(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: To determine the extracranial dose delivered by frameless stereotactic radiosurgery with the Elekta Gamma Knife Icon (GK-Icon) system. Extracranial dose is of concern for pediatric radiosurgery patients and is delivered both by the ��Co radiosurgery beams and the cone beam computed tomography (CBCT) positioning system.
Methods: The GK-Icon is a Gamma Knife Perfexion plus a CBCT positioning system. Measurements of the CBCT low-and high-dose protocols were obtained using a Farmer chamber with a buildup cap at several distances from a RANDO head phantom placed at the imaging isocenter. Raw chamber readings were scaled using the CBCT low-dose (2.5 mGy) and high-dose (6.4 mGy) protocol calibration measurements taken at isocenter using a conventional CTDI phantom. GK-Icon total extracranial dose was calculated by combining the CBCT dose measurements with published Gamma Knife Perfexion extracranial radiosurgery dose values (Lindquist and Paddick, Op. Neurosurgery 61(3), 130-141, 2007).
Results: Extracranial dose data were expressed as a percentage of the prescription dose by conservatively normalizing them to a 10 Gy prescription dose. If one high-dose and two low-dose CBCT images were acquired with the GK-Icon, the extracranial dose at 25 cm from isocenter was 0.04% of the prescription dose. At 25 cm away from isocenter, approximately 80 high-dose CBCT scans could be acquired in order for the GK-Icon extracranial dose to equal the extracranial dose given by the Novalis linac-based radiosurgery system. This is due to the fact that the dose fall off and leakage from the GK-Icon was sharper than the Novalis system. At distances greater than 25 cm from the isocenter, the Novalis system’s extracranial dose was an average of 7.5 times higher than that of the GK-Icon.
Conclusion: Extracranial GK-Icon doses were substantially lower than those reported with linear accelerator based radiosurgery, even with CBCT dose included.
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