Image-guided radiosurgery for spinal tumors: methods, accuracy and patient intrafraction motion.

Image-guided frameless extracranial radiosurgery has become an established treatment option; however, without a frame to restrict patient movements, intrafraction field mispositioning becomes more probable. The primary aim of this study is to determine the intrafraction motion of spinal radiosurgery patients. This aim was approached in two steps. First, a phantom study demonstrated that the system can detect movements accurately within 0.1 mm and rotational changes within 0.2 degrees. Second, patient positioning and monitoring were carried out for a group of 15 patients with 20 treatment sites. For the patient pool in the study, vertebral anatomy movement was observed to vary as much as 3 mm between sequential measurements and could occur in as little as 5 min. These results suggest a need for intrafraction patient monitoring and correctional shifts, even for patients whose overall treatment times are expected to be relatively short. Small relative rotations with standard deviations of less than 1.5 degrees were observed. The small relative rotational movements observed do not, alone, justify patient monitoring using the image-guidance system during the treatments of generally small radiosurgical targets.