Noninvasive patient fixation for extracranial stereotactic radiotherapy

Purpose: To evaluate the setup accuracy that can be achieved with a novel n oninvasive patient fixation technique based on a body cast attached to a re cently developed stereotactic body frame during fractionated extracranial s tereotactic radiotherapy. Methods and Materials: Thirty-one CT studies (greater than or equal to 20 s lices, thickness: 3 mm) from 5 patients who were immobilized in a body cast attached to a stereotactic body frame for treatment of paramedullary tumor s in the thoracic or lumbar spine were evaluated with respect to setup accu racy. The immobilization device consisted of a custom-made wrap-around body cast that extended from the neck to the thighs and a separate head mask, b oth made from Scotchcast. Each CT study was performed immediately before or after every second or third actual treatment fraction without repositionin g the patient between CT and treatment. The stereotactic localization syste m was mounted and the isocenter as initially located stereotactically was m arked with fiducials for each CT study. Deviation of the treated isocenter as compared to the planned position was measured in all three dimensions. Results: The immobilization device can be easily handled, attached to and r emoved from the stereotactic frame and thus enables treatment of multiple p atients with the same stereotactic frame each day. Mean patient movements o f 1.6 mm +/- 1.2 mm (laterolateral [LL]), 1.4 mm +/- 1.0 mm (anterior-poste rior [AP]), 2.3 mm +/- 1.3 mm (transversal vectorial error [VE]) and < slic e thickness = 3 mm (craniocaudal [CC]) were recorded for the targets in the thoracic spine and 1.4 mm +/- 1.0 mm (LL), 1.2 mm +/- 0.7 mm (AP), 1.8 mm +/- 1.2 mm (VE), and < 3 mm (CC) for the lumbar spine. The worst case devia tion was 3.9 mm for the first patient with the target in the thoracic spine tin the LL direction). Combining those numbers (mean transversal VE for bo th locations and maximum CC error of 3 mm), the mean three-dimensional vect orial patient movement and thus the mean overall accuracy can be safely est imated to be less than or equal to 3.6 mm. Conclusion: The presented combination of a body cast and head mask system i n a rigid stereotactic body frame ensures reliable noninvasive patient fixa tion for fractionated extracranial stereotactic radiotherapy and may enable dose escalation for less radioresponsive tumors that are near the spinal c ord or otherwise critically located while minimizing the risk of late seque lae. (C) 1999 Elsevier Science Inc.