Background: Radiosurgery is a focal brain irradiation technique that delivers, usually in a single session, high dose of ionizing radiation. The presence of lateral electronic disequilibrium and steep dose gradients in small fields demands special attention to the selection of a suitable detector with respect to its size, composition and response.

Materials and Methods : Small circular fields were produced by home-made collimators attached to a 9MV Neptun 10 PC linac ranging from 12.5mm to 25 mm at isocenter level in 2.5 mm
increment. Stereotactic beam data including percent depth dose, off axis ratio and output factor were measured using p-type silicon chip detector in a water phantom. Beam data were plotted for all available collimator sizes.

Results: Percent depth dose values at depth of 100 cm show 10 percent increase with enlargement of the field sizes from 12.5 mm to 25mm. Small overestimation of output factor has been observed using diode detector.

Conclusion: It is concluded for stereotactic radiosurgery with higher energy photon diode
detector could be a good and reasonable choice to measure percent depth dose and off axis ratio. Regarding the output factor, it is better to compare the results with those obtained by other
detectors. Iran . J. Radiat. Res., 2004; 2 (2): 79-84