Clinical use of dual image-guided localization system for spine radiosurgery.

The recently released Novalis TX linac platform provides various image guided localization methods including a stereoscopic X-ray imaging technique (ExacTrac) and a volumetric cone beam computed tomography (CBCT) imaging technique. The ExacTrac combined with the robotic six dimensional (6D) couch provides fast and accurate patient setup based on bony structures and offers "snap shot" imaging at any point during the treatment to detect patient motion. The CBCT offers a three dimensional (3D), volumetric image of the patient's setup with visualization of anatomic structures. However, each imaging system has a separate isocenter, which may not coincide with each other or with the linac isocenter. The aim of this paper was to compare the localization accuracy between Exactrac and CBCT for single fraction spine radiosurgery treatments. The study was performed for both phantom and patients (96 clinical treatments of 57 patients). The discrepancies between the isocenter between the ExacTrac and CBCT in four dimensions (three translations and one rotation) were recorded and statistically analyzed using two-tailed t-test.