Frameless immobilization system with roll correction for stereotactic radiosurgery of intracranial brain metastases

The objectives of this study were to develop a frameless immobilization system that allows roll rotation corrections and to investigate the performance of this system for stereotactic radiosurgery (SRS) treatment. We designed the support frame of a frameless immobilization system based on the commercial Brainlab immobilization system. The support frame consisted of a fixed component and a rotating component. With rack and pinion gears and guide holes installed in the system, the rotating component was configured to be rotated along the longitudinal axis of the patient with respect to the fixed component. To evaluate the performance of the system, the six degree-of-freedom (6D) positioning corrections (translational and rotational corrections) were assessed by image verification between planning computed tomography (CT) and cone-beam computed tomography (CBCT) images. The commercial immobilization system was evaluated in the same manner for comparison. The mean translational shifts for the commercial system were 0.68 ± 0.19 mm, 0.73 ± 0.24 mm and 0.78 ± 0.19 mm, while those for the developed system were 0.44 ± 0.31 mm, 0.43 ± 0.25 mm and 0.60 ± 0.14 mm in the lateral, longitudinal and vertical directions, respectively. The mean rotational shifts for the commercial system were 0.37° ± 0.12°, 0.32° ± 0.16° and 0.38° ± 0.14°, while those for the developed system were 0.04° ± 0.04°, 0.11° ± 0.06° and 0.15° ± 0.12° along the lateral, vertical and longitudinal axes of the patient, respectively. For institutions that do not have 6D robotic couches installed, the use of the developed immobilization system can provide 6D corrections, resulting in shorter treatment times and higher patient positioning accuracy.