Purpose/Objective: Conventional MRI for target definition of glioblastoma (GB) may receive inadequate radiation dose coverage of the nonenhanced (NE) hypercellular (HC) subvolume resulting in reduced therapeutic efficacy. It is a challenge to differentiate NE solid tumor from edema using conventional FLAIR and ADC (b≤1000 s/mm 2 ) images. This study aimed to develop a technique to identify the HC components of GB by using high b-value diffusion weighted imaging (DWI) and to investigate the

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... ionship of these components with the enhanced subvolumes, 95% prescribed radiation dose volume (95PDV) and progression-free survival (PFS). Materials and Methods: 21 patients (age: 23-76 years) with GB were treated by radiation therapy (RT) after surgical resection or biopsy. RT planning was based upon conventional MRI. During MRI simulation, DWI was acquired in 3 orthogonal directions with b-values of 0, 1000, and 3000 s/mm 2 . Gross tumor volume (GTV-Gd) and FLAIR abnormality volume (FLAIR TV) were defined on post-Gadolinium T1-weighted and T2 FLAIR images pre-RT. High b-value (3000 s/mm 2 ) DWI, in which fluid, edema, grey and white matter, to an extent, are suppressed, were used to define the HC volume (HCV) by using a threshold (mean intensity + 2 SD) calculated from a VOI in normal-appearing tissue most contralateral to the tumor (Fig 1) . All images, including planning CT (dose distribution) and MRI at recurrence, were registered to post-Gd T1WI pre-RT. Relationships between the HCV, GTV-Gd, FLAIR TV, 95PDV and PFS were analyzed.