Quality assurance of an image guided intracranial stereotactic positioning system

This work reports on the development and testing of an intracranial stereotactic patient positioning system (ISPPS) for Tomotherapy. The ISPPS consists of the combination of a head frame, head frame couch interface (HCI), megavoltage CT (MVCT), and optical tracking camera system. Three quality assurance tests were designed to quantify the positioning system's ability to localize an intracranial target. The first two of these tests were designed to determine (a) the ability of the MVCT to detect a known shift applied to an anthropomorphic phantom and (b) the precision of fixing the phantom to the treatment couch via a head frame and specially designed head frame couch interface. A system verification test, using a phantom and EDR2 film, was used to determine the overall delivery precision through comparison of a measured dose distribution on film to calculated dose. The average net translational difference between a known shift applied to a phantom and that detected by MVCT image fusion was 0.62 mm. Setup reproducibility of the head frame was measured with both MVCT and optical tracking. The frame setup precision was found to be well within 1 mm for translations as well as rotations. A system delivery verification test in phantom using film showed spatial agreement between planned and delivered dose distributions to within 1 mm.