Models A, B, and C

The Gamma Knife has evolved steadily since 1967.^11–13 In the first models (Model U or A), 201 cobalt sources were arranged in hemispherical array. However, these units present challenging cobalt 60 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). Gamma Knife radiosurgery usually involves multiple isocenters with different beam diameters to achieve a treatment plan that conforms to the irregular three-dimensional (3D) volumes of most lesions. The total number of isocenters may vary depending on the size, shape, and location of the target. Each isocenter has a set of three x, y, and z stereotactic coordinates corresponding to its location in 3D space as defined with 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 United States was installed at the University of Pittsburgh Medical Center in March 2000. This technology combined advances in dose planning with robotic engineering and used a submillimeter-accuracy automated positioning system (APS). It obviated the need to manually adjust each set of coordinates in a multiple-isocenter plan. The robotic positioning system moved the patient’s head to the target coordinates defined in the treatment plan. The robot eliminated the time spent removing the patient from the helmet, setting the new coordinates for each isocenter, and repositioning the patient in the helmet. This advance significantly reduced the total time spent to complete the treatment and also increased accuracy and safety. Because treatment time was shortened, a precise 3D plan could be generated by using multiple smaller beams to enhance volumetric conformality. Such an approach results in a steeper dose falloff extending beyond the target (higher selectivity). Other features of the Model C unit included an integral helmet changer, dedicated helmet installation trolleys, and color-coded collimators.^14–23 In 2005, the fourth-generation Leksell Gamma Knife (LGK) Model 4C was introduced. The first unit was installed at the University of Pittsburgh in January 2005. Model 4C was equipped with enhancements designed to improve workflow, increase accuracy, and provide integrated imaging capabilities. The integrated imaging, powered by Leksell GammaPlan (LGP), offered the ability to fuse images from multiple sources. These images can also be exported to a CD-ROM, so the referring physician could receive preoperative or postoperative images for reference and follow-up. The planning information could be viewed on both sides of the treatment couch. The helmet changer and robotic APS were faster and reduced total treatment time.

Leksell Gamma Knife PERFEXION

In 2002, Elekta Instrument AB, Stockholm, assembled a group of experts, including neurosurgeons, radiation oncologists, medical physicists, and engineers, who were assigned the task of defining specifications for the new LGK system. The group agreed on five critical features for the future system: (1) best dosimetry performance, (2) unlimited cranial reach, (3) best radiation protection for patient and staff, (4) full automation of the treatment process, and (5) patient and staff comfort. As a result of this development, a new system eventually called the LGK PERFEXION was officially introduced in 2006 (Fig. 256-1).²⁴ The radiation unit was redesigned with beam geometry entirely different from that of the previous Gamma Knife Models U, B, C, and 4C. A total of 192 ⁶⁰Co sources were arranged in a cylindrical configuration in five concentric rings. This differed substantially from the previous hemispherical arrangements and resulted in a different source-to-focus distance for each ring that varied from 374 to 433 mm. The primary and secondary collimators were replaced by a single large 120-mm-thick tungsten collimator array ring (Fig. 256-2). Consequently, no external collimator helmets were needed for the PERFEXION system.

FIGURE 256-1 Leksell Gamma Knife PERFEXION.

FIGURE 256-2 Leksell Gamma Knife PERFEXION radiation unit and collimator system. A, Cross section of the Leksell Gamma Knife PERFEXION radiation unit. B, Detailed view of sectors. Each sector holds 24 ⁶⁰Co sources and can be moved independently of other sectors in the desired position to define collimator size or block beams. C, Sector position in a 4-mm collimator. D, Sector position in an 8-mm collimator. E, Sector position in a 16-mm collimator.

The range of collimators (beam size) was also changed from previous Gamma Knife models. Only three collimators are available for the PERFEXION system. The 4- and 8-mm collimators remain, but the 14- and 18-mm collimators were replaced with a 16-mm collimator. The tungsten collimator array is subdivided into eight identical but independent sectors, each containing 72 collimators (24 collimators for 4 mm, 24 collimators for 8 mm, and 24 collimators for 16 mm). Collimator size for each sector is changed automatically by moving 24 sources over the selected collimator set. A sector with 24 sources can be moved independently of other sectors to obtain five different positions: (1) home position when the system is standby, (2) 4-mm collimator, (3) 8-mm collimator, (4) 16-mm collimator, and (5) sector-off position, defined as a position between the 4- and 8-mm collimators that provides blocking of all 24 beams for the sector (see Fig. 256-2). Sector movement is performed by servo-controlled motors with linear scales located at the rear of the radiation unit.

Treatment volume has been increased by more than 300% in comparison to previous models. However, because of a better collimator system (120-mm tungsten ring), the average distance from the cobalt source to the focus is very close to that of previous models, which results in similar output between the 18-mm collimator of previous models and the 16-mm internal collimator of the PERFEXION unit. The enhanced treatment volume (more than 3 times) allows greater mechanical treatment range in the x/y/z dimensions. The mechanical range in x/y/z dimension is 160/180/220 for the PERFEXION system as opposed to 100/120/165 for other Gamma Knife models. As a result, PERFEXION has unlimited cranial reach and is ideal for the treatment of multiple brain metastases because multiple tumors at different locations can be treated without any difficulty in terms of collisions with the collimator system.

The APS responsible for setting the stereotactic coordinates was replaced by the bed patient positioning system (PPS). Instead of patient head movement in the Model C and 4C units, the whole patient couch now moves into preselected stereotactic coordinates in the PERFEXION model. This provides better patient comfort and facilitates completion of treatment in just a single run. Docking of the patient into the PPS is done by means of an adaptor that attaches to the standard stereotactic Leksell G frame with three clips. The adapter is then directly docked to the PPS (Fig. 256-3). The patient can be locked into three different neck positions, gamma angles of 70 (extension), 90 (neutral), or 110 (flexion) degrees. The gamma angle is the only treatment parameter that requires manual setup. The PPS replaces the APS and manual trunnion stereotactic coordinate setup and has repeatable accuracy in excess of 0.05 mm.

FIGURE 256-3 The Leksell stereotactic frame is docked by means of a frame adapter in the Leksell Gamma Knife PERFEXION. A, Frame adapter attached to the Leksell G frame. B, The adapter is then directly docked to the PERFEXION patient positioning system.