Epidemiology

Dysembryoplastic neuroepithelial tumors are typically diagnosed in the first or second decade of life.1 Some isolated reports have been documented in the first 2 weeks3 and the sixth decade of life.4 DNETs appear to be more common in males, but definitive epidemiological data on incidence and prevalence is lacking.5 DNETs account for a significant minority of patients with medically refractory epilepsy.5 In a review of all neuroepithelial tumors at a single institution, DNETs were found in 0.6% of patients of all ages, 1.2% of patients under 20 years of age, and in 0.2% of patients 20 years of age or over.1,6

Clinical Presentation

Patients with DNET usually present with long-standing, medically refractory epilepsy without associated neurologic deficits. The seizures are typically complex-partial in nature and start prior to 20 years of age.5 Less common seizure types associated with DNET are generalized tonic-clonic, simple partial, or partial with secondary generalization. Chang et al7 reported these rates to be 32%, 22%, and 6%, respectively, in a series of 50 pediatric and adult DNET patients. DNETs are most often single supratentorial lesions confined to the cerebral cortex. They are more common in the temporal lobe cortex, either in the mesial or lateral aspect, but can occur anywhere in the cerebrum5 ( Fig. 29.1 ).

Natural History

Gross total resection (GTR) of the tumor is often curative and results in excellent seizure control. Without surgical resection, seizures arising from DNETs are difficult to control. Early series showed that biopsy alone does not result in good seizure control, whereas subtotal and gross total resection generally abolish or greatly reduce the frequency of seizures.8 Progression is rare after partial surgical resection, likely due to the indolent, slow-growing nature of the tumors.1,9–12 There are no reports of metastatic DNETs.

Associated Syndromes and Variants

Diagnostic Imaging

Although neuroimaging alone cannot establish the diagnosis of DNET, magnetic resonance imaging (MRI) studies often help to narrow the diagnostic possibilities and are crucial for surgical planning and assessment of treatment risks. A high-resolution MRI study is essential and should include the following sequences: T1- and T2-weighted, postcontrast, and fluid-attenuated inversion recovery (FLAIR). Additional sequences that may help with diagnosis and surgical planning include perfusion, diffusion-weighted images (DWI) with tractography, magnetic resonance (MR) spectroscopy, and functional MRI. Recent advances in imaging are able to define the anatomic relationship of the lesion to the adjacent cortex and underlying white matter. Double inversion recovery (DIR) MRI selectively images gray matter. By delineating the gray-white matter border, the exclusively cortical location of DNET can be appreciated with DIR.21 Other imaging modalities such as computed tomography (CT), positron emission tomography (PET), and single photon emission computed tomography (SPECT) may be useful in excluding other lesions (e.g., low-grade glioma, Taylor dysplasia, central neurocytoma, neuroepithelial cyst, ganglioglioma cyst, or pleomorphic xanthoastrocytoma) that share some radiographic features with DNET.22 DNETs are well-defined, nodular, heterogeneous lesions located in the cortical or subcortical region. There is minimal or no associated edema and little to no mass effect, although cystic changes are common ( Fig. 29.1 ).

On noncontrast CT scans, DNETs are hypodense and may have focal areas of hyperdense calcifications.1,23 Twenty percent of DNETs can have areas of nodular enhancement. Skull thinning, described as scalloping of the calvaria overlying the lesion, is also apparent on CT in up to 67% of patient.22,23

The MRI findings include the appearance of a thickened gyrus or a multinodular lesion.23 A precontrast T1-weighted sequence shows a heterogeneous mixed hypo- and isointense signal consistent with a pseudocystic structure. The T1-weighted postgadolinium sequence shows a hypointense lesion with rare focal areas of contrast enhancement. On T2-weighted sequences, the tumor appears hyperintense with little or no surrounding edema. Calcifications, if present, can be visualized as discrete areas of hypointensity within the tumor. A T2-weighted FLAIR sequence can identify a thin, well-defined, hyperintense “ring sign” that separates the lesion from surrounding normal brain, as described by Parmar et al.24 The authors attribute this ring-like appearance to neuroglial elements of the tumor. The “ring sign” was 82% sensitive and 90% specific for predicting DNET in this retrospective series of 11 patients with DNET and 21 patients with other gliomas. Rare case reports have documented the evolution of a DNET from a nonenhancing to an enhancing lesion over 15 years without obvious tumor growth25 ( Figs. 29.1 and 29.2; Table 29.1 ).

On PET imaging, DNETs demonstrate low metabolic activity.26 SPECT analysis shows hypoperfusion of the lesion with increased metabolism in the surrounding tissues,27 and on MRS, choline and N-acetylaspartate levels are normal within the tumor, though lactate may occasionally be detected.5,22