Different techniques of small field stereotaxic external beam radiotherapy, also known as stereotactic radiosurgery, or intracranial lesions will be compared by means of dose distributions calculated using a CT-based, 3- dimensional treatment planning computer program. Radiosurgical treatment of intracranial tumors and vascular malformations has proven to be a very successful means of curing life-threatening and debilitating lesions in thousands of patients. These results have been achieved by a small number of radiosurgical modalities with specific treatment protocols. Recently, a number of new radiosurgical modalities have been implemented or proposed, and the use of these techniques is spreading rapidly. In order to evaluate different radiosurgical modalities, a systematic comparison of dose distributions will be performed, and the results evaluated by means of dose-volume histograms. Radiation types that will be studied are charged particles (protons, helium ions, carbon ions, and neon ions) and photons (Co 60 gamma-rays and megavoltage photons from linear accelerators). Irradiation geometries (the number, orientation, and weighting of beamports) that will be studied are the currently-used charged particle methods at Lawrence Berkeley Laboratory (LBL) and the Harvard Cyclotron, the Co 60 Gamma Knife, and a number of Linac-based rotational arc methods, including the methods used in Heidelberg, Boston, and Montreal. The dose to the target volume and to regions of normal tissue will be compared. The effects of the target size, shape, and location on the patterns of dose distribution will be studied in order to determine classes of lesions best treated by particular radiosurgical methods. Use will be made of clinical data at LBL in order to help determine dose-volume relationships of clinical effects.