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 cranial 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 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.