Accuracy and stability of positioning in radiosurgery: Long term results of the Gamma Knife system

Abstract

The primary aim of this investigation was to determine the long term overall accuracy of an irradiation position of Gamma Knife systems. The mechanical accuracy of the system as well as the overall accuracy of an irradiation position was examined by irradiating radiosensitive films. To measure the mechanical accuracy, the GafChromic film was fixed by a special tool at the unit center point (UCP). For overall accuracy the film was mounted inside a phantom at a target position given by a two-dimensional cross. Its position was determined by CT or MRI scans, a treatment was planned to hit this target by use of the standard planning software and the radiation was finally delivered. This procedure is named "system test" according to DIN 6875-1 and is equivalent to a treatment simulation. The used GafChromic films were evaluated by high resolution densitometric measurements. The Munich Gamma Knife UCP coincided within x; y; z: -0.014 +/- 0.09 mm; 0.013 +/- 0.09 mm; -0.002 +/- 0.06 mm (mean +/- SD) to the center of dose distribution. There was no trend in the measured data observed over more than ten years. All measured data were within a sphere of 0.2 mm radius. When basing the target definition in the system test on MRI scans, we obtained an overall accuracy of an irradiation position in the x direction of 0.21 +/- 0.32 mm and in the y direction 0.15 +/- 0.26 mm (mean +/- SD). When a CT-based target definition was used, we measured distances in x direction 0.06 +/- 0.09 mm and in y direction 0.04 +/- 0.09 mm (mean +/- SD), respectively. These results were compared with those obtained with a Gamma Knife equipped with an automatic positioning system (APS) by use of a different phantom. This phantom was found to be slightly less accurate due to its mechanical construction and the soft fixation into the frame. The phantom related position deviation was found to be about +/- 0.2 mm, and therefore the measured accuracy of the APS Gamma Knife was evidently less precise by additional +/- 0.2 mm. These measurements demonstrate that an irradiation position defined by a CT scan can be hit within the intrinsic system precision. In radiosurgery with the Gamma Knife, a fixation with the Leksell stereotactic frame is applied. As this frame is considered to add no further uncertainties due to patient movements, the measured accuracy applies to a real patient treatment situation. The major contribution to the overall accuracy of an irradiation position is given by the MRI scans.

Effect of fiducial marker defects on stereotactic target localization in the Leksell stereotactic system

“…The automatic positioning system of Gamma Knife Model C or 4C provides an additional error of ± 0.2 mm [11]. Wu et al [24] showed that the measured mechanical accuracy of Gamma Knife Model U was 0.3 mm.…”

Assessment and characterization of the total geometric uncertainty in Gamma Knife radiosurgery using polymer gels

“…Relevant published studies have added scientific creditability to this belief and an accuracy level better than 0.5 mm is widely acknowledged within the GK user community irrespective of the specific model. [4][5][6][7] On the other hand, the MR-related spatial distortions, which have been recognized as a major contributor to GK geometric accuracy degradation, 4, 7-11 vary from system to system and depend on the gradient coil design, the magnetic field strength, and the particular imaging parameters. Over the recent years, a considerable body of work has been performed on image distortion measurement and correction techniques [12][13][14][15][16][17][18] in an effort to exploit the comparative advantages of MR relative to CT images for radiation treatment planning purposes.…”

Current concepts in stereotactic radiosurgery - a neurosurgical and radiooncological point of view

“…Reasons for the failure of stereotactic radiosurgery in achieving an adaequate tumor control include an inadequate visualization of the tumor, a lack of intraoperative 3-D (volumetric) imaging, or an insufficient or limited dose (e.g. due to proximity to the brainstem) [21,29,33,47,51,58,68,145,154]. …”