MR and PET strategies identify optimal biopsy sampling sites for malignant gliomas

A study comparing six MR and PET brain imaging strategies has found cell proliferation and microcirculation to be best suited for marking the hot spot to optimally grade malignant gliomas.Accurately grading gliomas is difficult because of the heterogeneous pathophysiology of glioblastomas and other malignant brain tumors, said Dr. Marc-André Weber in his abstract presentation at the 2007 International Society for Magnetic Resonance in Medicine meeting in May.

Clinicians can choose from several sophisticated MR and PET imaging strategies to identify where the most anaplastic area of a tumor lies. Weber and colleagues from the German Cancer Research Center and nuclear medicine and radiology departments at the University of Heidelberg in Germany examined six approaches performed on 23 patients with gliomas. MRI was performed on a 1.5T scanner the day before PET imaging, using the following protocols:

• Na-23 MRI to map sodium concentrations in the tumor and surrounding tissue
• proton MR spectroscopy to measure multivoxel metabolic activity
• dynamic contrasted-enhanced MRI
• dynamic susceptibility-weighted MRI to measure the degree of contrast enhancement and microvascular permeability with diffusion mapping and diffusion tensor imaging
• F18-fluorothymidine (FLT) PET to measure cell proliferation
• FDG-PET to measure glucose metabolism

Weber, a senior physician in the cancer center's radiology department, reported that FLT- and FDG-PET and dynamic contrast-enhanced MRI and MRS each generated evidence of heterogeneity seen in all 18 high-grade gliomas but absent in five patients with proven grade II tumors.

The study found the methods that singled out regions of the highest microcirculation and cell proliferation were best suited for identifying the hot spot for tumor biopsies. High FLT uptake with PET and a high choline/

N

-acetylaspartate peak with MRS both are known from previous studies to single out regions of high cell proliferation. As expected, a strong correlation between FLT uptake and a high choline/NAA peak on MRS was observed. Elevated microcirculation, as shown with DCE-MRI, appeared in the same areas. A high microcirculation rate reflected the need for an amply nutrient supply for growing tumor tissue, Weber said.The results advised against Na-23 MRI, diffusion tensor imaging, and FDG-PET. Their performance indicated that they are poorly suited for identifying anaplastic tumor. The most intense signal from Na-23 MRI originated from the tumors' necrotic cores, while investigators had difficulty seeing tumor borders during FDG-PET because of avid FDG uptake in normal tissue around the tumors, Weber said.For more information from the Diagnostic Imaging archives:

Report from SNM: FLT-PET predicts survival in patients with brain tumors

PET brings new definition to brain tumor diagnostics

Functional MR imaging maps brain function and plasticity