FINAL DIAGNOSIS: DYSEMBRYOPLASTIC NEUROEPITHELIAL TUMOR (DNT)
DNTs are typically supratentorial intracortical lesions, and patients become symptomatic mostly in the first two decades of life. About 100 cases have been reported in the literature since the first description in 1988 (1). Clinically, patients suffer chronic, intractable seizures (often the partial complex type). The favored locations for these lesions are the temporal or frontal lobes; in the case of multifocal disease, the parietal lobe might be involved. Other sites are very rare. Usually the lesions are stable clinically and radiologically for years.
Cysts or focal calcifications might be present. MRI scans underline the intracortical location and the well-defined margins usually without edema and obvious mass effect. Signal intensity is very low compared with the gray matter on T1-weighted images and hyperintense on T2-weighted images. Post-gadolinium images rarely show scant, focal, and peripheral enhancement (2).
Grossly, DNTs are mucinous or gelatinous multinodular lesions of very friable consistency, making these lesions especially susceptible to artefactual fragmentation and loss of diagnostic tissue during handling and processing (2).
Microscopically, all DNTs exhibit multiple intracortical nodules of varying size. The matrix is rich in acidic mucopolysaccharides staining strongly with Alcian blue. Most of the tumor cells closely resemble those of a well-differentiated oligodendroglioma. Unlike oligodendrogliomas, however, the cells are often arranged in clusters or intricate patterns. In other nodules, microcystic appearance predominates. The neurons are also well-differentiated and of variable size, lying randomly scattered in clear spaces (so-called "floating" neurons or neuroglial elements). As in this case, an astrocytic component or clusters of bipolar astrocytes reminiscent of pilocytic astrocytomas may occasionally be seen. The adjacent cerebral cortex often exhibits cortical dysplasia with disturbed lamination and disarray in architecture. Necrosis and endothelial proliferation are rare, and mitoses are uncommon. Proliferation studies have shown most cases to have MIB-1 labeling indices of <1% (3,4).
The principal differential diagnoses of DNTs are oligodendrogliomas and gangliogliomas. Oligodendrogliomas can be differentiated by the presence of more polymorphic cells, chromasia, and more pronounced perineuronal satellitosis. In gangliogliomas, the astrocytic component is more pronounced, the stroma is collagenous reticulin-rich (featuring inflammatory cells), and the neurons are more pleomorphic. In this case, the differential diagnosis of the lesion before stereotactic biopsy was that of a craniopharyngioma or low-grade glioma according to the radiological findings. The correct diagnosis could be established, based on the examination of very small (stereotactic) specimens, with the help of knowing the clinical data (seizures, long anamnesis) and neuroimaging features (intracortical lesion). However, it may be impossible to distinguish DNTs from differential diagnoses with small, non-representative (stereotactic) specimens.
Contributed by Bernd F.M. Romeike, MD and Wolfgang Feiden, MD, University Clinics of the Saarland, Department of Neuropathology