Initial performance characterization and clinical implementation of a novel image-guided system for Perfexion

Mark Ruschin1, Paul De Jean2, Steve Ansell2, Greg Bootsma2, Caroline Chung1, Cynthia Menard2, Young-Bin Cho2, David Jaffray2

1Toronto, Canada 2Radiation Medicine Program, Princess Margaret Hospital, Toronto, ON, Canada

Keywords: gamma knife, image guidance, technique, computed tomography, radiosurgery

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     A novel cone-beam CT (CBCT) image-guidance system has been installed on a Perfexion unit at our institution and has been used for patient imaging. 
     The purpose of this study is to describe the initial performance and clinical experience with this image-guided Perfexion (IGP) system.
     Our initial clinical protocol was designed to allow CBCT images of patients with brain metastases to be acquired immediately prior to and after treatment and retrospectively analyzed for setup accuracy and intra-fraction motion.
     The initial CBCT prototype could achieve a 188-degree scan. Adjustable imaging parameters included: beam quality (tube potential and filter), patient dose, scan speed, reconstruction resolution, and number of projections. The optimal beam quality was determined by measuring the contrast-to-noise ratio in known objects at different tube potentials and filter combinations. With the optimized beam quality, the minimum required patient dose for sufficient image quality was then determined by reviewing images of anthropomorphic phantoms. Target localization accuracy was determined by simulating the treatment planning process in end-to-end testing in phantoms using CBCT images defined in Gammaplan. 
     The optimal beam quality – taking into account contrast and patient dose – was determined to be 90 kV with a bowtie filter plus 0.1mm copper. Using 0.5mAs per projection and 188 projections (1 per degree) resulted in a CBCT dose of approximately 1cGy to the centre of a 16cm head phantom. Low-contrast details such as polyethylene inserts (CT number=-100) in water could be detected. High contrast resolution of 7-8lp/cm and 3-4lp/cm was attained using 0.5mm and 1.0mm reconstructed cubic voxels respectively. The scan time was set to 1min, after which the reconstructed volume is available in <30s if using 1.0mm cubic voxels. Retrospective reconstruction at 0.5mm voxels has been possible using the saved projection images. The mean absolute targeting error in phantom was 0.3mm ± 0.2mm (range: 0.1 – 0.6mm). 
     Setup accuracy for the first CBCT patient was determined to be within 1mm in all cardinal directions, with an intra-fraction motion of less than 0.6mm in any direction. A benchmark for this concept has not yet been defined.
     A novel CBCT system for image-guided Perfexion has been clinically implemented and is in use for patient imaging.
     This concept will require additional testing and use to determine indications and best practice.


Project Roles:

M. Ruschin (), P. Jean (), S. Ansell (), G. Bootsma (), C. Chung (), C. Menard (), Y. Cho (), D. Jaffray ()