|The standard investigations in diagnosis and treatment planning of brain
gliomas are MRI and CT. Pathologic changes are characterized on MRI and CT by increased
water content (edema) and blood-brain barrier (BBB) disruption, visualized as contrast
enhancement. Low-grade gliomas show low density or isodensity on CT images and
hypointensity or occasionally isointensity on T1-weighted MRI. They are hyperintense on
T2-weighted images. Peritumoral edema is minimal or absent. A disruption of the BBB is
usually absent. High-grade gliomas are hypodense on CT and show low-signal intensity on
T1-weighted MRI and high signal intensity on T2-weighted MRI. Peritumoral edema is
frequently seen. High-grade gliomas usually show contrast enhancement, correlated with the
extent of neovascularization and loss of integrity of the BBB owing to tumor infiltration
and production of vascular endothelial growth factor. However, both contrast enhancement
and hyperintensity areas on T2-weighted MRI are not always a real measure of tumor
extension in low- or high-grade gliomas. Tumor cells have been detected beyond the margins
of contrast enhancement in the surrounding edema and even in the adjacent normal-appearing
brain tissue. After neurosurgery or RT, BBB disturbances and edema can also be treatment
related and cannot be differentiated from persistent tumor on CT or MRI.
Glioma cells avidly take up radiolabeled amino acids, but only low uptake by normal cerebral tissue is present. Several years ago, histologic studies demonstrated that positron emission tomography (PET) using the radiolabeled amino acid carbon-11 methionine provides a more accurate delineation of intracerebral gliomas than CT and MRI. However, because of the short physical half life of carbon-11 (20 min), an on-site cyclotron is required for production of this radiopharmaceutical agent. In contrast to methionine, IMT can be imaged with the much more widely available single photon emission CT (SPECT). It has been demonstrated in several studies that IMT-SPECT allows visualization of gliomas with high contrast Furthermore, IMT-SPECT has been found to be a specific test for the differentiation of tumor recurrence and therapy-induced changes.
The aim of the present study was to determine how the findings on IMT-SPECT can influence RT planning after surgical resection of gliomas. In 66 postoperative patients with gliomas who were scheduled to undergo conformal RT, IMT-SPECT images were coregistered with T1- and T2-weighted MRI data sets. On these ''fusion'' images, residual tumor volume was delineated using the combined information of IMT-SPECT and MRI. This tumor volume was compared with the volume of contrast enhancement in T1-weighted MRI studies and the volume defined by the hyperintensity on T2-weighted images.
In patients with surgically resected brain gliomas, the size and location of residual IMT uptake differs considerably from the abnormalities found on postoperative MRI. Because of the known high specificity of IMT uptake for tumor tissue, the findings on IMT-SPECT may significantly modify the target volumes for radiotherapy planning. This will help to focus the high irradiation dose on the tumor area and to spare normal brain tissue.