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Dose Adaptation with a Virtual 6D Couch for Frameless Brain Radiosurgery Via a Dedicated Gamma-Ray Device

J Cheung

J Cheung*, S Braunstein , P Sneed , M McDermott , P Theodosopoulos , L Ma , University of California San Francisco, San Francisco, CA


SU-F-J-106 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

The couch for Gamma Knife Radiosurgery (GKSRS) is limited to translational movements. With frameless designs, patient positional rotations may become an important factor. Therefore, we investigated the effect of rotations for GKSRS and the ability to correct for these rotations with simple couch translations.

Seven previously treated GKSRS patients with different disease types (tumor volumes 0.01-12cc) were selected for this study. The planning MRI images were processed in order to digitally rotate the stereotactic frame (Leksell G-frame) to ±1, ±2, ±3, ±5, and ±10-degrees in the transverse plane from the nominal position. These images were imported back into the clinical planning system (LGP 10.2) and dose from the original treatment plan was calculated with these rotational errors. Translation corrections for each shot based on the rotation transform matrix were applied to the plan and coverage and Paddick conformity index (PCI) were evaluated for the uncorrected and corrected plans.

The rotational errors resulted in large decreases in plan coverage, with some small tumors being missed completely with rotations of ±3-degrees or more. The rotated plans resulted in an average (range) decreased coverage of 8% (0-33%), 28% (0-87%), 43% (0-100%), 57% (0-100%), and 72% (5-100%) for ±1, ±2, ±3, ±5, and ±10-degrees, respectively. For the corrected plans, the average (range) decrease in coverage was only 1% (0-4%), even for the rotations of ±10-degrees. The PCI differences improved as well, with the rotated plans resulting in decreased PCI of 2% (0-11%), 13% (0-32%), 22% (0-50%), 31% (1-56%), 42% (4-67%) for ±1, ±2, ±3, ±5, and ±10-degrees, respectively, which improved to 1% for all of the corrected plans with a range from 5% decrease to 13% increase in PCI.

The applications of simple 3D couch translations were successful in recovering rotation uncertainties up to ±10-degrees for frameless GKSRS.

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