Characterization and Dosimetric Effects of Intra-fraction Motion During Cranial Radiosurgery

Abstract

The purpose of this study is to retrospectively review intra-fraction patient motion during image-guided cranial radiosurgery and evaluate the effects of the intra-fraction motion on the delivered dose for the subset of patients with motion greater than one millimeter. Patient alignment data were collected for all consecutive single and five fraction radiosurgery treatments at our institution during an approximately one-year period (n=1016 radiographs). These data, comprised of three-dimensional coordinates were recorded at initial alignment, at intervals during the treatment, and at treatment completion. The differences between the initial alignment and the subsequent radiographs represent patient motion. In cases with motion greater than one millimeter in any orthogonal direction, an automated script was used to simulate delivered treatment, based on the prescribed treatment plan and a shift in patient position based on the measured patient motion. The delivered treatment plans were then analyzed based on their conformity (ratio of volume at prescription dose over total planned target volume), heterogeneity (max dose over prescription dose), and gradient (volume at 50% prescription dose over volume at prescription dose) indices. 73 patients received 270 1-5 fraction radiosurgery treatments. 3D magnitude error between initial alignment and subsequent films was 0.4 mm ± 0.3 mm. 2.5% of shifts were greater than 1 mm, and 0.2% greater than 2 mm. There were 8 treatment fractions with errors greater than 1 mm in at least one direction, some with multiple radiographs greater than 1 mm. Among these 8 treatments, the median motion magnitude was 1.27 ± 0.31 mm. For seven out of the eight cases, intra-fraction motion changed conformality index by less than 5%. In one case 75% of the delivered treatment was altered by intra-fraction motion with a 3D magnitude value of 1.86 mm. This motion resulted in a change in the planned conformity, heterogeneity, and gradient indices from 3.13, 1.18, and 5.83, respectively, to 0.24, 1.04, and 71.01. On clinical follow-up, this patient had a partial response. In the other 7 cases, there was one case of radiation necrosis with the balance achieving partial or complete responses. Clinically relevant intra-fraction motion during image-guided cranial radiosurgery was evaluated for all consecutive patients during an approximately one-year period. 2.5% of intrafraction shifts were greater than 1 mm, and 0.2% greater than 2 mm. Shifts greater than 1 mm in a given direction were analyzed, and found to have limited impact on the dose distribution and clinical followup.