AbstractID: 8908 Title: Independent evaluation of the radiation parameters of each of the 201 Cobalt-60 sources installed in a Gamma Knife The Gamma Knife is a radiation therapy device used for stereotactic radiosurgery of intracranial lesions. Treatments typically involve several applications of radiation, called shots, each a combination of highly collimated beams emitted from 201 cobalt-60 sources focused to a central point. The intensity and size of each shot are modified by varying the time of exposure and by applying one of four differently sized collimator helmets. The resulting dose distribution is calculated by a planning system based on an algorithm that assumes each source contributes equally in terms of its output and beam profile to the total dose. The aim of this study is to evaluate the validity of this assumption and gain a perspective on the overall accuracy of the dose calculation. A 16 cm diameter tungsten carbide sphere with two preformed holes running perpendicularly through its central axes was built. For each measurement, the larger hole was precisely aligned with a single source. A pinpoint ion chamber placed in the smaller perpendicular hole enabled an evaluation of individual source output. Based on the design of the sphere and use of 200 extra helmet plugs, the contribution to the ion chamber reading from the 200 sources not under investigation was insignificant. For profile evaluation, several pieces of radiochromic film were exposed perpendicularly within the hole that was lined up with a source and subsequently analyzed. Both of these procedures were repeated for each source in turn. A full characterization of the radiation parameters of all sources will be presented.