First described in 1988 by Daumas-Duport et al,1 dysembryoplastic neuroepithelial tumor (DNT) is a rare benign neoplasm of the central nervous system affecting young people and is clinically characterized by drug-resistant partial seizures and a normal neurologic examination. The latest World Health Organization classifies DNTs into the same group as neuronal and mixed glioneuronal neoplasms and defines them as tumor WHO grade I.2 Because DNT is characterized by distinctive biological and clinicopathological natures, a correct preoperative diagnosis is helpful for planning surgical strategies and improving prognosis. The MR images of 13 cases, as well as magnetic resonance spectroscopy (MRS) results for five of the 13 cases, with surgically and pathologically proved DNT were retrospectively reviewed in this study. The purpose of this study was to characterize DNT using magnetic resonance imaging (MRI) and MRS, and to analyze the value of these two techniques in the diagnosis of DNT.
Imaging studies from 2001 to 2007 of 13 patients with DNT were retrospectively reviewed. All cases were postsurgically diagnosed on the basis of pathologic findings. Seven male and six female patients were included in the study. The age at surgery ranged from 8 months to 29 years (mean age, 15 years). Seizure was the main clinical manifestation in all cases. Seizure duration ranged from 1 month to 18 years, with a mean duration of 8 years before surgery. With the exception of three cases in which seizure duration was 1, 6 and 8 months prior to detection of the foci by MRI and subsequent tumor resection, the seizures were drug-resistant (ten cases). Seizures began before the age of 20 years in eleven cases. Simple partial seizures occurred in two cases, complex partial seizures in eleven cases. Partial seizures were subsequently generalized in five cases. One patient presented with dizziness and anomalopia, two with mental retardation, three with memory impairment and one with calculation decline. No other neurologic deficits were found. No febrile convulsions or status epilepticus occurred in any of the cases.
MR plain scanning using 1.0-T or 1.5-T MR scanners (Siemens Magnetom Impact and Sonata, Germany) was performed on all patients. The following conventional sequences were performed: sagittal and axial, spin echo, T1-weighed sequences (repetition time ms/echo time ms = 450/15, 6-mm sections); sagittal and axial, spin echo, T2-weighted sequences (repetition time ms/echo time ms = 4000/90); axial, fluid attenuated inversion recovery (FLAIR) sequence (repetition time ms/inversion time ms/echo time ms = 8500/180/135). In addition coronal scanning was performed in eight patients. Nine patients received gadopentetate dimeglumine (0.1-0.2 mmol/kg). In five patients, single-voxel point-resolved spectroscopy (PRESS) was performed with a voxel size of 15-25 mm3 and imaging parameters of 1500 ms/135 ms (TR/TE) and 196 averages. The spectra were obtained from the central portion of the tumor. All data postprocessing were performed with the software provided by the manufacturer.
Using the various sequences described above, the images were assessed with respect to the distribution and extent of the lesions, signal features, mass effects of the tumor, calcification and enhancement characteristics. MRS postprocessing included zero-filling, Gaussian apodization, Fourier transformation, water reference processing, frequency shift correction, and phase and baseline correction. Peak integral values for N-acetylaspartate (NAA), choline-containing compounds (Cho) and creatine (Cr) were determined using a curve fitting algorithm at their respective ppms. Peak integral values were normalized to the internal Cr peak.
Location and appearance
All tumors were located in the supratentorium. Cortical involvement was present in all cases; additional involvement of the subcortical white matter was found in two. The tumor was located in the temporal lobe in nine patients (69.2%), the frontal lobe in three (23.1%), and the boundary between the temporal and occipital lobes in one (7.7%). In one frontal tumor, abnormal gray matter was distributed in the area between the DNT lesion and the lateral ventricle.
The DNT lesions consisted of a single cyst or multiple cysts in 11 cases and a thick gyriform configuration in two patients. DNT demonstrated a triangular pattern of distribution in five patients (Figure 1), a rounded pattern in three patients, and an irregular pattern in five patients.
On T1-weighted images, the lesions all appeared hypointense. On T2-weighted images, the lesions were always of high signal intensity. The margins between the DNT and normal tissue were sharp in eight cases, whereas five lesions had poorly defined borders. The lesions appeared to be isointense or hyperintense on FLAIR images. In all but four cases, the lesions seemed to be divided by thin septa. These septa were best seen on FLAIR sequences. FLAIR images revealed a hyperintense ring around the tumors in 9 patients (Figure 2).
Hemorrhage and calcification
Hemorrhage or calcification was not showed in any case.
Peritumoral edema and mass effect
Peritumoral edema was not found in any of the cases. Slight effacement of the ventricles was seen in two cases with a diameter of more than 25 mm (27.4%) but no displacement of the midline was shown. Significant mass effect was not found in the remaining lesions. A deformation of the overlying skull was observed in six cases.
Contrast enhancement was performed on nine patients but enhancement was absent in five of these cases (Figure 3A-3C). In the four cases which responded to enhancement, it revealed a nodular appearance in two patients, a central rimlike area in one patient and patchyness of the lesion in one patient.
The MRS showed a low NAA peak, a normal Cr peak and a lack of elevation of the choline-containing component (Cho) or Cho-Cr ratio (Cho/Cr) in the five patients on which this test was performed (Figure 3).
