This review provides a complete technical description of the CyberKnife ® VSI™ System, the latest addition to the CyberKnife product family, which was released in September 2009. This review updates the previous technical reviews of the original system version published in the late 1990s. Technical developments over the last decade have impacted virtually every aspect of the CyberKnife System. These developments have increased the geometric accuracy of the system and have enhanced the dosimetric accuracy and quality of treatment, with advanced inverse treatment planning algorithms, rapid Monte Carlo dose calculation, and post-processing tools that allow trade-offs between treatment efficiency and dosimetric quality to be explored. This review provides a system overview with detailed descriptions of key subsystems. A detailed review of studies of geometric accuracy is also included, reporting a wide range of experiments involving phantom tests and patient data. Finally, the relationship between technical developments and the greatly increased range of clinical applications they have allowed is reviewed briefly.

Robotic radiosurgery using more than one circular collimator can improve treatment plan quality and reduce total monitor units (MU). The rationale for an iris collimator that allows the field size to be varied during treatment delivery is to enable the benefits of multiple-field-size treatments to be realized with no increase in treatment time due to collimator exchange or multiple traversals of the robotic manipulator by allowing each beam to be delivered with any desired field size during a single traversal. This paper describes the Iris variable aperture collimator (Accuray Incorporated, Sunnyvale, CA, USA), which incorporates 12 tungsten-copper alloy segments in two banks of six. The banks are rotated by 30 degrees with respect to each other, which limits the radiation leakage between the collimator segments and produces a 12-sided polygonal treatment beam. The beam is approximately circular, with a root-mean-square (rms) deviation in the 50% dose radius of <0.8% (corresponding to <0.25 mm at the 60 mm field size) and an rms variation in the 20-80% penumbra width of about 0.1 mm at the 5 mm field size increasing to about 0.5 mm at 60 mm. The maximum measured collimator leakage dose rate was 0.07%. A commissioning method is described by which the average dose profile can be obtained from four profile measurements at each depth based on the periodicity of the isodose line variations with azimuthal angle. The penumbra of averaged profiles increased with field size and was typically 0.2-0.6 mm larger than that of an equivalent fixed circular collimator. The aperture reproducibility is < or =0.1 mm at the lower bank, diverging to < or =0.2 mm at a nominal treatment distance of 800 mm from the beam focus. Output factors (OFs) and tissue-phantom-ratio data are identical to those used for fixed collimators, except the OFs for the two smallest field sizes (5 and 7.5 mm) are considerably lower for the Iris Collimator. If average collimator profiles are used, the assumption of circular symmetry results in dose calculation errors that are <1 mm or <1% for single beams across the full range of field sizes; errors for multiple non-coplanar beam treatment plans are expected to be smaller. Treatment plans were generated for 19 cases using the Iris Collimator (12 field sizes) and also using one and three fixed collimators. The results of the treatment planning study demonstrate that the use of multiple field sizes achieves multiple plan quality improvements, including reduction of total MU, increase of target volume coverage and improvements in conformality and homogeneity compared with using a single field size for a large proportion of the cases studied. The Iris Collimator offers the potential to greatly increase the clinical application of multiple field sizes for robotic radiosurgery.

A robotic image-guided radiosurgical system has been modified to treat extra-cranial sites using implanted fiducials and skeletal landmarks to locate the treatment targets. The system has been used to treat an artero-venous malformation in the cervical spine, a recurrent schwannoma of the thoracic spine, a metastatic adenocarcinoma of the lumbar spine, and three pancreatic cancers. During each treatment, the image guidance system monitored the position of the target site and relayed the target coordinates to the beam-pointing system at discrete intervals. The pointing system then dynamically aligned the therapy beam with the lesion, automatically compensating for shifts in target position. Breathing-related motion of the pancreas lesions was managed by coordinating beam gating with breath-holding by the patient. The system maintained alignment with the spine lesions to within +/- 0.2 mm on average, and to within +/- 1 mm for the pancreatic tumors. This experience has demonstrated the feasibility of using image-guided robotic radiosurgery outside the cranium.

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A robotic image-guided radiosurgical system has been modified to treat extra-cranial sites using implanted fiducials and skeletal landmarks to locate the treatment targets. The system has been used to treat an artero-venous malformation in the cervical spine, a recurrent schwannoma of the thoracic spine, a metastatic adenocarcinoma of the lumbar spine, and three pancreatic cancers. During each treatment, the image guidance system monitored the position of the target site and relayed the target coordinates to the beam-pointing system at discrete intervals. The pointing system then dynamically aligned the therapy beam with the lesion, automatically compensating for shifts in target position. Breathing-related motion of the pancreas lesions was managed by coordinating beam gating with breath-holding by the patient. The system maintained alignment with the spine lesions to within +/- 0.2 mm on average, and to within +/- 1 mm for the pancreatic tumors. This experience has demonstrated the feasibility of using image-guided robotic radiosurgery outside the cranium.