AbstractID: 9371 Title: Multi-Institutional Study of Gamma Knife Dosimetry
Delivering high doses to small treatment volumes requires accurate knowledge of the output and dose distributions of the particular
unit to be used for the treatment. The Leksell Gamma Knife® is an ideal choice for small treatment volumes, since the targets can be
as small as 4mm in diameter. A number of authors have published experimentally measured output factors of the four collimator
helmets for single Gamma Knife® units, but a consistent QA characterization has not been performed on a volume of different units to
determine if variations exist. The aim of this study is to experimentally measure the output factors and dose distributions for a number
of Gamma Knife® units to determine the level of consistency from one unit to another, and for a comparison to the manufacturer's
published values. The output factors and dose distributions were measured with Gafchromic® HS film (ISP), an Exradin A16
Microchamber, and continuous arrays of individually sorted TLDs. By having 361 TLD microcubes in a 1.9cm x 1.9cm square, a
dose distribution with resolution of one millimeter is obtained that can be closely compared to the film measurements. The results of
the dose distributions show non-uniformities in the radiation field within the 50% isodose lines for the two largest collimator helmets,
and the initial output factors were found to be as low as 0.786 ± 0.002 for the 4 mm helmet, a 9.7 % difference from the value
published by Elekta of 0.87.