The Gamma Knife has evolved steadily since 1967. In the first models (model U or A), 201 cobalt sources were arranged in a hemispherical array. These units presented challenging ⁶⁰Co loading and reloading issues. To eliminate this problem, the unit was redesigned so that sources were arranged in a circular (O-ring) configuration (models B, C, and 4C) (Fig. 8.1).

Gamma Knife radiosurgery usually involves multiple isocenters of different beam diameters to achieve a treatment plan that conforms to the irregular three-dimensional volumes of most lesions. The total number of isocenters may vary depending upon the size, shape, and location of the target. Each isocenter has a set of three x, y, z stereotactic coordinates corresponding with its location in three-dimensional space as defined using a rigidly fixed skull stereotactic frame. In terms of actual dose delivery, this means several changes in the patient’s head position within the helmet. In 1999, the model C Gamma Knife was introduced. The first model C in the USA was installed at the University of Pittsburgh Medical Center in March 2000. This technology combined advances in dose planning with robotic engineering and uses a submillimeter accuracy automatic positioning system (APS) (Fig. 8.2). This technology obviates the need to manually adjust each set of coordinates in a multiple isocenter plan. The robotic positioning system moves the patient’s head to the target coordinates defined in the treatment plan. The robot eliminates the time spent removing the patient from the helmet, setting the new coordinates for each isocenter, and repositioning the patient in the helmet. This has significantly reduced the total time spent to complete the treatment and also increases accuracy and safety [8–13]. Because the treatment time is shortened, a precise three-dimensional (3D) plan can be generated using multiple smaller beams achieving volumetric conformality. Such an approach results in a steeper dose fall-off extending beyond the target (higher selectivity). The other features of the model C unit include an integral helmet changer, dedicated helmet installation trolleys, and color-coded collimators. In 2005, the fourth-generation Leksell Gamma Knife, model 4-C, was introduced (Fig. 8.3). The first unit was installed at the University of Pittsburgh in January 2005. The model 4-C is equipped with enhancements designed to improve workflow, increase accuracy, and provide integrated imaging capabilities. The integrated imaging, powered by Leksell GammaPlan, offers the ability to fuse images from multiple sources. The planning information can be viewed on both sides of the treatment couch. The helmet changer and robotic APS are faster and reduce total treatment time.