In: Neuro-Oncology 2006;8(4): Seventh Congress of the European Association for Neuro-Oncology;
14 Sep 2006-17 Sep 2006; Vienna, Austria. 2006. p. 371-371.
BACKGROUND: Conventional interpretation of methionine PET (Met-PET) scans in suspected
brain tumors uses the ratio of the tracer uptake within the lesion to the corresponding
contralateral area. The precise location at which the region of interest used to calculate
the reference value is placed is vital, because local variations in methionine uptake
may significantly alter the calculated ratio. Identifying a precise mirror region
is complicated by the distorting effect of the tumor and the need for manual realignment
of the image. METHOD: Patients with low-grade primary brain tumors or benignlesions
were identified on the basis of a tissue diagnosis or surveillance neuroimaging that
excluded a high-grade tumor. These conditions were selected because they primarily
involve a single hemisphere, with methionine uptake in the unaffected hemisphere being
essentially normal. A total of 180 Met-PET scans performed during 2003 – 2005
were identified from the database at the Max Planck Institute for Neurological Research,
coded, and anonymized for analysis. Scans demonstrating midline lesions, significant
mass effect, or evidence of substantial previous surgery were then excluded. A methionine
template was prepared using data from patients who had undergone both FDG and Met-PET
scans within eight weeks, with normalization to a previously developed FDG template.
Methionine scans were coregistered to the template, after masking of any tumor, and
the diseased hemispheres stripped. Mean uptake maps for each hemisphere were calculated
on a voxel-by-voxel basis and merged to create the normal methionine uptake map. Scans
unsuitable for inclusion into the normal map were reanalyzed using the contralateral
hemisphere and the normal uptake map for reference values, allowing the methods to
be compared. RESULTS: Good correlation was found between uptake ratios using reference
values calculated by both methods. Reference values could be reliably calculated in
tumors that were previously problematic to analyze, such as those that cross the midline.
Coregistration of the normal map was impaired in some cases by loss of the normal
architecture, but valid reference values were obtained despite this. CONCLUSION: Use
of a normal uptake may facilitate calculation of PET uptake ratios in brain tumors.
Further research is required to evaluate the correlation with histological findings
and the accuracy of image coregistration in the presence of distorting tumors.