Spine Stereotactic Body Radiotherapy Utilizing Cone-Beam CT Image-Guidance With a Robotic Couch: Intrafraction Motion Analysis Accounting for all Six Degrees of Freedom

Purpose: To evaluate the residual setup error and intrafraction motion following kilovoltage cone-beam CT (CBCT) image guidance, for immobilized spine stereotactic body radiotherapy (SBRT) patients, with positioning corrected for in all six degrees of freedom. Methods and Materials: Analysis is based on 42 consecutive patients (48 thoracic and/or lumbar metastases) treated with a total of 106 fractions and 307 image registrations. Following initial setup, a CBCT was acquired for patient alignment and a pretreatment CBCT taken to verify shifts and determine the residual setup error, followed by a midtreatment and posttreatment CBCT image. For 13 single-fraction SBRT patients, two midtreatment CBCT images were obtained. Initially, a 1.5-mm and 1 degrees tolerance was used to reposition the patient following couch shifts which was subsequently reduced to 1 mm and 1 degrees degree after the first 10 patients. Results: Small positioning errors after the initial CBCT setup were observed, with 90% occurring within 1 mm and 97% within 1 degrees. In analyzing the impact of the time interval for verification imaging (10 +/- 3 min) and subsequent image acquisitions (17 +/- 4 min), the residual setup error was not significantly different (p > 0.05). A significant difference (p = 0.04) in the average three-dimensional intrafraction positional deviations favoring a more strict tolerance in translation (1 mm vs. 1.5 mm) was observed. The absolute intrafraction motion averaged over all patients and all directions along x, y, and z axis (+/- SD) were 0.7 +/- 0.5 mm and 0.5 +/- 0.4 mm for the 1.5 mm and 1 mm tolerance, respectively. Based on a 1-mm and 1 degrees correction threshold, the target was localized to within 1.2 mm and 0.9 degrees with 95% confidence. Conclusion: Near-rigid body immobilization, intrafraction CBCT imaging approximately every 15-20 min, and strict repositioning thresholds in six degrees of freedom yields minimal intrafraction motion allowing for safe spine SBRT delivery. (C) 2012 Elsevier Inc.