Dose Profile Measurements Of Composite Gamma-Knife Dose Distributions - Preliminary Results

Dose distributions for Leksell Gamma-Knife are small and of nearly spherical shape. They result from cross-firing of 201 individual small photon beams. In the radiosurgery, usage of radiation beams is essential dosimetry task for measurement and evaluation of dose profiles of clinical photon beams. Since the dosimetry of individual photon beams in Gamma - Knife radiosurgery is not possible, due to construction of Leksell Gamma - Knife Unit, only composite dose distributions could be measured and evaluated. In this work a simple method for high resolution dose profile measurements of composite Gamma-Knife dose distribution was proposed and the preliminary results of measurements were presented. For that purpose, dose profiles were measured in referent spherical phantom with three types of detectors (PTW Semiflex and PinPoint ionization chambers and p - type silicon detector PTW Diode P) for 18 mm, 14 mm, 8 mm and 4 mm helmet collimator. Profiles were scanned in lateral - lateral (LL) and superior-inferior directions (SI) with the resolution of 0.2 mm. The volumetric averaging effect of detectors response was observed especially for small collimators (8 mm and 4 mm helmet). The size of profiles (full width at half maximum, FWHM) and penumbra values (distance between 20 percent and 80 percent isodose) were compared to referent published values and the values simulated with Gamma-Knife treatment planning system (GK TPS). A possible deconvolution for correction of volumetric averaging effect was suggested. (author).