title: Maintaining accuracy in stereotactic radiosurgery.
abstract: OBJECTIVE
  To provide the manufacture's specification for the base phantom of a commercially available stereotactic radiosurgery system so that its accuracy can be confirmed, and to describe a calibration device that allows the accuracy of the base phantom to be verified quickly and on a routine basis. Modifications to the target pointer system that make matching the pointer tips easier and less likely to damage the pointer tips are also described.
  
  
  METHODS
  In stereotactic radiosurgery, spatial accuracy is the key factor for successful dose delivery. With some commercially available systems, this accuracy depends on the accuracy of the base phantom coordinate system, how closely the tip of the target pointer can be matched to the tip of the base phantom pointer, and how accurately the coordinates set on the isocentric subsystem match those set on the base phantom. Two major problems, usually overlooked when evaluating system accuracy are, first, the base phantom, which establishes the stereotactic coordinate system, is assumed to be completely accurate. This is a dangerous assumption because the base phantom is used frequently for routine patient treatments and for standard quality assurance tests. To exacerbate the problem, no independent device is provided with stereotactic systems to check the accuracy of the base phantom. Second, the accuracy of the isocenter coordinates set on the head support stand depends upon how closely the target pointer and the base phantom pointer can be aligned. The hardware provided with the system is difficult to use and easily leads to damage of the pointer tips.
  
  
  RESULTS
  In this work, we provide the manufacturer's specifications for a popular stereotactic system, describe a device that can be used to check quickly and easily the accuracy of the base phantom, and describe a modification to the transfer pointer system that allows the pointer tips to be more easily aligned with reduced possibility of damage to the pointer tips.
  
  
  CONCLUSIONS
  The methods and apparatus described in this paper should be useful to anyone using a base phantom for testing radiosurgery accuracy.
authors.affiliation: Department of Therapeutic Radiology-Radiation Oncology, University of Minnesota Hospital and Clinics, Minneapolis 55455, USA.
authors.forename: B J
authors.initials: BJ
authors.lastname: Gerbi
authors.forename: D M
authors.initials: DM
authors.lastname: Roback
authors.forename: S D
authors.initials: SD
authors.lastname: Humphrey
authors.forename: W A
authors.initials: WA
authors.lastname: Hall
mesh_terms: D004867:Equipment Design / Q000592:standards; D011785:Quality Assurance, Health Care; D016634:Radiosurgery / Q000295:instrumentation* / Q000592:standards