- Normal examination: As a rule, results of the neurological examination of children with DNET are normal, without focal deficits or signs of increased ICP.
- No expected abnomalities: In general, no special laboratory tests are necessary in children with DNET.
Although CT scan is not the examination of first choice in patients with suspected DNET or other gliomatous tumors, in many patients presenting to an emergency room after a seizure, CT scan may be easier to obtain than MRI, especially in artificially ventilated patients.
- Cortical- and subcortical-based hypodensity: On CT scan, a DNET is visible as a hypodense area in the cortical and subcortical regions. There is a lack of enhancement with iodinated contrast agents. Calcifications are rare, contrary to oligodendrogliomas (39, 40). In some cases scalloping of the overlying skull can be seen.
- Gyriform tumor: On MRI, the fact that the tumor occupies both cortical and subcortical regions is easily demonstrated (6, 15, 20). The tumor has a triangular shape or can be gyriform, and it is not surrounded by edema. Mass effect is minimal compared to that of gliomas. Pseudocysts can be small or large and multiple.
- T1-weighted image: Hypointensity with gadolinium enhancement is seen in approximately one-third of patients. Enhancement can be nodular, ring-like, or heterogeneous. Jensen et al. (17) described a case where gadolinium enhancement appeared only after 15 years of observation. The contrast enhancement was not related to malignant transformation but possibly to epileptic activity.
- T2-weighted image: Hyperintense.
- FLAIR image: Hyperintense. Some authors describe a hyperintense “ring sign” corresponding to the loose matrix at the margins (28).
- Differentiating DNETs from other tumor types: Differential diagnosis with pilocytic and low-grade astrocytomas can be difficult, but the subcortical location and the aspect of the cysts can give some clues. Oligodendrogliomas typically also have a subcortical location and are usually more calcified. Pleomorphic xanthoastrocytomas display a characteristic superficial enhancement with gadolinium, while angiocentric gliomas extend into the ventricle.
fMRI, DTI, and other MR imaging techniques
- Functional imaging: Functional imaging of adjacent eloquent areas and long tracts can be done with fMRI and DTI, similar to the use of these modalities in glioma imaging. MR spectroscopy and perfusion- and diffusion-weighted imaging have been described (4), but the roles of these modalities have to be further investigated in the differential diagnosis with glial tumors.
Nuclear Medicine Tests
Metabolic imaging (PET, SPECT)
- Amino acid markers: Typically, DNETs show low uptake of radiolabeled amino acid markers that are incorporated in proteins, like methionine and fluoroethyltyrosine (FET) (22, 29, 34).
- AMT: A high uptake of AMT (alpha methyl tryptophan) has been described by Juhasz et al. illustrating tryptophan catabolism (18). PET scanning can be useful for distinguishing DNETs that have important contrast enhancement on MRI from malignant glioma.
- SISCOM: Metabolic imaging can also be useful in the context of the planning of epilepsy surgery. SISCOM (subtraction of ictal and interictal SPECT coregistered to MRI) shows strong hyperperfusion in epileptic foci and can therefore be useful to detect perilesional areas of cortical dysplasia that appear normal on MRI but have to be resected to obtain control of intractable seizures (41).
Correlation of Tests
- Imaging usually sufficient for oncological management: Based on the imaging tests, a provisional diagnosis of DNET can sometimes be made. In other cases, the imaging will suggest a low-grade glioma. In both cases, this will be sufficient for an “oncological” resection of the lesion, with image guidance to maximize resection and minimize damage to surrounding tissue.
- Functional MRI, PET, and EEG useful for seizure management: In epileptic children, an oncological resection of the lesion may not be sufficient. In these cases, other functional imaging tests, metabolic imaging, video-EEG-monitoring, etc. are indicated. If all tests point in the same direction and indicate that the DNET is the epileptic focus (100 % concordance), it will generally suffice to resect the lesion. In cases with less concordance, extralesional dysplastic cortex may be responsible for the seizures and has to be resected as well. Infrequently, other foci at a distance from the DNET may also be found. Surgery in these cases must be very thoroughly discussed within the epilepsy surgery team, with the parents, and, from a certain age, also with the child.