Room: Exhibit Hall
Purpose: Gamma Knife Iconáµ€á´¹ has a new CBCT localization feature which can be also used for the frame localization and by doing it â€“ potentially remove the errors connected to spatial distortions of the indicator markers and effect of a frame itself. The phantom study is needed to evaluate both the dosimetric uncertainty of CBCT localization and to compare it with the traditional indicator box localization technique.
Methods: A modified Elekta phantom with three film holders that was positioned in Leksell frame was analyzed using both indicator box and CBCT localization. The CT images of the phantom were imported to Gamma Plan and localized using CT markers from the indicator box and compared with CBCT localization. Pieces of EBT3 Gafchromicáµ€á´¹ films were placed in those holders and small mechanical holes were made in the center of films. After irradiation films were scanned using Epson 850 flatbed scanner with the 400 dpi resolution. Delta values analysis of differences between mechanical and dosimetrical film centers was done using J-image software.
Results: Delta values for the CBCT localization averaged between 3 films for x, y, z axes in mm were accordingly: 0.25 Â± 0.11; 0.14 Â± 0.06; 0.25 Â± 0.05. 2D radial deltas for XZ/XY/YZ were 0.35Â± 0.12; 0.29 Â± 0.13; 0.29 Â± 0.08. The 3D displacement vector was 0.38 mm. The differences between stereotactic coordinates for two ways of localization on Gamma Plan in right/left, anterior/posterior and superior/inferior directions were 0.7/0.4/0.2 mm accordingly.
Conclusion: The end-to-end test for the CBCT localization showed less than 0.4 mm of the 3D vector displacement which is in manufacture tolerance. The frame localization technique showed a max stereotactic coordinate difference of 0.7 mm in X direction which can give emphasize a point of using CBCT localization even for the frame patients.
Funding Support, Disclosures, and Conflict of Interest: Consultant for Elekta AB