Abstract
The BANG™ polymer gel dosimeter was used to evaluate 3D absorbed dose distributions in tissue delivered with Gamma Knife stereotactic radiosurgery systems. We compared dose distributions calculated with Leksell GammaPlan (LGP) 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. The Hahn spin‐echo sequence with two echoes was used for the MRI scans. Calibration relations between the spin‐spin relaxation rate and the 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 31×31×31 dose matrix elements. With the 3D dose data we calculated the 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. Tumor control probability 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.PACS numbers: 87.53.‐j, 87.53.Dq, 87.53.Ly
Highlights
Many investigators are actively studying polymer gel dosimeters as a dosimetric tool that provides true 3D dose distributions in a tissue-equivalent medium.[1]
Ter.[8,9,10] Comparison of 1D line profiles and 2D dose distributions between measurements and Leksell GammaPlan (LGP) calculations was undertaken by some investigators.[2,3,4,5,6,7] There are other studies for which Monte Carlo simulation methods were used to evaluate the LGP calculations.[11,12,13] The results of previous studies show that the distance between the measured center of shots and the mechanical center is 1 mm or smaller
Recent studies of polymer gel dosimetry for Gamma Knife used small polymer gel-filled containers embedded in a head-shaped solid phantom.[5,7] These setups introduce unknown effects of material inhomogeneity on dose distributions and may lead to large measurement uncertainties
Summary
Many investigators are actively studying polymer gel dosimeters as a dosimetric tool that provides true 3D dose distributions in a tissue-equivalent medium.[1]. Some studies examined the shot placement accuracy of Gamma Knife units by measuring the distance between the location of the dose maximum and the mechanical cen-. Ter.[8,9,10] Comparison of 1D line profiles and 2D dose distributions between measurements and LGP calculations was undertaken by some investigators.[2,3,4,5,6,7] There are other studies for which Monte Carlo simulation methods were used to evaluate the LGP calculations.[11,12,13] The results of previous studies show that the distance between the measured center of shots and the mechanical center (or the unit center point) is 1 mm or smaller. Our results, which are not presented in this paper but have been published in part elsewhere,(14,15) are consistent with the observations by other investigators concerning 1D or 2D comparisons
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