Target localization accuracy in frame-based stereotactic radiosurgery: Comparison between MR-only and MR/CT co-registration approaches

Abstract

Purpose: In frame-based Gamma Knife (GK) stereotactic radiosurgery two treatment planning workflows are commonly employed; one based solely on magnetic resonance (MR) images and the other based on magnetic resonance/computed tomography (MR/CT) co-registered images. In both workflows, target localization accuracy (TLA) can be deteriorated due to MR-related geometric distortions and/or MR/CT co-registration uncertainties. In this study, the overall TLA following both clinical workflows is evaluated for cases of multiple brain metastases.

Methods: A polymer gel-filled head phantom, having the Leksell stereotactic headframe attached, was CT-imaged and irradiated by a GK Perfexion unit. A total of 26 4-mm shots were delivered at 26 locations directly defined in the Leksell stereotactic space (LSS), inducing adequate contrast in corresponding T2-weighted (T2w) MR images. Prescribed shot coordinates served as reference locations. An additional MR scan was acquired to implement the "mean image" distortion correction technique. The TLA for each workflow was assessed by comparing the radiation-induced target locations, identified in MR images, with corresponding reference locations. Using T1w MR and CT images of 15 patients (totaling 81 lesions), TLA in clinical cases was similarly assessed, considering MR-corrected data as reference. For the MR/CT workflow, both global and region of interest (ROI)-based MR/CT registration approaches were studied.

Results: In phantom measurements, the MR-corrected workflow demonstrated unsurpassed TLA (median offset of 0.2 mm) which deteriorated for MR-only and MR/CT workflows (median offsets of 0.8 and 0.6 mm, respectively). In real-patient cases, the MR-only workflow resulted in offsets that exhibit a significant positive correlation with the distance from the MR isocenter, reaching 1.1 mm (median 0.6 mm). Comparable results were obtained for the MR/CT-global workflow, although a maximum offset of 1.4 mm was detected. TLA was improved with the MR/CT-ROI workflow resulting in median/maximum offsets of 0.4 mm/1.1 mm.

Conclusions: Subpixel TLA is achievable in all workflows. For the MR/CT workflow, a ROI-based MR/CT co-registration approach could considerably increase TLA and should be preferred instead of a global registration.

Keywords: Gamma Knife; SRS; distortion; frame; registration; spatial accuracy; stereotactic space.

FIGURE 1

Screenshots of the GammaPlan treatment planning system depicting a transversal magnetic resonance (MR) slice of the irradiated phantom intersecting five Gamma Knife (GK) shots, after performing the spatial co‐registration to the Leksell stereotactic space (LSS), following the (A,B) MR‐only, (C,D) MR/CT, and (E,F) MR‐corrected workflows. To increase visibility, the central GK shot is enlarged in the bottom row panels (B,D,F). Legend: Green solid line: the 14‐Gy isodose for each GK shot delivered (corresponding centroids serving as reference points); light blue, dark blue, and red lines: contours of the automatically generated target structures, defined using the radiation‐induced polymerization areas, for the assessment of target localization accuracy, following the MR‐only, MC/CT, and MR‐corrected workflows, respectively. All data shown are registered to the LSS coordinate system

FIGURE 2

(A,B,C) An indicative brain lesion, contoured on the contrast‐enhanced T1‐weighted magnetic resonance (T1w MR) images and localized in the Leksell stereotactic space (LSS), following all four frame‐based workflows. Transformed structures are shown in 3D, in pairs of two for visibility and comparison purposes. (D) The position of the centroid corresponding to each workflow, localized in the LSS coordinate system and corresponding radial distances $d=\sqrt{\mathrm{\Delta }{x}^2+\mathrm{\Delta }{y}^2+\mathrm{\Delta }{z}^2}$. x‐, y‐, z‐axes correspond to the normal axes of the LSS

FIGURE 3

Box‐whisker plots derived from the patient study results, implementing the (A) magnetic resonance (MR) only, (B) magnetic resonance/computed tomography (MR/CT) global, and (C) MR/CT‐region of interest (ROI) workflows. Red lines indicate median detected offsets on each axis, whereas blue boxes range from the first to third quartile. Whiskers depict the remaining data or extend up to 1.5 times the interquartile range in either direction. In each dataset, remaining outliers (if any) are shown by the red marks. Legend: x, y, z correspond to the normal axes of the Leksell stereotactic space (LSS); x, left‐right direction; y, posterior‐anterior direction; z, inferior‐superior direction; R, the radial distance between reference and evaluated points, i.e., $R=\sqrt{\mathrm{\Delta }{x}^2+\mathrm{\Delta }{y}^2+\mathrm{\Delta }{z}^2}$

FIGURE 4

Detected radial offset between reference and evaluated centroid locations in the Leksell stereotactic space (LSS), plotted against distance to the magnetic resonance (MR) isocenter, for each lesion. (A) MR‐only workflow, (B) magnetic resonance/computed tomography (MR/CT)‐global workflow, (C) MR/CT‐region of interest (ROI) workflow. Using the Spearman's correlation coefficient, a statistically significant positive correlation was revealed only for the MR‐only workflow (see Section 3.2)