ESTRO 2024 - Abstract Book

S848

Clinical - CNS

ESTRO 2024

MLCs or BrainLab tungsten cones. All plans were normalized to achieve target coverage of 97%. The conformity index (CI) and gradient index (GI) were calculated for each plan and compared across different techniques while the sphericity of the targets was calculated using the Matlab (v.R2023b) codes (Anton Semechko).

Definition of Sphericity

Sphericity, a key parameter in this study, is a measurement of how closely the shape of an irregular volume matches that of a sphere. It is quantified by the formula:

Sphericity = V p / V MBS

where V p represents the volume of the irregular shape of interest, and V MBS denotes the volume of the minimum bounding sphere that encompasses the shape. Sphericity values range from 0 to 1, with a value of 1 indicating a perfect sphere and values closer to 0 representing more irregular or non-spherical shapes.

Results:

This study revealed a significant correlation between the sphericity of the target volume and its corresponding CI (R = -0.8647, p = 8.74e-7) in the cone-based SRS plans (figure 1). The slope of the regression model was -1.440 (p = 8.74e-7) and the y-intercept was 2.135 (p = 4.918e-13). With this model, the CI can be predicted in cone-based SRS by calculating the sphericity of the target before planning. For example, the sphericity has to be smaller than 0.58 to achieve a cone-based plan with conformity index below 1.30. On the other hand, there were no statistically significant regression models for the MLC-based plans, suggesting that the sphericity of the targets did not have a significant impact on the conformity indices in these plans.

Figure 1 Conformity Index vs Sphericity for the three techniques