Stereotactic Radiosurgery involves the use of highly focused ionizing radiation beams to treat localized cancer tumours and lesions. Due to the damaging effects of radiation on healthy tissue, quality assurance checks must take place before treatment to ensure accurate delivery. This is of critical importance in cases like brain tumours, in which the healthy tissue at risk is in close proximity to the target volume. A dodecahedral radiosurgical phantom was designed and fabricated to measure the isocentre variation of a linear accelerator at an isocentric irradiation facility. It was shown that the phantom can localize individual treatment beams to within an uncertainty of 0.2mm. Due to the intrinsic accuracy of the phantom, it was found that careful phantom design and manufacturing as well as an accurate and complex characterization, in terms of measurements, positioning and computer modeling, must take place. This accurate characterization of the phantom is crucial to ensure the accurate treatment of stereotactic radiosurgery. This research is part of a larger project to further develop the phantom we have introduced in order to exploit a wider set of functions pertaining to maintaining accurate treatment delivery.