It is a challenge to differentiate non-enhanced components of glioblastoma (GBM) from edema and normal tissue using conventional MRI, including diffusion weighted imaging with b-value of 1000 s/mm² or less. An ill-differentiation between non-enhanced tumor from edema and normal tissue could lead to inadequate treatment for GBM by surgery and radiation therapy. This study aimed to develop a method to identify the hypercellular component of GBM using high b-value diffusion weighted (DW) imaging, and investigate its relationship to gross tumor volume defined on post-Gd T1 weighted images (GTV_Gd) and abnormality volume on T2 FLAIR images (AV_FLAIR). Also, the clinical values of the HCV for tumor target definition of radiation therapy and for prediction of outcomes were assessed.Forty patients (median age: 54 years) with newly diagnosed GBM were included in this IRB approved study. All patients had MRI scans prior to concurrent chemoradiation therapy (CRT). All scans were acquired on a 3T scanner (Skyra, Siemens). DW images were acquired in 3 orthogonal directions with b-values of 0, 1000, and 3000 s/mm². Target definition for radiation treatment was based upon post-contrast T1-weighted and T2 FLAIR images according to the ASTRO guideline. The hypercellularity volume (HCV) of GBM was defined on the high b-value (3000) DW images by suppressing fluid, edema, grey matter, and white matter to an extent. An intensity threshold (mean intensity + 2SD) obtained from a VOI in normal appearing tissue and contralateral to the tumor was used to determine the HCV in each individual patient. After co-registration of DW, FLAIR and post-contrast T1-weighted images pre RT, planned dose volumes and MRI at recurrence, spatial overlaps of the HCV with the GTV-Gd, AV-FLAIR, 95% radiation dose volume (95PDV) and pattern of failure were compared. The predictive value of HCV for progression free survival (PFS) was analyzed by univariate proportional hazards regression. For 31 of the 40 patients, the image data have been analyzed and were reported here.Of the 31 patients, the HCVs varied from 0.1 to 66 cc with a median of 9.3 cc. The HCVs overlapped with the GTVs-Gd from 3% to 98% (a median of 68%), indicating that large portions of the HCVs were non-enhanced. The HCVs were much smaller than abnormality volumes defined on T2 FLAIR (a median of 70 cc and ranged from 17.8 to 359.8 cc), but extended beyond the AV_FLAIRs in 6 patients. The 95% prescribed radiation dose volumes did not cover the HCVs completely, and in 6 patients the uncovered HCVs were greater than 1 cc, ranging from 1 cc to 25cc (Fig 1). For the first 21 patients who have been followuped more than 18months after RT, 15 had tumor progression. The pre-RT HCV and non-enhanced HCV were significant negative prognostic indicators for PFS (p<0.002 and p<0.01, respectively). The portion of the HCV that was not covered by the 95PDV was also a significant predictor for PFS (p<0.05).High b-value DW imaging could provide a means to extract the hypercellularity components of GBM and aid in target definition for surgery and RT. The HCV identified by high b-value diffusion weighted imaging could be an aggressive component of GBM. An intensified treatment of HCV could prolong the time for progression. Using FLAIR images to define CTV could over-treat brain tissue and cause toxicity (Fig 2). Further follow-ups and evaluations with pattern of failure and clinical outcomes are ongoing.