title: Dosimetric and clinical analysis of spatial distribution of the radiation dose in gamma knife radiosurgery for vestibular schwannoma.
abstract: OBJECTIVE
  We investigated variations in the distribution of radiation dose inside (dose inhomogeneity) and outside (dose falloff) the target volume during Gamma Knife (GK) irradiation of vestibular schwannoma (VS). We analyzed the relationship between some parameters of dose distribution and the clinical and radiological outcome of patients.
  
  
  METHODS
  Data from dose plans of 203 patients treated for a vestibular schwannoma by GK C using same prescription dose (12 Gy at the 50% isodose) were collected. Four different dosimetric indexes were defined and calculated retrospectively in all plannings on the basis of dose-volume histograms: Paddick conformity index (PI), gradient index (GI), homogeneity index (HI), and unit isocenter (UI). The different measures related to distribution of the radiation dose were compared with hearing and tumor outcome of 203 patients with clinical and radiological follow-up of minimum 2 years.
  
  
  RESULTS
  Mean, median, SD, and ranges of the four indexes of dose distribution analyzed were calculated; large variations were found between dose plans. We found a high correlation between the target volume and PI, GI, and UI. No significant association was found between the indexes of dose distribution calculated in this study and tumor control, tumor volume shrinkage, hearing worsening, loss of functional hearing, or complete hearing loss at last follow-up.
  
  
  CONCLUSIONS
  Parameters of distribution of the radiation dose during GK radiosurgery for VS can be highly variable between dose plans. The tumor and hearing outcome of patients treated is not significantly related to these global indexes of dose distribution inside and around target volume. In GK radiosurgery for VS, the outcome seems more to be influenced by local radiation dose delivered to specific structures or volumes than by global dose gradients.
authors.affiliation: Gamma Knife Center, Hospital Erasme, Brussels, Belgium. nmassage@ulb.ac.be
authors.forename: Nicolas
authors.initials: N
authors.lastname: Massager
authors.forename: Sarah
authors.initials: S
authors.lastname: Lonneville
authors.forename: Carine
authors.initials: C
authors.lastname: Delbrouck
authors.forename: Nadir
authors.initials: N
authors.lastname: Benmebarek
authors.forename: Françoise
authors.initials: F
authors.lastname: Desmedt
authors.forename: Daniel
authors.initials: D
authors.lastname: Devriendt
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D005260:Female; D006309:Hearing / Q000528:radiation effects; D034381:Hearing Loss / Q000209:etiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D009464:Neuroma, Acoustic / Q000473:pathology / Q000601:surgery*; D011829:Radiation Dosage; D016634:Radiosurgery / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods*; D012542:Scattering, Radiation; D018709:Statistics, Nonparametric; D047368:Tumor Burden / Q000528:radiation effects; D055815:Young Adult