title: Cone beam computed tomography image guidance system for a dedicated intracranial radiosurgery treatment unit.
abstract: OBJECTIVE
  Image guidance has improved the precision of fractionated radiation treatment delivery on linear accelerators. Precise radiation delivery is particularly critical when high doses are delivered to complex shapes with steep dose gradients near critical structures, as is the case for intracranial radiosurgery. To reduce potential geometric uncertainties, a cone beam computed tomography (CT) image guidance system was developed in-house to generate high-resolution images of the head at the time of treatment, using a dedicated radiosurgery unit. The performance and initial clinical use of this imaging system are described.
  
  
  METHODS
  A kilovoltage cone beam CT system was integrated with a Leksell Gamma Knife Perfexion radiosurgery unit. The X-ray tube and flat-panel detector are mounted on a translational arm, which is parked above the treatment unit when not in use. Upon descent, a rotational axis provides 210° of rotation for cone beam CT scans. Mechanical integrity of the system was evaluated over a 6-month period. Subsequent clinical commissioning included end-to-end testing of targeting performance and subjective image quality performance in phantoms. The system has been used to image 2 patients, 1 of whom received single-fraction radiosurgery and 1 who received 3 fractions, using a relocatable head frame.
  
  
  RESULTS
  Images of phantoms demonstrated soft tissue contrast visibility and submillimeter spatial resolution. A contrast difference of 35 HU was easily detected at a calibration dose of 1.2 cGy (center of head phantom). The shape of the mechanical flex vs scan angle was highly reproducible and exhibited <0.2 mm peak-to-peak variation. With a 0.5-mm voxel pitch, the maximum targeting error was 0.4 mm. Images of 2 patients were analyzed offline and submillimeter agreement was confirmed with conventional frame.
  
  
  CONCLUSIONS
  A cone beam CT image guidance system was successfully adapted to a radiosurgery unit. The system is capable of producing high-resolution images of bone and soft tissue. The system is in clinical use and provides excellent image guidance without invasive frames.
authors.affiliation: Department of Radiation Oncology, University of Toronto, Radiation Medicine Program, Princess Margaret Hospital, Toronto, Ontario, Canada. Mark.Ruschin@sunnybrook.ca
authors.forename: Mark
authors.initials: M
authors.lastname: Ruschin
authors.forename: Philip T
authors.initials: PT
authors.lastname: Komljenovic
authors.forename: Steve
authors.initials: S
authors.lastname: Ansell
authors.forename: Cynthia
authors.initials: C
authors.lastname: Ménard
authors.forename: Gregory
authors.initials: G
authors.lastname: Bootsma
authors.forename: Young-Bin
authors.initials: YB
authors.lastname: Cho
authors.forename: Caroline
authors.initials: C
authors.lastname: Chung
authors.forename: David
authors.initials: D
authors.lastname: Jaffray
mesh_terms: D059925:Anatomic Landmarks / Q000000981:diagnostic imaging; D001921:Brain / Q000000981:diagnostic imaging; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods*; D016371:Cranial Irradiation / Q000295:instrumentation / Q000379:methods*; D004867:Equipment Design; D006801:Humans; D019047:Phantoms, Imaging; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D061106:Radiotherapy Setup Errors; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation / Q000379:methods*; D012399:Rotation; D012886:Skull / Q000000981:diagnostic imaging