## 4 Discussion
### Clinical Prognosis Based on Tractography-Derived Metrics
Such variations could include evaluating algorithms where multiple b-values (rather than one b-value) could be employed, either varying the \(\text{b}_{\text{max}}\) or employing multiple shells of diffusion acquisition. Alternately, varying spatial resolutions (e.g., voxel sizes) and number of gradient directions could be used to further minimize signal-to-noise in acquired images. In the present study, these parameters were fixed at specific values as part of pre-planned pulse sequences such that individual parameters were not subject to investigation. Such future directions may hold benefits for dwMRI and tractography analyses of heterogeneous tumors like glioblastoma in which different tissue components (e.g., tumor core versus shell, necrotic regions, tumor margin, and surrounding parenchyma containing invading cells) have varied diffusional properties corresponding to differing extents of migration and cellularity [6; 16; 17; 18]. On the other hand, higher intra-tumoral mean and axial diffusivity values were associated with longer overall survival, which has been reported previously [62]. While the interpretation of these metrics in the context of a variably necrotic core in high-grade glioma may be difficult to discern, less intact architectural boundaries and greater \"liquidation\" within the assessed tumor ROI may lead to reduced directional diffusivity (perpendicular or axial to involved fiber bundles within the tumor), and therefore portend greater migration activity away from the tumor core and poorer outcomes [67]. This explanation would explain both findings in the current study (i.e., lower mean and axial diffusivity in addition to longer total and projecting mean tract lengths) with respect to our tract-associated cellular migration hypothesis. Even so, studies have suggested that mean diffusivity does not always correlate uniformly with tumors that typically have more intra-tumoral architectural integrity and/or less necrosis; conventional dwMRI metrics alone may perform better at predicting outcomes (or pathology) when combined with other radiomic markers [36; 38; 68]. Further study of tractography-derived versus basic dwMRI metrics head-to-head would be an interesting future direction as new tractography-derived analyses emerge. Additionally, the presence of edema around the tumor could theoretically introduce a free water effect which affects diffusion metrics particularly adjacent to invasion-affected white matter tracts, which are hypothesized to coincide with observed tumor-intersecting tractography. Accordingly, any observed directionality may be mitigated because of that edema, reducing directional specificity. Algorithms which incorporate free water elimination or multi-tissue techniques could help adjust for any possible signal loss due to invasion-associated edema. Finally, while we observed statistically significant associations of tractography-derived metrics with survival time, the implications of the location and distribution of invading tumor cells within the brain on glioblastoma outcomes remain preliminary. Further, it is possible, given all patients in this study had invasive disease by pathologic definition, that certain outcome associations may be non-significant specifically because of homogenous disease severity, or that the observed significant correlations of outcomes with certain tract pathways (corpus callosum, inferior longitudinal fasciculus, and/or inferior frontal-occipital fasciculus) and overall tract length are actually driven by the likelihood of earlier recurrence or functional deterioration when those pathways are involved. Future clinical outcome studies should perform multivariable analyses which adequately control for multiple additional variables including recurrence time, recurrence location, symptomatic progression, chemoradiation treatment effects, and others which were not collected by the present study, and which may better explain the relationships observed within the present univariate analyses.