SU-E-J-45: Validation of the ExacTrac Virtual Isocenter Based Target Localization Method

Medical Physics
Bo ZhaoJian-Yue Jin

Abstract

With no stable landmarks available for localization, a 'virtual isocenter' "'or surrogate landmark near the target'" can be used for image guidance. However, using a virtual isocenter in ExacTrac has not been thoroughly validated. This study evaluates its target localization accuracy and investigates the impact of two different couch correction sequences. A CT scan was acquired on an anthropomorphic thoracic phantom with a 2mm-diameter ball bearing (BB) marker implanted in thelung region. A treatment plan was created with isocenter placed at the BB center, and exported to ExacTrac. In ExacTrac, a virtual isocenter wasplaced on a spine vertebral body where three translational shifts (8.8cm laterally, 1.5cm longitudinally and 6cm vertically) were present. A series ofcouch rotations (+/-3 degrees, 1 degree increment) was intentionally applied to simulate angular setup variations. For each rotation, two stereoscopic x-rayimages were acquired and fused using the ExacTrac 6D registrationalgorithm. Calculated shifts were applied using two sequences: (1)automatic 5D corrections (three translations/two robotic couch rotations) followed by manual couch rotation; (2) manual couch rotation then automatic 5D corrections. After each ExacTrac...Continue Reading

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