The Leksell gamma knife Model U versus Model C: a quantitative comparison of radiosurgical treatment parameters.

OBJECTIVE: We present a quantitative comparison of radiosurgery treatments for cavernous sinus tumors using the Leksell gamma knife Model U versus the Model C with automatic positioning system (APS) (Elekta Instruments, Norcross, GA).
METHODS: At our medical center from August 1994 through May 2000, the Model U was used to treat 96 patients (37 men [39%] and 59 women [61%]; median age, 54.5 yr) with benign cavernous sinus tumors: 43 meningiomas (45%), 48 pituitary tumors (50%), and 5 others (5%). From June 2000 through April 2002, the Model C with APS treated 45 patients (20 men [44%] and 25 women [56%]; median age, 51.4 yr) with 15 meningiomas (33%), 29 pituitary tumors (65%), and 1 schwannoma (2%). The two groups had similar treated tumor volumes (Model U mean, 4.3 cm(3); Model C mean, 4.2 cm(3)), equivalent tumor distances from critical structures (optic nerve, chiasm, and pons), comparable distributions in Sekhar tumor grades, and the same median prescribed dose of 15 Gy to the 50% isodose line at the tumor periphery. All planning and treatments were performed by the same radiosurgery team to minimize dosage to adjacent critical tissues and to optimize conformity index.
RESULTS: Analysis of multiple treatment parameters showed that the Model C plans were superior. Model C treatments had an improved conformity index (Model U mean, 1.7; Model C mean, 1.6; P < 0.02) and a lower underdosed tumor volume (Model U mean, 0.4 cm(3); Model C mean, 0.1 cm(3); P < 0.004). The total treated volume and the excess treated volume were similar. The Model C group had a reduction in optic chiasm dose (Model C mean dose, 3.8 Gy; Model U mean dose, 5.3 Gy; P < 0.0001). The average number of isocenters was slightly higher for the Model C group (6.7 versus 6 for the Model U), but with a lower mean number of collimator sizes (1 versus 2 for the Model U). Model C plans required a mean of 93 fewer plugs per treatment, thus contributing to an estimated 67.6 minutes saved per treatment session.
CONCLUSION: Comparison of radiosurgery treatments using the Leksell gamma knife Model U versus the Model C with APS was performed by quantitative analysis of treatment parameters on a cohort of benign cavernous sinus tumors. Treatment plans using the Model C resulted in better tumor coverage (improved conformity, less underdosed tumor volume) and decreased optic chiasm dose. An estimated average of 1 hour was saved per treatment when using the Model C with APS.