Room: AAPM ePoster Library
Frameless Gamma Knife (GK) stereotactic radiosurgery (SRS) has becoming an effective supplement to the traditional frame-based method due to its non-invasive nature and allowance for fractionation, however, at a sacrifice of the immobilization accounting for intra-fraction motion. While the intra-fraction motion could be monitored during treatment with a tracking system, details of the motion were lost when only a single external surrogate was used. The purpose of this study was therefore to comprehensively evaluate the intra-fraction motion using cone-beam CTs (CBCTs).
Patients treated for frameless GK SRS with multiple CBCTs acquired during one fraction were retrospectively identified. As patients might be re-positioned due to treatment interruptions, the CBCTs, which were acquired while a patient remained in the mask and therefore contained intra-fraction motion information, were selected for analysis. The registration between every two consecutive CBCTs was determined based on the clinical accepted registrations. The translational and rotational shifts and the rotation axis of each registration were then calculated. Principal component analysis (PCA) was conducted to derive the most dominant intra-fraction motion patterns.
Among 492 CBCTs of 57 GK frameless SRS patients, 170 qualified CBCT pairs were identified. The median absolute translational shifts were 0.39, 0.66, and 0.38 mm in medial-lateral, anterior-posterior, and superior-inferior directions, respectively. The median absolute pitch, roll, and yaw angles were 0.06, 0.13, and 0.01 degree, respectively. PCA revealed that the first principal component of rotation axis was along (1.00, -0.03, 0.02) direction, indicating that most common intra-fraction rotation was the flexion and extension of the neck, while there was no significant motion pattern for the translational shift.
The propose method allows the assessment of infra-fraction motion pattern during frameless GK SRS. The most dominant motion pattern was found as the neck flexion and extension, which agreed with the clinical observation.