White matter pathlength maps from diffusion-weighted MRI tractography for radiotherapy target planning in glioblastoma.

PURPOSE: Conventional radiotherapy target delineation for glioblastoma (GBM) includes an isotropic expansion from gross tumor visible on anatomic MRI to an empirically defined clinical target volume (CTV) for coverage of microscopic infiltrative disease. GBM spreads preferentially along white matter tracts, but this information has not previously been systematically incorporated into radiotherapy planning. We investigated using white matter tractography from diffusion-weighted MRI (dwMRI) to inform target delineation. METHODS: Thirteen patients with GBM underwent 55-directional dwMRI at the time of post-operative radiation planning MRI. Whole-brain tractography was performed, and streamlines passing within 5 mm of gross disease were used to generate maps representing white matter path length from gross disease. Clinical target volumes were generated using tractography (CTVtract) and conventional isotropic expansion (CTVisotropic). MRI at time of GBM recurrence was registered to the radiation planning MRI, and coverage of the recurrence volume was compared between CTVtract and CTVisotropic. RESULTS: CTVtract demonstrated non-isotropic expansion of the primary tumor along regional white matter tracts while respecting natural anatomic boundaries. CTVtract volumes using a 2 cm path-length were a median of 67 cc smaller (-19%) than the paired 2 cm CTVisotropic volumes (p = 0.003). 10/13 recurrence volumes were included in CTVisotropic, while 12/13 recurrence volumes were included in CTVtract. CONCLUSION: Tractography can be used to define CTV that may provide superior coverage of areas at risk of recurrence while simultaneously reducing target volume.