PRECISION DOSIMETRY IN NARROW COLLIMATED RADIATION BEAMS OF THE LEKSELL GAMMA-KNIFE

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 i s individually collimated so that the beams converge precisely on a co mmon focal region at the centre, where the tumour is positioned. With the smallest collimation of 4 mm focus diameter the radiation field ha s a volume of less than 10 mm(3). In such a geometry precise determina tion of the absorbed dose rate (of the order of 3.5 Gy.min(-1)), essen tial for the result of tumour treatment, is difficult. Comparative mea surements are described with a miniaturised ionisation chamber of 125 mm(3), a calibrated semiconductor diode of 0.8 mm(3) and TLD-100 powde r 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 extrapolat ion between the different methods allows for precise determination of the absolute absorbed dose rate, even for the smallest focus, traceabl e to the primary standards via the ionisation chamber measurements. Us ing the semiconductor diode, accurate dose rate profiles can be establ ished. The method has been tested on two Gamma Knife installations at the Neurosurgical University Clinics at Graz and Vienna.