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The ⁶⁰Co sources are distributed on a drive mechanism (Fig. 2) that places the sources over the collimator openings. The drive mechanism is referred to as a sector, twenty-four sources are placed onto each sector and 8 sectors are available for treatment planning. The sources are aligned on the sector in the following pattern: 6, 4, 5, 4, 5. This pattern for all sectors can be broken into 5 rings of sources (Fig. 3). The distribution of the rings is approximately 15° from the normal, with the angles at 75, 60, 45, 30, and 15°. This means that no sources are parallel opposed. This distribution of sources ensures that the dose maximum will remain in the target and no hotspots will be outside of it. The Gamma Knife has four collimator settings: 16, 8, 4 mm and a block. Each ring of sectors 1 through 8 has a certain geometric distance to the point of intersection for each collimator; this point of intersection is referred to as the radiologic focal point (RFP; Table 1).

Table 1. Distance from rings to the RFP and the associated relative and effective output factor for the 16 mm collimator. Courtesy of Elekta.

Fig. 2. Source drive mechanism with sources attached. Courtesy of Elekta.

Fig. 3. Source distribution within a sector on film.