A novel voxel based homogeneity index: Rationale and clinical implications for whole-brain radiation therapy

A homogeneity index (HI) measures the uniformity of a dose distribution within a given target volume. Traditional HIs only use a limited number of dose-volume histogram data-points for calculation. A voxel-based homogeneity index (VHI) is proposed which utilizes the entire information of the three-dimensional dose distribution. We compared the VHI with existing HIs and analyzed if VHI results were associated with treatment outcomes in patients who underwent therapeutic WBRT.

The VHI analyzes deviations from the prescribed dose in each voxel of the target volume. We retrospectively analyzed WBRT treatment plans. Overall survival (OS), CNS progression-free-survival (CNS PFS) and hazard rates were compared for tertile-split levels of the VHI using the Kaplan-Meier methods and multivariable Cox-regression analysis.

WBRT treatment plans (n = 770) were used for HIs comparison. OS and CNS PFS were assessed for 430 patients. The VHI showed a higher sensitivity for dose inhomogeneities. Lower OS and CNS PFS were observed for higher levels of VHI_Underdosage, particularly in patients with good performance status (KPS >70%) (OS: Log-rank P = .007, HR = 1.37 95%CI [1.09, 1.72]).

Higher sensitivity and feasibility to assess treatment plan quality using the VHI were demonstrated. First clinical implications were found in terms of compromised OS/CNS PFS for WBRT with radiation underdosage.