title: Factors that may determine the targeting accuracy of image-guided radiosurgery.
abstract: OBJECTIVE
  The AAPM TG-135 report is a landmark recommendation for the quality assurance (QA) of image-guided robotic radiosurgery. The purpose of this paper is to present results pertaining to intentionally offsetting the phantom as recommended by TG-135 and to present data on targeting algorithm accuracy as a function of imager parameters in less than ideal circumstances, which had not been available at the time of publication of TG-135.
  
  
  METHODS
  All tests in this study were performed at the Cooper University Hospital CyberKnife Center in Mt. Laurel, NJ. For intentional offsets, initial tests were performed on the Accuray-supplied anthropomorphic head and neck phantom, whereas for subsequent tests, the Accuray-supplied alignment quality assurance (AQA) phantom was used. To simulate the effects of imager parameters for larger patients, slabs of Blue Water (Standard Imaging, Inc., Middleton, WI) were added to attenuate the x-ray images in some of the tests. In conjunction with attenuated x-ray tests, the number of fiducials was varied by systematically deselecting them one at a time at the CyberKnife console.
  
  
  RESULTS
  Tests using the AQA phantom verified that submillimeter alignments were consistently achieved even with intentional shifts and rotations of up to 10.0 mm and 1.0°, respectively. An analysis of 17 months of daily QA alignment tests showed that submillimeter alignments were achieved more than 99% of the time even with such intentional shifts and rotations of the phantom. When additional slabs of Blue Water were added to simulate patient attenuation of the x-ray images, targeting errors could be induced depending on imager parameters and the amount of Blue Water used. A series of consecutive tests showed that two helpful variables to ensure good accuracy of the system were (1) the fiducial extraction confidence level (FECL) system parameter and (2) the number of targeted fiducials. When fewer than four fiducials were used, the FECL reported by the CyberKnife was sometimes high even when a false lock occurred, so using multiple fiducials helped to ensure reliable targeting.
  
  
  CONCLUSIONS
  Radiosurgery requires the highest degree of targeting accuracy, and in our experience, the CyberKnife has been able to maintain submillimeter accuracy consistently. It has been verified that our CyberKnife can correct for phantom shifts of up to 10.0 mm and rotations of up to 1.0°. It has also been discovered that false locks are more likely to occur with a single fiducial than with multiple fiducials. Although targeting accuracy can only be measured on a phantom, the insight gained from analyzing the QA tests can help us in devising better strategies for achieving the best treatment for our patients.
authors.affiliation: Bott Cancer Center, Holy Redeemer Hospital, Meadowbrook, Pennsylvania 19046 and Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey 08103.
authors.forename: Gopal
authors.initials: G
authors.lastname: Subedi
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey 08103.
authors.forename: Todd
authors.initials: T
authors.lastname: Karasick
authors.affiliation: Bott Cancer Center, Holy Redeemer Hospital, Meadowbrook, Pennsylvania 19046.
authors.forename: Jimm
authors.initials: J
authors.lastname: Grimm
authors.affiliation: Bott Cancer Center, Holy Redeemer Hospital, Meadowbrook, Pennsylvania 19046.
authors.forename: Sheena
authors.initials: S
authors.lastname: Jain
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey 08103.
authors.forename: Jinyu
authors.initials: J
authors.lastname: Xue
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey 08103.
authors.forename: Qianyi
authors.initials: Q
authors.lastname: Xu
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey 08103.
authors.forename: Yan
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey 08103.
authors.forename: Sucha
authors.initials: S
authors.lastname: Asbell
authors.affiliation: First Radiation Oncology Group, Jacksonville, Florida 32216.
authors.forename: Niraj
authors.initials: N
authors.lastname: Pahlajani
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey 08103.
authors.forename: Tamara
authors.initials: T
authors.lastname: LaCouture
mesh_terms: D000465:Algorithms; D011786:Quality Control; D012371:Robotics; D025321:Surgery, Computer-Assisted / Q000295:instrumentation / Q000379:methods*