which were not in accordance with accuracy in treatment with Gamma Knife radiosurgery. Moreover, the findings of Monte Carlo calculation revealed that the applied simulation code (EGSnrc) was a proper tool for evaluation of 3 D dose distribution in Gamma Knife unit.