[Effect on treatment planning based on properties of cobalt-60 stereotactic radiosurgery units]

The brand-new version of gamma knife, Perfexion, is equipped with an automatic collimator arrangement system that does not require manual collimator exchange and a couch-traveling system that is approximately ten times faster than Model C, so treatment time with multiple shots is assumed to remain within a clinically acceptable range. In this study, the treatment plans for Model C and Perfexion were compared from the viewpoint of number of shots, coverage, selectivity, conformity, and gradient in planning target volume (PTV) coverage. We enrolled 187 and 89 patients with vestibular schwannomas treated by Model C and Perfexion in the study. Treatment planning was created on a Leksell GammaPlan workstation. The mean PTV was 5.2 ml (range 0.1-18.4 ml) in Model C and 4.1 ml (range 0.1-32.1 ml) in Perfexion. The mean shot number for Model C and Perfexion was 11 (range 2-27) and 16 (range 1-41) at the isodose contour of 40-60%, respectively. The mean PTV coverage was 94% (range 73-100%) and 98% (range 91-100%), and the mean PTV selectivity was 83% (range 46-98%) and 87% (range 63-97%) for Model C and Perfexion, respectively. The mean conformity index was 1.15 (range 0.81-2.02) and 1.14 (range 0.97-1.57), and the mean gradient index was 2.82 (range 2.37-3.35) and 2.91 (range 2.55-4.48) for Model C and Perfexion, respectively. In Perfexion, better PTV coverage and selectivity were achieved by using an excessively large number of shots. In addition, the use of a small collimator in Perfexion produced a steeper dose gradient. Our comparative research demonstrated the greater clinical usefulness of Perfexion.