Review: PET useful in radiation treatment planning of brain tumors
PET using 11C-methionine (MET) and 18F-fluoroethyl-L-tyrosine (FET) can be used for gross tumor volume delineation in brain gliomas and for the differentiation between treatment-related changes and residual/recurrent tumor and has higher sensitivity and specificity in comparison to CT and MRI, according to a review published in the September issue of Radiotherapy and Oncology.

The role of PET using amino acids for gross tumor volume (GTV) delineation in brain gliomas, metastases and benign cranial tumors such as meningiomas and glomus tumors was reviewed by Anca-Ligia Grosu, MD, from the department of radiation oncology and Wolfgang A. Weber, MD, from the department of nuclear medicine, University of Freiburg in Freiburg, Germany.

MET, 123I-alpha-methyltyrosine (IMT) and FET are the most important radiolabeled amino acids used in the diagnosis of brain tumors. The amino acid-PET tracers are accumulated by L and A amino acid transporters. Tumor cells take up radiolabeled amino acids at a high rate, while there is only a relatively low uptake in normal cerebral tissue, which are independent from blood-brain-barrier disturbance, according to Grosu and Weber.

The higher sensitivity and specificity of amino acid-PET in the diagnosis of gliomas in comparison to CT and MRI was demonstrated in many clinical trials, according to Grosu and Weber. Summarizing the data from the literature between 1983 and 2008, Grosu and Weber identified 45 trials including 1,721 patients, which investigated the role of MET-PET in the diagnosis of gliomas. Eleven studies including 706 patients were based on PET/MRI/CT stereotactic biopsies.

Between 2000 and 2008, 12 trials including 361 patients evaluated the role of FET-PET in the diagnosis of brain gliomas. In three studies with a combined total of 126 patients, the results were based on PET/MRI/CT stereotactic biopsies. All these studies have shown that the specificity of MET-PET and FET-PET for malignant gliomas was significantly higher (85 to 95 percent) in comparison to MRI, which has a high sensitivity but a lower specificity (50 percent).

Grosu and Weber also analyzed the similarity between MET and FET-PET for tumor visualization and investigated 42 patients with gliomas and metastases on the same day with MET and FET-PET. They found an almost identical sensitivity and specificity of both tracers and the intensity of tracer uptake and the extension of gliomas with FET-PET were closely correlated with the extension with MET-PET. Thus, these studies indicate that the results of MET-PET can be extrapolated to FET-PET, concluded Grosu and Weber.

The review also suggested that the role of PET in the visualization of the biological characteristics of the tumors for proliferation using 18F-fluorothymidine (FLT-PET); hypoxia using 18F-fluoromisonidazole (FMISO-PET) and angiogenesis/ peptide expression using arginine-glycine-aspartic acid (RGD-PET) should be investigated in future studies.