INTER- AND INTRAFRACTION PATIENT POSITIONING UNCERTAINTIES FOR INTRACRANIAL RADIOTHERAPY: A STUDY OF FOUR FRAMELESS, THERMOPLASTIC MASK-BASED IMMOBILIZATION STRATEGIES USING DAILY CONE-BEAM CT

Purpose: To determine whether frameless thermoplastic mask-based immobilization is adequate for image-guided cradinal radiosurgery. Methods and Materials: Cone-beam CT localization data from patients with intracranial tumors were studied using daily pre- and posttreatment scans. The systems studied were (1) Type-S IMRT (head only) mask (Civco) with head cushion; (2) Uni-Frame mask (Civco) with head cushion, coupled with a BlueBag body immobilizer (Medical Intelligence); (3) Type-S head and shoulder mask with head and shoulder cushion (Civco); (4) same as previous, coupled with a mouthpiece. The comparative metrics were translational shift magnitude and average rotation angle; systematic inter-, random inter-, and random intrafraction positioning error was computed. For strategies 1-4, respectively, the analysis for interfraction variability included data from 20, 9, 81, and 11 patients, whereas that for intrafraction variability included a subset of 7, 9, 16, and 8 patients. The results were compared for statistical significance using an analysis of variance test. Results: Immobilization system 4 provided the best overall accuracy and stability. The mean interfraction translational I shifts (+/- SD) were 2.3 (+/- 1.4), 2.2 (+/- 1.1), 2.7 (+/- 1.5), and 2.1 (+/- 1.0) mm whereas intrafraction motion was 1.1 (+/- 1.2), 1.1 ( 1.1), 0.7 (+/- 0.9), and 0.7 (+/- 0.8) mm for devices 1-4, respectively. No significant correlation between intrafraction motion and treatment time was evident, although intrafraction motion was not purely random. Conclusions: We find that all frameless thermoplastic mask systems studied are viable solutions for image-guided intracranial radiosurgery. With daily pretreatment corrections, symmetric PTV margins of 1 mm would likely be adequate if ideal radiation planning and targeting systems were available. (C) 2011 Elsevier Inc.