Leksell Gamma Knife Instrument Model 4C was introduced to our hospital in Jan. 2009. The dose profile for the collimator is very important. However, it is very difficult to obtain the accurate dose profile for a very small irradiation field.

     We attempted the dose profile measurement using an ionization chamber with 0.125 cm3 volume.

      

     The ionization chamber was used PTW Model 31010. The dimensions of the chamber were 5.5-mm diameter by 6.5-mm in length with a sensitive volume of 0.125 cm3. The phantom was a polystyrene plastic sphere of 16-cm in diameter supplied by Elekta. The electrometer was used PTW Model UNIDOS E. The applied voltage for the ionization chamber was added -400 V. The phantom inserted to the ionization chamber at the center of the plastic sphere was mounted on a gamma knife treatment instrument by trunnions system. The dosimeter orientation was a sagittal(x-y) plane.

     The x-direction dose profiles for each collimator were measured by the ionization chamber with 0.125 cm3 volume. The standard dose profiles for each collimator by radiochromic films at gamma knife setup were used. All dose profiles were normalized at maximum dose rate. The ionization chamber dose profile corresponded closely with the radiochromic film’s one for 18-mm collimator. But there was a slight difference at the shoulder of dose profile. The shoulder profile of the ionization chamber was lower than the radocromic film one. However, the ionization chamber and radiochromic dose profiles for the other collimators were different.

     This is a retrospective study.

     For 18-mm collimator, the x-direction dose profile of the ionization chamber corresponded closely with the radiochromic film was observed.

     However, the dosimeter with the smaller sensitive volume is indispensable to measure the dose profile for the smaller irradiation filed.