Effect of composite sector collimation on average dose fall-off for Gamma Knife Perfexion

Abstract

The new capability of composite sector collimation in Gamma Knife Perfexion produces complex, nonspherical, and nonelliptical dose distributions. In this study, the authors investigated the effect of composite sector collimation on average dose fall-off compared with the previous Gamma Knife model. A general formalism was derived to describe the peripheral dose distribution of all Gamma Knife models in the form of (V/V(0)) = (D/D(0))(gamma), where V is the volume of the peripheral isodose line with the value of D, V(0) is the reference prescription isodose volume, D(0) is the prescription dose, and gamma is the fitting parameter that determines how fast the dose falls off near the target. Based on this formula, the authors compared 40 cases involving patients treated with Gamma Knife Perfexion with 40 similar cases involving patients treated with Gamma Knife model 4C. The cases were grouped based on the use of the sector collimators in the treatment planning process. For each group as well as all cases combined, the mean gamma values were compared by means of the Student t-test for varying ranges of the peripheral dose distribution-from 100% of the prescription dose to 75, 50, and 25% of the prescription dose. The fit of general formula to the data was excellent for both Gamma Knife Perfexion and Gamma Knife 4C with R(2)> 0.99 for all the cases. The overall gamma values (mean +/- 2 standard deviations) were as follows: gamma = -1.74 +/- 0.47 (Model 4C) versus -1.77 +/- 0.40 (Perfexion) within 100-75% of the prescription dose; gamma = -1.57 +/- 0.26 (Model 4C) versus -1.58 +/- 0.25 (Perfexion) within 100-50% of the prescription dose; gamma = -1.47 +/- 0.18 (Model 4C) versus -1.50 +/- 0.16 (Perfexion) within 100-25% of the prescription dose. No statistical significance between the mean differences for Gamma Knife Perfexion and Model 4C was found within these ranges. The probability values were 0.65, 0.84, and 0.22, respectively. The use of composite sector collimators in Gamma Knife Perfexion demonstrated no statistically significant effects on the volume-averaged dose fall-off near a target periphery for typical treatment cases.