Targeting accuracy at couch kick for a frameless image guided radiosurgery system

Purpose/Objectives: Targeting accuracy at all possible couch angles needs to be carefully evaluated prior to initiating a frameless image-guided stereotactic radiosurgery program on a Linac for treating functional disorders such as trigeminal neuralgia. In this study, we report positioning accuracy with stereoscopic x-ray imaging over the complete range of couch rotation using anthropomorphic head phantoms. Materials/Methods: An anthropomorphic head phantom with three 5 mm tungsten BBs as hidden targets was CT simulated. A group of 7 arcs was planned with couch angles from 0 to 90 degrees in 15 degrees increments. A pair of stereoscopic x-ray images that auto-matches to planning CT according to bony anatomy was utilized to position each BB to machine isocenter at all planned couch angles. Targeting accuracy was measured by stereoscopic x-ray imaging of the BB itself, which provides the distance from the centroid of BB to the x-ray imaging isocenter. For each BB, the hidden target test was repeated 5 times at couch 0 degrees and 3 times at other couch angles, resulting in a total of 69 measurements, each with random initial setup deviation. Following the same workflow, a second anthropomorphic head phantom with two 5 mm BBs was utilized to evaluate localization accuracy at couch angles of 0 through 270 degrees in 15 degrees increments, resulting in another 18 measurements. Results: Residual setup deviation following image guidance in the first head phantom was 0.6 +/- 0.1, 0.4 +/- 0.1, and 0.4 +/- 0.1 mm, respectively, at the three BBs, and 0.5 +/- 0.1 mm overall (N=69). Comparable results of 0.4 +/- 0.1 mm (N=18) were achieved with the second head phantom. Combining results from both phantoms, the targeting accuracy was 0.5 +/- 0.1 mm (range 0.2-0.8 mm). No apparent correlation was observed between targeting accuracy and couch rotation. Conclusions: Accurate positioning within 1 mm can be achieved with stereoscopic x-ray imaging at any couch angle. Frameless image-guided stereotactic radiosurgery could achieve targeting accuracy similar to that of frame-based systems for high dose treatment of trigeminal neuralgia.