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Neuroradiology Case of the Week

Case 65

Karin Westesson and P-L Westesson MD, PhD, DDS

Clinical Presentation: MR imaging of the brain was obtained in a 36-year-old male with a long history of epilepsy to evaluate for mesial temporal sclerosis or a left temporal lobe mass.

Radiological Findings:  MR imaging showed a well circumscribed, non-enhancing lobulated 1.8 cm mass lesion in the left hippocampus. The lesion was hyperintense on FLAIR and T2-weighted and had low T1 signal.

Diagnosis: Dysembryoplastic Neuroepithial Tumor (DNET)

Discussion:  A DNET is a rare, benign neoplasm that is occasionally cystic and superficially located in temporal lobe [1]. It almost always presents with partial complex seizure disorders and with no focal neurological deficit [2]. Since it is slow growing as seen by the scalloping of the orbital roof and has a low proliferative potential, there is often skull remodeling in the patient [2,3]. To be considered a DNET histologically, the lesion must show at least one of the following characteristics: a specifically glioneuronal element, a nodular component, or an association with cortical dysplasia [4]. Because the tumors are generally cortical, convex DNETs may erode the inner table of the calvarium.
    DNETs originate embryonically from abnormal dysplastic cells in the germinal matrix, most likely during the 2nd trimester [2]. They extend along the migratory path of the neurons toward the cortex, which explains why DNETs are associated with cortical dysplasia (PR). Histologically, DNETs consist of a mixture of neuroepithelial cells with a multinodular architecture. The glial nodules and masses of oligodendrocyte-like cells are dispersed around the mucinous cysts [5].
    Imaging of DNETs is relatively non-specific and there are many differential diagnoses: ganglioglioma, astrocytoma and oligodentroglioma. On CT scans, the DNET will appear as a cortical mass with low attenuation without edema and most do not enhance with contrast [6]. It may appear as a stroke but does not have the temporal evolution to atrophy and has minimal mass effect. On MR imaging, it appears well-circumscribed and pseudocystic. It is moderately hyperintense on T2 weighted images (PR), and on T1 it appears mixed hypointense. DNET’s appear as benign cysts with increase signals on proton density weighted imaging. It is virtually impossible to differentiate between a low-grade astrocytoma, a ganglioglioma and a DNET on MRI (4). When a DNET is partially cystic and calcified, it is also indistinguishable from a low-grade oligodendroglioma [3].
    Surgical resection of the epileptogenic foci is generally the preferred treatment and DNETs generally don’t come back, resulting in a low recurrence rate.

References:

1. Zimmerman RA, Bilaniuk L. Brain tumors. IN: Neuroimaging: Clinical and Physical Principles. Eds. Robert A. Zimmerman, Wendell A. Gibby, Raymond F. Carmody. New York: Springer, 2000.
2. Barkovich AJ. Pediatric Neuroimaging, 3rd Ed. Philadelphia: Lippincott, Williams & Wilkins, 2000.
3. Atlas SW, Lavi E, Fisher PG. Intraaxial brain tumors. IN: Magnetic Resonance of the Brain and Spine, 3rd ed. Ed. Scott W. Atlas. Philadelphia: Lippincott Williams, Wilkins, 2002.
4. Friedland RJ, Bronen RA. Epilepsy. IN: Clinical Magnetic Resonance Imaging, 2nd ed. Eds. Robert R. Edelman, Michael B. Zlalgkin, John R. Hesselink. Philadelphia: W. B. Saunders, 1996.
5. Ellison D, Love S, Chimelli L, Harding B, Lowe J, Roberts GW, Vinters HV. Neuropathology. St. Louis: Mosby, 1998.
6. Osborn AG. Meningiomas and other nonglial neoplasms. IN: Diagnostic Neuroradiology. St. Louis: Mosby, 1994.