Evaluation of dose delivery accuracy of Gamma Knife by polymer gel dosimetry

BANG polymer gel dosimeter was used to evaluate three-dimensional (3D) absorbed dose distributions in tissue delivered with Gamma Knife stereotactic radiosurgery systems. We compared dose distributions calculated with Leksell GammaPlan treatment-planning software with dose distributions measured with the polymer gel dosimeter for single shot irradiations. Head-sized spherical glass vessels filled with the polymer gel were irradiated with Gamma Knife. The phantoms were scanned with a 1.0T MRI scanner. Hahn spin-echo sequence with two echoes was used for the MRI scans. Calibration relations between spin-spin relaxation rate and absorbed dose were obtained by using small cylindrical vials, which were filled with the polymer gel from the same batch as for the spherical phantom. We made voxel-by-voxel comparisons of measured and calculated dose distributions for 31x31x31 dose matrix elements. With the 3D dose data we calculated tumor control probability (TCP) and normal tissue complication probability (NTCP) for a simple model. For the maximum dose of 100 Gy the mean and one standard deviation of differences between the measured and the calculated doses were the following: -0.384.63 Gy, 1.492.77 Gy, and -1.034.18 Gy for 8-mm, 14-mm, and 18-mm collimators, respectively. TCP values for measurements were smaller than the calculations by 0 to 7 %, whereas NTCP values were larger by 7 to 24 % for four of six experiments.