With the development of neuroimaging techniques and epilepsy surgery in recent years, more and more attention has been paid to DNT. The clinical presentation of DNT is stereotypical: a long history of partial refractory seizures beginning before the age of 20 years old in adolescents and young adults, and the absence of progressive neurologic deficit.1,3,4 In this study, only 2 patients had their first seizure after 20 years of age, which is consistent with rare reports of cases with late seizure onset in the literature. The epileptogenic mechanism of DNT is still unclear. DNT may cause seizures by its intrinsic epileptogenicity as well as by compressing or irritating the adjacent cortex like other glioneuronal tumors.5 Other epileptogenic mechanisms, such as secondary hippocampus sclerosis, may exist.6 From a clinical perspective, it is also noteworthy that DNT is frequently associated with cortical dysplasia (CD) which is known to have intrinsic epileptogenicity. One study showed that seizures often recur postoperatively as a result of DNT with CD. This finding showed that CD associated with DNT might be a part of the epileptogenic zone.5 Therefore, it seems necessary to resect the CD along with the DNT, using invasive monitoring/intraoperative ECoG in order to achieve seizure control. In general, no neurologic deficit is found in patients with DNT. When present, neurologic deficit is dependent on the location of the tumor.
Pathologically, DNT is a benign, predominantly intracortical lesion composed mainly of a population of heterogeneous cellular components, such as oligodendrocyte-like cells with admixtures of mature neurons and astrocytes, which are located in a myxoid or mucinous interstitial matrix.7 Involvement of the cortical gray matter and a multinodular growth pattern are typical findings. The cortex adjacent to a DNT shows a disordered architecture. Generally, pathological examination leads to the diagnosis of DNT;8 however, some oligodendrogliomas harboring cystic changes and entrapping neurons may mimic DNT.9 For these reasons, Fernandez et al10 emphasized the need for a multidisciplinary approach comprised of clinicians, neuroradiologists, and pathologists to diagnose DNT.
High-resolution MR imaging has played a significant part in recognition of low-grade tumors and tumor-like lesions as a major cause of chronic focal epilepsy.11 Modern MR techniques with high resolution and high contrast permit a much more detailed evaluation of the various tumor components and reflect the lesions' intrinsic pathological changes. Because of this, MR should be routinely used to examine patients to guide their treatment and to identify those patients who are likely to benefit from epilepsy surgery.
MRI findings of DNT are characterized by a triangular pseudocystic appearance predominantly in an intracortical location. The triangular appearance results from a tumor width which is maximal at the cortical level and decreases toward the ventricles, leading to a triangular pattern of distribution, usually best seen on coronal images. Because DNT is generally accepted as dysembryoplastic in origin, this triangular pattern may be related to neuronal development along radiating glial fibers.10 In this study, all cases were located in the cortex. The expected triangular pseudocystic appearance appeared in five cases. Typically, DNT displays hypointensity on T1-weighted MR images and hyperintensity on T2-weighted MR images. On the FLAIR sequence, the lesion appeared to be isointense or hyperintense.10 In contrast with low-grade gliomas, a hyperintense ring sign at the periphery of the tumor and internal septations on FLAIR images in patients with DNT are characteristic.8,10
In this study, MRI manifestations of the hyperintense ring sign and internal septations were detected in nine cases. These appear to be true septa which are not caused by reflections of sulci into the tumor. Instead, they may indicate that the architecture of the DNT is nodular and may reveal the boundaries between normal cortex, cellularly dense glial nodules, and zones of loosely packed specific glioneuronal element (SGNE).8,10,12 Typically, edema and mass effects on midline structures are not associated with DNTs, and contrast enhancement is absent.1 In the present study, prominent nodular, central rimlike or patchy enhancements were detected in four patients. These enhancements were similar to those reported by Fernandez et al.10 Contrast enhancements in DNT may be associated with pathological paraplastic blood vessels within the tumor. Fernandez et al10 also showed that an MRI manifestation of a triangular pseudocystic appearance and internal septations was helpful in making a DNT diagnosis when the pathological findings were obscure, making diagnosis difficult. Studies1,13,14 showed that calcified portions had been identified in DNTs. No calcifications were found in this study; however this may be explained by the relative insensitivity of the MRI technique to small calcifications.15
MRS is a noninvasive technique that can detect metabolic abnormalities in brain tissue.16 Previously reported MR spectroscopic findings in brain tumors included a decrease in NAA, a marker of neuronal integrity and an increase in the Cho, resulting from increased cell membrane and myelin turnover.17,18 Others have suggested that the presence of a Cho signal in a proton MRS reflects cellular proliferation in tumors.19 In this study, the spectra showed a low NAA peak, a normal Cr peak and the lack of an elevated Cho or Cho/Cr in the five patients. The low NAA in DNT is probably due to a reduction in the number of neurons per unit volume resulting from the presence of specific glioneuronal elements and mucinous material in the tumor.20 The specific finding that DNT lacks an elevated Cho-Cr ratio suggests a lack of cellular proliferative activity in DNT. This result, supplemented with conventional MRI findings, can aid in the diagnosis of DNT. Because only five patients were studied by MRS in this study, further study of the MRS findings of DNT will be necessary in the future.
The MRI findings of DNT are stereotypical. When coupled with typical clinical presentations and a history of chronic focal epilepsy, MR findings may strongly suggest the preoperative diagnosis. MRS is helpful for making a correct presurgical diagnosis of DNT.
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