[Measurements of Gamma-Knife helmet output factors using a radiophotoluminescent glass rod dosimeter and a diode detector].

A radiophotoluminescent (RPL) glass rod dosimeter (GRD) and a small active volume p-type silicon diode detector (stereotactic field detector, SFD) were used for the measurement of Gamma-Knife output factors. All measurements were done using a 16 cm diameter spherical polystyrene phantom with the detector at the focal spot of Gamma-Knife. The GRD system consists of small rod-shaped glass chip detectors and an automatic readout device. The output factors measured with GRD of the 14, 8 and 4 mm helmets relative to the 18 mm helmet are 0.981, 0.942 and 0.877, respectively. Similarly, the corresponding output factors measured with SFD are 0.980, 0.949 and 0.867, respectively. These output factors are comparable with the values given in a recent publication and the values recommended by Elekta, the manufacture. The angular dependence of these detectors is also measured using a linear accelerator-based stereotactic radiosurgery system. For the Gamma-Knife angle ranging from 6 to 36 degrees from the vertical axis, the measured angular dependence of the GRD is approximately 1.0% at a 4 MV x-ray beam. The response of SFD indicates approximately 3-4% directional dependence for the same angle range for a 6 MV x-ray beam. The Gamma-Knife helmet output factors measured with SFD are corrected for angular dependence. In summary, GRD can be a good candidate in measuring small field output factor. Due to an angular dependence the small p-type diode detector needs care when calculating the Gamma-Knife dose.