Radiosurgery aims to deliver a high radiation dose to a small target volume while sparing surrounding healthy tissues. However, since the target volume is often large and irregularly-shaped, a significant amount of healthy tissue is irradiated. To improve conformity of the dose volume to the target volume, we propose to optimize the field shape by trimming the field described by the radiosurgery cone with the accelerator jaws for a given arc. We have measured output factors (OF), tissue-maximum ratios (TMR), off-axis ratios (OAR) and penumbrae for 40, 32.5 and 24 mm cone fields trimmed by the lower (i.e., X jaws) and /or upper (i.e., Y jaws) collimator jaws. The smallest field was 8 mm large, and length was limited by the cone size. The average penumbra due to the cone field is 2.8 mm, and 4.1 and 6.1 mm for those due to the X and Y jaws, respectively. Moreover, the penumbrae due to the X and Y jaws are independent of jaw position within the radiosurgical field. Because of the large penumbra involved with the Y jaws, radiosurgical fields should be trimmed by the X1 and/or X2 jaws only. The measured OF's have been fitted with a hyperbolic function. All of the fitted OF's fall within +/- 0.5% of the measured OF's. The TMR values obtained with trimmed fields do not change much, except for the smallest fields (up to 10% at a depth of 20 cm). Therefore, using trimmed radiosurgical fields requires straightforward dosimetric changes and provides a level of beam shaping for large cone fields (> 20 mm in diameter) without introducing additional hardware.