Residual Setup Errors in Cranial Stereotactic Radiosurgery without 6DoF Robotic Couch

Abstract

This IRB-approved study was to evaluate the residual interfractional setup errors and intrafractional motion of patients treated with cranial stereotactic radiosurgery without 6 degree of freedom (DoF) couch. We compared a frameless non-invasive vacuum-suction immobilization to a rigid immobilization. Twenty consecutive patients treated by medical linear accelerator or tomotherapy were selected for data collection. The dose and number of fractions received by each patient ranged from 18Gy in 1 fraction (SRS) to 25Gy in 5 fractions (SRT). 12 patients were immobilized using PinPoint, a frameless suction system (Aktina Medical, New York) and 8 patients were immobilized using the Talon rigid screw system. Customized head cushions were used in all patients. 6 out of 12 Atkina patients received pre and post treatment cone beam CT (CBCT) to evaluate the intrafractional motion of the Aktina system. The intrafractional motion with the Talon rigid screw system was previously reported to be negligible thus not repeated in this study. All patients received pre treatment CBCT or megavoltage CT (MVCT) to assess interfractional setup accuracy. Shifts to the final treatment position were determined based on matching bony anatomy in the pre-treatment setup CT and the planning CT. Residual rotational and translational errors were analyzed retrospectively using image registration software. For frameless Aktina system, mean and standard deviation of the intrafractional motions were -0.5±0.7mm (lateral), 0.1±0.9mm (vertical), -0.5±0.6mm (longitudinal), -0.04±0.18°(pitch), -0.1±0.23°(yaw), and -0.03±0.17°(roll) indicating negligible intrafractional motion. Interfractional rotation errors were 0.20±0.69° (pitch), 0.34±0.56° (yaw), 0.35±0.82° (roll) for frameless vacuum-suction immobilization versus -0.10±0.25° (pitch), -0.08±0.16° (yaw), -0.20±0.41° (roll) for Talon rigid screw immobilization showing rigid immobilization setup was more reproducible than frameless immobilization. Without rotational correction by 6DoF couch, residual setup error exists and increases from the image registration center. In a 3D vector space, a tumor located 5cm from the center of image fusion would require a 0.9 mm margin with the Talon system and a 2.1 mm margin with Aktina. With image-guided radiotherapy, translational setup errors can be corrected by image registration between pre-treatment setup CT and planning CT. However, without 6 DoF couch, rotational errors cannot be eliminated. Our study showed that the frameless Aktina immobilization system had negligible intrafractional motion. The interfractional rotation setup error using Aktina was larger than rigid immobilization with the Talon system. For a single lesion away from the center of image registration or for multiple lesions far apart, additional margin may be needed to account for the un-correctable rotational setup errors without 6 DoF couch.