title: Technical Note: Dose gradients and prescription isodose in orthovoltage stereotactic radiosurgery.
abstract: OBJECTIVE
  The purpose of this work is to examine the trade-off between prescription isodose and dose gradients in orthovoltage stereotactic radiosurgery.
  
  
  METHODS
  Point energy deposition kernels (EDKs) describing photon and electron transport were calculated using Monte Carlo methods. EDKs were generated from 10  to 250 keV, in 10 keV increments. The EDKs were converted to pencil beam kernels and used to calculate dose profiles through isocenter from a 4π isotropic delivery from all angles of circularly collimated beams. Monoenergetic beams and an orthovoltage polyenergetic spectrum were analyzed. The dose gradient index (DGI) is the ratio of the 50% prescription isodose volume to the 100% prescription isodose volume and represents a metric by which dose gradients in stereotactic radiosurgery (SRS) may be evaluated.
  
  
  RESULTS
  Using the 4π dose profiles calculated using pencil beam kernels, the relationship between DGI and prescription isodose was examined for circular cones ranging from 4 to 18 mm in diameter and monoenergetic photon beams with energies ranging from 20 to 250 keV. Values were found to exist for prescription isodose that optimize DGI.
  
  
  CONCLUSIONS
  The relationship between DGI and prescription isodose was found to be dependent on both field size and energy. Examining this trade-off is an important consideration for designing optimal SRS systems.
authors.affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wisconsin 53705.
authors.forename: Jessica M
authors.initials: JM
authors.lastname: Fagerstrom
authors.affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wisconsin 53705.
authors.forename: Edward T
authors.initials: ET
authors.lastname: Bender
authors.affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wisconsin 53705.
authors.forename: Wesley S
authors.initials: WS
authors.lastname: Culberson
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D009010:Monte Carlo Method; D017785:Photons; D011874:Radiometry / Q000379:methods; D016634:Radiosurgery / Q000379:methods; D011879:Radiotherapy Dosage