Precise ex vivo histological validation of heightened cellularity and diffusion-restricted necrosis in regions of dark apparent diffusion coefficient in 7 cases of high-grade glioma

Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults and is highly lethal.1,2 The current standard of care produced a median survival of 14.6 months when first described,3 with small improvements reported through aggressive protocols for treating recurrence in the ensuing years. Local recurrence following initial treatment is the most common form of relapse due to infiltrating neoplastic cells invisible to current contrast-enhanced imaging techniques. Because of this, fluid attenuated inversion recovery (FLAIR) or T2-weighted images have recently been included in imaging protocols for the evaluation of tumor progression. The criteria of the Response Assessment in Neuro-Oncology deem “significant increase in non-enhancing lesion” grounds for tumor progression.4 Nonenhancing lesion consists of regions of abnormal FLAIR or T2 hyperintensity, which generally indicates the presence of vasogenic edema or infiltrative glioma. Edema results from tumor breakdown of the endothelial junctions, allowing intravascular proteins and fluid to penetrate the cerebral extracellular space, often spreading down white matter tracts. One of the troubling aspects of T2/FLAIR hyperintensity is the vast extent to which it often reaches beyond the T1-weighted contrast-enhancing tumor. In cases of invasive glioblastoma, malignant tumor cells have been found 4 cm away from contrast-enhancing tumors,5 and 96% of patients recur within 3 cm of their initial lesion following surgery.6,7 Furthermore, patients on the anti–vascular endothelial growth factor (VEGF) drug bevacizumab commonly exhibit disease progression in nonenhancing lesions rather than in areas of traditional enhancement, as the drug “seals” the blood–brain barrier and prevents contrast agent from leaking out of the capillary bed.8 It has also been suggested that the drug changes the tumor phenotype from angiogenic to mesenchymal.9,10 It is therefore imperative to be able to monitor and detect tumor infiltration beyond the contrast-enhancing boundary. MRI-based diffusion weighted imaging (DWI) has shown promise in neuro-oncology. DWI-derived apparent diffusion coefficient (ADC) measures the average diffusion of water molecules within each voxel. ADC has been shown to decrease in the presence of hypercellularity.11–19 Some debate remains as to whether or not ADC is predictive of tumor grade, as some studies have found a correlation,11,19–22 while others have found considerable overlap between tumor grades,18,23 or no correlation at all.24 Complicating the issue, ADC is sensitive to other tissue characteristics, including the presence of extracellular fluid due to vasogenic edema or tumor destruction of the extracellular matrix. It is therefore unclear how restricted diffusion need be to indicate “hypercellularity” and how to localize such regions outside of contrast-enhancing tumor. This study accomplishes 2 objectives. First we examine cases of noninfiltrative meningioma to determine normal ADC values within vasogenic edema known to not contain an infiltrative tumor component. The ADC threshold from these patients is then used to define regions of significantly decreased ADC within FLAIR hyperintensity (ie, ADC-FLAIR mismatch [AFMM]) in 7 cases of high-grade glioma. Six of these patients underwent extensive treatment, while the seventh refused treatment. Following death, each patient was evaluated with ex vivo histology to precisely compare ADC and cell density voxel by voxel. We hypothesized that regions with ADC below the minimum ADC threshold and colocalized with T2/FLAIR hyperintensity would have increased cell density compared with regions with ADC above threshold and colocalized with T2/FLAIR hyperintensity.