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Dosimetric Impact of Rotational Setup Errors On Multiple Brain Targets Treated with Single Isocenter Volumetric Modulated Arc Therapy

J Xue*, D No , J Zhang , H Wang , D Barbee , S Lymberis , J Silverman , I Das , NYU Langone Medical Center, New York, NY


(Wednesday, 8/1/2018) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 3

Purpose: This study was to investigate dosimetric variation of simulated multiple brain targets using single isocenter VMAT taking into account rotational setup uncertainties derived from cone beam CT (CBCT) and 6 degrees of freedom (6DoF) couch.

Methods: Seven targets were delineated in the CT image of a patient’s brain scan. Each gross tumor volume (GTV) was drawn as a sphere of 1 cm in diameter. Each GTV target was then expanded with 2 mm margin to be a planning tumor volume (PTV).A VMAT plan with single isocenter was optimized by 6X, 6XFFF and 10XFFF beams respectively to cover each PTV with 100% prescription dose. All targets were then simulated to have a rotational setup error by one degree, i.e., the rotation relative to the isocenter along pitch, roll or yaw by +/- 1°. Dosimetric variations were analyzed for each PTV and GTV targets imitating the delivery with the specified errors.

Results: Dosimetric variations were observed with the given setup errors. Minimum dose and PTV coverage decreased as much as 10% for some targets. PTV mean dose went up about 1% (0.5 – 1.2%) and GTV coverage had less than 0.5% decrease for every target. The magnitude of deviations is dependent on the location of a target, the orientation of target rotation and slightly on beam energy. Some PTVs actually gained the volume coverage by as much as 6%.

Conclusion: Stereotactic radiosurgery (SRS) using single isocenter VMAT can be an efficient treatment for multiple brain lesions. The uncertainty of target positioning is associated with the accuracy of CBCT and 6DoF couch. This study demonstrated that the total rotational tolerance should be maintained below 1° in physics QA, and it is also necessary to have a minimum margin of 2 mm to ensure an adequate GTV coverage.


Setup Errors, Stereotactic Radiosurgery, Target Localization


TH- External beam- photons: intracranial stereotactic/SBRT

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