The influence of anisotropy on the clinical target volume of brain tumor patients

Abstract

Objective.Current radiotherapy guidelines for glioma target volume definition recommend a uniform margin expansion from the gross tumor volume (GTV) to the clinical target volume (CTV), assuming uniform infiltration in the invaded brain tissue. However, glioma cells migrate preferentially along white matter tracts, suggesting that white matter directionality should be considered in an anisotropic CTV expansion. We investigate two models of anisotropic CTV expansion and evaluate their clinical feasibility.Approach.To incorporate white matter directionality into the CTV, a diffusion tensor imaging (DTI) atlas is used. The DTI atlas consists of water diffusion tensors that are first spatially transformed into local tumor resistance tensors, also known as metric tensors, and secondly fed to a CTV expansion algorithm to generate anisotropic CTVs. Two models of spatial transformation are considered in the first step. The first model assumes that tumor cells experience reduced resistance parallel to the white matter fibers. The second model assumes that the anisotropy of tumor cell resistance is proportional to the anisotropy observed in DTI, with an 'anisotropy weighting parameter' controlling the proportionality. The models are evaluated in a cohort of ten brain tumor patients.Main results.To evaluate the sensitivity of the model, a library of model-generated CTVs was computed by varying the resistance and anisotropy parameters. Our results indicate that the resistance coefficient had the most significant effect on the global shape of the CTV expansion by redistributing the target volume from potentially less involved gray matter to white matter tissue. In addition, the anisotropy weighting parameter proved useful in locally increasing CTV expansion in regions characterized by strong tissue directionality, such as near the corpus callosum.Significance.By incorporating anisotropy into the CTV expansion, this study is a step toward an interactive CTV definition that can assist physicians in incorporating neuroanatomy into a clinically optimized CTV.

Keywords: brain tumors; clinical target volume; glioma.

Figure A1:

RGB-colormap displaying the directionality of the metric tensors. Left: Original metric tensor image overlay the atlas T1w. Right: Warped metric tensor image overlaying the patient T1w. The images are color coded as follows: red right-left (RL), green anterior-posterior (AP) and blue superior-inferior (SI).

Figure 1:

(a) Example of a brain segmentation after skull stripping: white matter (level=1), gray matter (level=2) and CSF (level=3) segmentations. The GTV and corpus callosum are shown in yellow and blue, respectively. (b) Shortest path map (in mm) for the corresponding patient with the GTV as the origin.

Figure 2:

Comparison of the DDO CTV to the standard and NU CTVs, in the axial plane (a) and coronal plane (b). Green: standard CTV, red: NU CTV, and white: DDO CTV. From left to right: resistance coefficients $\rho$ equal to 0.5, 0.1, and 0.05. As $\rho$ decreases, the DDO and NU CTV expansion increases along the principle direction of the white matter tissue compared to the standard CTV. The yellow and blue masks show the GTV and corpus callosum, respectively.

Figure 3:

Boxplots showing dice dissimilarity (1 – DSC) between the FD CTV with the standard CTV and NU CTV on the left and right, respectively. Each boxplot represents the distribution of patients for a specific combination of the model parameters.

Figure 4:

Boxplots showing Hausdorff 95% percentile index (HD95) between the DDO CTV with the standard CTV and NU CTV on the left and right, respectively. Each boxplot represents the distribution of patients for that specific combination of the model parameters.

Figure 5:

Comparison of the FD CTV to the standard and NU CTVs, in the axial plane (a) and coronal plane (b). Green: standard CTV, red: non-uniform CTV, and white: anisotropic CTV. From left to right: resistance coefficients $\rho$ equal to 0.5, 0.1, and 0.05. The FD CTV are shown for two values of the anisotropy weighting parameter $\gamma$ equal to 1 (dashed lines) and 3 (solid lines). Increasing $\gamma$ yields a CTV expansion to the contralateral hemisphere through the corpus callosum.

Figure 6:

Boxplots showing dice dissimilarity (1 – DSC) between the FD CTV with the standard CTV and NU CTV in (a) and (b), respectively. Each boxplot represents the distribution of patients for a specific combination of the model parameters.

Figure 7:

Boxplots showing Hausdorff 95% percentile index (HD95) between the FD CTV with the standard CTV and NU CTV in (a) and (b), respectively. Each boxplot represents the distribution of patients for that specific combination of the model parameters.