title: Investigation of intracranial peripheral dose arising from the treatment of large lesions with Leksell GammaKnife Perfexion.
abstract: This investigation involves quantifying the extent of intracranial peripheral dose arising from simulated targets situated in the skull-base or upper-spine region using the Leksell GammaKnife Perfexion treatment unit. For each of three spherical target volumes--denoted as Vs (4 cm3), VM (18 cm3), and VL (60 cm3)--three treatment plans were manually generated, one for each of the three collimator sizes--4, 8, and 16 mm. Each of the plans was delivered to a spherical dosimetry phantom with an insert containing EBT Gafchromic film. The total dose at 70 mm from the targets' edges, %D(70 mm), was measured as a function of elevation angle and expressed as a percentage of the prescription dose. The film insert was placed centered in the median sagittal plane (Leksell X = 100) and %D(70 mm) was measured for the angular range from 0 degree (superior/along Z axis) to 90 degrees (anterior/along Y axis). For a given collimator i, the irradiation time ti to treat a spherical target of volume V using the 50% isodose line was observed to follow a power-law relationship of the form ti = Ai(V/ Vi)n where Ai was the maximum dose divided by collimator dose rate and Vi was the volume encompassed by the 50% isodose line for a single shot. The mean value of n was 0.61 (range: 0.61-0.62). Along the superior (Z) direction (angle=0 degree) and up to angles of around 30 degrees, the %D(70 mm) was always highest for the 4 mm plans, followed by the 8 mm, followed by the 16 mm. In this angular range, the maximum measured %D(70 mm) was 1.7% of the prescription dose. The intracranial peripheral dose along the superior direction (combined scatter and leakage dose) resulting from irradiation of upper-spine or base-of-skull lesions is measured to be less than 2% of the prescription dose, even for very large (60 cm3) targets. The results of this study indicate that, for a given target volume, treatment plans consisting of only 4 mm shots yield larger peripheral dose in the superior direction than 8 mm shot only plans, which in turn yield larger peripheral dose than 16 mm shot only plans.
authors.affiliation: Radiation Medicine Program, Princess Margaret Hospital, University Health Network, Toronto, Ontario M5G 2M9, Canada. mark.ruschin@rmp.uhn.on.ca
authors.forename: Mark
authors.initials: M
authors.lastname: Ruschin
authors.forename: Håkan
authors.initials: H
authors.lastname: Nordström
authors.forename: Per
authors.initials: P
authors.lastname: Kjäll
authors.forename: Young-Bin
authors.initials: YB
authors.lastname: Cho
authors.forename: David
authors.initials: D
authors.lastname: Jaffray
mesh_terms: D001822:Body Burden; D001932:Brain Neoplasms / Q000503:physiopathology* / Q000601:surgery*; D003198:Computer Simulation; D006801:Humans; D053685:Laser Therapy; D008954:Models, Biological; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012062:Relative Biological Effectiveness