title: Comparison of radiation dose spillage from the Gamma Knife Perfexion with that from volumetric modulated arc radiosurgery during treatment of multiple brain metastases in a single fraction.
abstract: OBJECTIVE
  The objective of this study was to examine radiation dose distributions created by 2 competing radiosurgery modalities for treating multiple brain metastases: single-isocenter volumetric modulated arc radiosurgery (VMAS) and Gamma Knife Perfexion (GKP). In addition, the effectiveness of multiple radiosurgery quality metrics was evaluated and compared between these advanced treatment modalities.
  
  
  METHODS
  Seven anonymized MRI data sets, each showing 2-5 metastases, were used to create plans on each system. The GammaPlan (version 10.1) program was used for planning of GKP. A neurosurgeon contoured the volumes to be treated, and no planning target volume expansion was used. A prescription dose coverage of ≥ 99% was achieved for each tumor volume. The Philips Pinnacle (version 9.2) program was used for planning of VMAS, using the SmartArc optimization algorithm for delivery on a Varian iX linear accelerator. Contours were transferred from GammaPlan, and again no planning target volume expansion was used. Between 2 and 5 arcs with table angles of 90°-270° were used. Again, a V100% of ≥ 99% was achieved for each tumor volume. After planning, the MRI scans, tumor volumes, and dose information from each plan were exported according to the Digital Imaging and Communications in Medicine standard to the VelocityAI program for analysis. Brain dose-volume histograms (DVHs) for normal brain tissues were generated, and the volume of these tissues receiving 20%-90% of the prescription dose was tabulated. Finally, the prescription isodose to tumor volume ratio (PITV; Shaw et al., 1993), conformity index (CI; Paddick, 2000), gradient index (GI, Paddick and Lippitz, 2006), and conformity/gradient index (CGI, Wagner et al. 2003) were calculated for each plan. Both the PITV and CI have ideal values of 1, while the GI and CGI have ideal values of lowest and highest achievable, respectively.
  
  
  RESULTS
  The DVHs consistently showed that with VMAS a higher amount of normal brain tissues received each dose level than with GKP. These increases were largest for lower isodose levels, with the volumes of normal brain that received 20%-50% and 60%-90% of the prescription dose showing average increases of 403% and 227%, respectively. Prescription isodose conformality showed only minor differences between the 2 modalities. Radiosurgery quality metrics including measures of the dose gradient (GI and CGI) indicated that the GKP plan was superior in each case, with respective average GI and CGI values of 3.04 and 57.75 for GKP and of 10.22 and 10.85 for VMAS. Metrics evaluating prescription isodose conformality alone differed only slightly between the modalities. Average respective PITV and CI values were 2.13 and 0.53 for GKP and 2.27 and 0.51 for VMAS.
  
  
  CONCLUSIONS
  Stereotactic radiosurgery plans for the treatment of multiple metastases with VMAS delivered significantly more dose to the normal brain tissues than plans for GKP. Radiosurgery quality metrics including a measure of the dose gradient are better suited to providing contrast between modern radiosurgery treatment platforms.
authors.affiliation: Departments of 1 Radiation Oncology and.
authors.forename: Daniel
authors.initials: D
authors.lastname: McDonald
authors.forename: John
authors.initials: J
authors.lastname: Schuler
authors.forename: Istvan
authors.initials: I
authors.lastname: Takacs
authors.forename: Jean
authors.initials: J
authors.lastname: Peng
authors.forename: Joseph
authors.initials: J
authors.lastname: Jenrette
authors.forename: Kenneth
authors.initials: K
authors.lastname: Vanek
mesh_terms: D000465:Algorithms; D001921:Brain / Q000473:pathology / Q000528:radiation effects / Q000601:surgery; D001932:Brain Neoplasms / Q000556:secondary* / Q000601:surgery*; D016208:Databases, Factual; D006801:Humans; D058996:Quality Improvement; D011829:Radiation Dosage; D016634:Radiosurgery / Q000379:methods* / Q000592:standards; D012720:Severity of Illness Index; D047368:Tumor Burden
keywords: CGI = conformity/gradient index; CI = conformity index; DVH = dose-volume histogram; GI = gradient index; GK4C = Gamma Knife model 4C; GKP = Gamma Knife Perfexion; Gamma Knife; MLC = multileaf collimator; PITV = prescription isodose to tumor volume ratio; Perfexion; VMAS = volumetric modulated arc radiosurgery; WBRT = whole-brain radiotherapy; brain metastases; conformality; quality metrics; stereotactic radiosurgery; volumetric modulated arc radiosurgery