title: Does dose rate affect efficacy? The outcomes of 256 gamma knife surgery procedures for trigeminal neuralgia and other types of facial pain as they relate to the half-life of cobalt.
abstract: OBJECTIVE
  Gamma Knife surgery (GKS) is a treatment option for patients with refractory typical trigeminal neuralgia (TN), TN with atypical features, and atypical types of facial pain. The Gamma Knife's 201 60Co sources decay with a half-life of 5.26 years. The authors examined whether the decrease in dose rate over 4.6 years between Co source replacements affected the control rates of facial pain in patients undergoing GKS.
  
  
  METHODS
  The authors collected complete follow-up data on 239 of 326 GKS procedures performed in patients with facial pain. Patients were classified by their type of pain. The isocenter of a 4-mm collimator helmet was targeted at the proximal trigeminal nerve root, and the dose (80-90 Gy) was prescribed at the 100% isodose line. Patients reported the amount of pain control following radiosurgery by answering a standardized questionnaire. Eighty percent of patients experienced greater than 50% pain relief, and 56% of patients experienced complete pain relief after GKS. Neither dose rate nor treatment time was significantly associated with either the control rate or degree of pain relief. A significant association between the type of facial pain and the pain control rate after GKS was observed (p < 0.001; Pearson chi-square test). In their statistical analysis, the authors accounted for changes in prescription dose over time to prevent the dose rate from being a confounding variable. There was no observable effect of the dose rate or of the treatment duration within the typical period to source replacement.
  
  
  CONCLUSIONS
  Patients with facial pain appear to receive consistent treatment with GKS at any time during the first half-life of the Co sources.
authors.affiliation: Department of Neurosurgery, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157-1030, USA.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Balamucki
authors.forename: Volker W
authors.initials: VW
authors.lastname: Stieber
authors.forename: Thomas L
authors.initials: TL
authors.lastname: Ellis
authors.forename: Stephen B
authors.initials: SB
authors.lastname: Tatter
authors.forename: Allan F
authors.initials: AF
authors.lastname: Deguzman
authors.forename: Kevin P
authors.initials: KP
authors.lastname: McMullen
authors.forename: James
authors.initials: J
authors.lastname: Lovato
authors.forename: Edward G
authors.initials: EG
authors.lastname: Shaw
authors.forename: Kenneth E
authors.initials: KE
authors.lastname: Ekstrand
authors.forename: J Daniel
authors.initials: JD
authors.lastname: Bourland
authors.forename: Michael T
authors.initials: MT
authors.lastname: Munley
authors.forename: Michael
authors.initials: M
authors.lastname: Robbins
authors.forename: Charles
authors.initials: C
authors.lastname: Branch
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003037:Cobalt Radioisotopes; D004307:Dose-Response Relationship, Radiation; D005157:Facial Pain / Q000601:surgery*; D005260:Female; D005500:Follow-Up Studies; D006207:Half-Life; D006801:Humans; D008297:Male; D008875:Middle Aged; D010147:Pain Measurement; D011851:Radioactivity; D016634:Radiosurgery; D016896:Treatment Outcome; D014277:Trigeminal Neuralgia / Q000601:surgery*
chemical_list: D003037:Cobalt Radioisotopes