Frame-less and mask-less cranial stereotactic radiosurgery: a feasibility study

Currently, high-precision delivery in stereotactic radiosurgery (SRS) is achieved via high-precision target localization and rigid patient immobilization. Rigid patient immobilization can result in, however, patient discomfort, which is exacerbated by the long duration of SRS treatments and may induce patient movement. To address this issue, we developed a new SRS technique that is aimed to minimize patient discomfort while maintaining high-precision treatment, based on a less-rigid patient immobilization combined with continuous patient motion monitoring. In this paper, we examine the feasibility of this new technique. An anthropomorphic head phantom is used to check the accuracy of a 3D surface imaging system that provides the monitoring. Volunteers are used to study patient motion inside a new type of head mold that is used for minimal immobilization. Results show that for different couch angles, the difference between the phantom positions recorded by the surface imaging system and by an infrared optical tracking system was within 1 mm in displacements and 1 deg. in rotation. The motion detected by both systems during couch shifts is within 1 mm agreement. The average maximum volunteer head motion in the head mold during the 20 min interval in any direction was 0.7 mm (range: 0.4-1.1 mm). Patient motion due to couch motion was always less than 0.2 mm. We conclude that motion inside the minimally immobilizing head mold is small and can be accurately detected by real-time surface imaging.