MAGAT gel and EBT2 film-based dosimetry for evaluating source plugging-based treatment plan in Gamma Knife stereotactic radiosurgery

Abstract

This work illustrates a procedure to assess the overall accuracy associated with Gamma Knife treatment planning using plugging. The main role of source plugging or blocking is to create dose falloff in the junction between a target and a critical structure. We report the use of MAGAT gel dosimeter for verification of an experimental treatment plan based on plugging. The polymer gel contained in a head-sized glass container simulated all major aspects of the treatment process of Gamma Knife radiosurgery. The 3D dose distribution recorded in the gel dosimeter was read using a 1.5T MRI scanner. Scanning protocol was: CPMG pulse sequence with 8 equidistant echoes, TR = 7 s, echo step = 14 ms, pixel size = 0.5mm × 0.5mm, and slice thickness of 2 mm. Using a calibration relationship between absorbed dose and spin-spin relaxation rate (R2), we converted R2 images to dose images. Volumetric dose comparison between treatment planning system (TPS) and gel measurement was accomplished using an in-house MATLAB-based program. The isodose overlay of the measured and computed dose distribution on axial planes was in close agreement. Gamma index analysis of 3D data showed more than 94% voxel pass rate for different tolerance criteria of 3%/2 mm, 3%/1 mm and 2%/2 mm. Film dosimetry with GAFCHROMIC EBT 2 film was also performed to compare the results with the calculated TPS dose. Gamma index analysis of film measurement for the same tolerance criteria used for gel measurement evaluation showed more than 95% voxel pass rate. Verification of gamma plan calculated dose on account of shield is not part of acceptance testing of Leksell Gamma Knife (LGK). Through this study we accomplished a volumetric comparison of dose distributions measured with a polymer gel dosimeter and Leksell GammaPlan (LGP) calculations for plans using plugging. We propose gel dosimeter as a quality assurance (QA) tool for verification of plug-based planning.

Figure 1. Treatment planning created in LGP with MR images of the MAGAT gel‐filled phantom: (a) plan created with three shots without plugging; (b) plan modified by placement of plugs near the shots represented by small circles in blue color.

Figure 2. EBT2 film exposed to radiation pattern (a) according to the plan created for plugging. Gel exposure to the same plan (b) seen in the MR image of the gel phantom. Small circles above and below the phantom represent the calibration vials.

Figure 3. Residual plot for the R2 dose response of the MAGAT polymer gel dosimeter.

Figure 4. Dose distribution comparison in axial plane. Comparison of MAGAT polymer gel: MRI‐measured (blue line) and LGP‐calculated (red line) relative dose distributions of 26% (4.2 Gy), 50% (8 Gy), 80% (12.8 Gy), on an axial plane at Z=101 mm.

Figure 5. Dose‐volume comparisons (a) of measurements (green dotted line) and LGP calculations (blue solid line). Note that 100% in horizontal axis corresponds to 16 Gy. A diagram (b) showing the difference in differential dose‐volume histograms for measured and computed dose distributions. Relative number of voxels vs. dose at the voxel.

Figure 6. The dose‐dependent–dose‐difference diagram (D4 diagram) generated using percent difference ratio. The diamond symbols indicate the mean dose difference. The error bars are for one standard deviation. Note that the 100% corresponds to 16 Gy, which is the maximum dose in the calculation matrix.

Figure 7. Histogram showing the gamma index distribution for MAGAT gel measurement. The criterion for the gamma index calculations was 2%/2 mm in axial plane. The calculation volume was limited to a subspace x=70 to 115, y=65 to 115, and z=90 to 114. More than 90% voxels included in the calculation had the value smaller than unity.

Figure 8. Isodose overlay comparison of gel measured (dotted line) and computed dose distributions (solid line) on axial plane at Z=101 mm. Isodose lines (a) for 5% (0.8 Gy), 26% (4.2 Gy), 50% (8 Gy) 70% (11.2 Gy), and 90% (14.4 Gy) of measurements and calculations are plotted. The units of x‐ and y‐axes are in mm. Gamma index map (b) for 2%/2 mm tolerance criteria in the axial plane. Comparison shows low gamma index values in the irradiated areas around the shot centers.

Figure 9. Optical density vs. absolute dose curves of EBT2 film in the 0–10 Gy dose range for green, red, and blue channels.

Figure 10. Isodose overlay comparison of film‐measured (dotted line) and computed dose distributions (solid line) on transverse plane. Isodose lines (a) for 5% (0.2 Gy), 26% (1.04 Gy), 50% (2 Gy), and 70% (2.8 Gy) and 90% (3.6 Gy) of measurements and calculations are plotted. The units of x‐ and y‐axes are in mm. Gamma index (b) map for 2%/2 mm tolerance criteria in the axial plane.

Figure 11. Histogram showing the gamma index distribution for EBT2 film measurement. The criterion for the gamma index calculations was 2%/2 mm in axial plane. The calculation region was limited to a subspace x=70 to 115, y=65 to 115, and z=101.

Figure 12. The arrows indicating the direction in which the horizontal (smooth line) and vertical profile (dashed line) were obtained for comparison between Leksell GammaPlan (LGP), gel dosimeter, and EBT2 film.

Figure 13. Comparison of experimentally obtained dose distributions represented by dose profiles in axial images (profiles were taken in the direction as shown in (a) representing the vertical profile between LGP calculated, film measured and gel measured doses; (b) representing horizontal profile between LGP‐calculated, film‐measured, and gel‐measured dose.