(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Linac-based SRS has grown in popularity, however there are uncertainties in positioning and dose delivery that mandates treatment margins for adequate tumor coverage. The goal of this study was to assess overall treatment uncertainty of SRS treatments, via end-to-end testing.
Methods: total of 17 irradiations were performed using a LUCY® 3D QA phantom. Radiochromic film was prepared with fiducial markers (<1mm thickness) placed on the film to mark the target position (1 or 3 targets), and the film was placed in the LUCY phantom insert. Target contours were defined on CT images by a uniform expansion of 1 cm diameter from the center of the fiducial markers. Based on a SRS prescription dose of 12 Gy, treatment plans were computed using Multi-Mets or Cranial SRS modes with Brainlab Elements (ver 2.0) and Dynamic Conformal Arc or VMAT plan with Eclipse (ver 15.5). An identical non-coplanar arc template was used in all as to limit geometric plan variability. All plans were delivered using CBCT and/or ExacTrac based localization on a Varian Truebeam STx.
Results: average separation between marked target center and radiation center was 0.513±0.156 mm with CBCT localization alone, 0.652±0.135 mm when using CBCT and ExacTrac (0.7 mm/0.7 deg tolerance). The average separation was 0.364±0.133 mm when both imaging techniques were used but ExacTrac localization was set to 0.5 mm/ 0.5 deg tolerance, and finally 0.41±0.103 mm when ExacTrac localization (0.5 mm/ 0.5deg) was solely used.
Conclusion: on the uncertainties for the imaging localization and positioning, a 0.5mm target margin is warranted. Moreover, ExacTrac imaging tolerances showed higher accuracy in overall localization when used as the primary localization technique or in conjunction with CBCT, especially when used with smaller imaging tolerances.
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