The Leksell Gamma Knife is a stereotactic radiosurgical instrument for non-invasive cerebral surgery without opening the skull. It contains some 200 Co-60 Y radiation sources of, totally, 220 TBq. Each source is individually collimated so that the beams converge precisely on a common focal region at the centre, where the tumour is positioned. With the smallest collimation of 4 mm focus diameter the radiation field has a volume of less than 10 mm(3). In such a geometry precise determination of the absorbed dose rate (of the order of 3.5 Gy.min(-1)), essential for the result of tumour treatment, is difficult. Comparative measurements are described with a miniaturised ionisation chamber of 125 mm(3), a calibrated semiconductor diode of 0.8 mm(3) and TLD-100 powder in capsules of 21.1 and 28.7 mm(3) respectively, taken at different focal diameters of 4, 8, 14 and 18 mm. It can be shown that extrapolation between the different methods allows for precise determination of the absolute absorbed dose rate, even for the smallest focus, traceable to the primary standards via the ionisation chamber measurements. Using the semiconductor diode, accurate dose rate profiles can be established. The method has been tested on two Gamma Knife installations at the Neurosurgical University Clinics at Graz and Vienna.
