Abstract
The purpose of this study was to evaluate the stability of the Leksell Frame G in Gamma Knife radiosurgery (GKR). Forty patients undergoing GKR underwent pretreatment stereotactic MRI for GKR planning and stereotactic CT immediately after GKR. The stereotactic coordinates of four anatomical landmarks (cochlear apertures and the summits of the anterior post of the superior semicircular canals, bilaterally) were measured by two evaluators on two separate occasions in the pretreatment MRI and post‐treatment CT scans and the absolute distance between the observations is reported. The measurement method was validated with an independent group of patients who underwent both stereotactic MRI and CT imaging before treatment (negative controls; n: 5). Patients undergoing GKR for arteriovenous malformations (AVM) also underwent digital subtraction angiography (DSA), which could result in extra stresses on the frame. The distance between landmark localization in the scans for the negative control group (0.63 mm; 95% CI: 0.57–0.70; SD: 0.29) represents the overall consistency of the evaluation method and provides an estimate of the minimum displacement that could be detected by the study. Two patients in the study group had the fiducial indicator box accidentally misplaced at post‐treatment CT scanning. This simulated the scenario of a frame displacement, and these cases were used as positive controls to demonstrate that the evaluation method is capable of detecting a discrepancy between the MRI and CT scans, if there was one. The mean distance between the location of the landmarks in the pretreatment MRI and post‐treatment CT scans for the study group was 0.71 mm (95% CI: 0.68–0.74; SD:0.32), which was not statistically different from the overall uncertainty of the evaluation method observed in the negative control group (p=0.06). The subgroup of patients with AVM (n: 9), who also underwent DSA, showed a statistically significant difference between the location of the landmarks compared to subjects with no additional imaging: 0.78 mm (95% CI: 0.72–0.84) vs. 0.69 mm (95% CI: 0.66–0.72), p=0.016. This is however a minimal difference (0.1 mm) and the mean difference in landmark location for each AVM patient remained submillimeter. This study demonstrates submillimeter stability of the Leksell Frame G in GKR throughout the treatment procedure.PACS number(s): 87.53.‐j, 87.53.Ly, 87.56.Fc
Highlights
Gamma Knife radiosurgery (GKR) has traditionally relied on a rigid immobilization system to stereotactically converge multiple beams of ionizing radiation at a defined intracranial target
The second category refers to inaccurate definition of the target which results from erroneous imaging technique selection, inappropriate interpretation of the images, or geometrical inaccuracies of the scans due to distortion or other technical factors.[3]. This latter error has been well described and is usually assessed in standard QA procedures.[4]. The third source of error, which is the main scope of this study, is the possibility that the actual location of a target within the stereotactic system at treatment differs from its calculated location due to displacement of the reference frame between the imaging procedure and the treatment
The mean frame-on time defined as time difference between the beginning of the pre-GKR MRI and post-GKR CT scan in the study subjects was 157.8 min [89–298]
Summary
Gamma Knife radiosurgery (GKR) has traditionally relied on a rigid immobilization system to stereotactically converge multiple beams of ionizing radiation at a defined intracranial target. The second category refers to inaccurate definition of the target which results from erroneous imaging technique selection, inappropriate interpretation of the images, or geometrical inaccuracies of the scans due to distortion or other technical factors.[3] This latter error has been well described and is usually assessed in standard QA procedures.[4] The third source of error, which is the main scope of this study, is the possibility that the actual location of a target within the stereotactic system at treatment differs from its calculated location due to displacement of the reference frame between the imaging procedure and the treatment Such displacement could occur from instability due to inadequate frame placement or from stresses induced in the frame between imaging and treatment. Most studies investigating the stability of fixation systems are laboratory- and phantom-based and standard QA procedures are not capable of detecting potential frame displacement throughout the clinical procedure.[5]
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