Clinical case

History : 30 years old man with new seizure

T2 and FLAIR images demostrate left frontal lobe cortical signal abnormality with subcortical cystic areas, this lesion has hypointense T1 signal and doesn't shows post contrast enanchement.

Diagnosis: Dysembryoplastic neuroepithelial tumour (DNET)

Dysembryoplastic neuroepithelial tumour (or DNET) is a benign (WHO Grade I) slow growing tumour arisig from either cortical (vast majority) or deep grey matter. They are thought to arise from secondary germinal layers and are frequently (upto 80% of cases) associated with co-existent cortical dysplasia, and is characteristically the cause of intractable partial seizures (see temporal lobe epilepsy).
They demonstrates essentially no growth over time, although very gradual increase in size has been described. As expected prognosis is excellent and even though these lesions are often incompletely resected, tumour progression is uncommon. Additionally even in cases of incomplete ressection, seizure frequently cease.
Location
The temporal lobe is the most common location, but all parts of the CNS containing grey matter are potential locations.
-temporal lobe: over 60% of cases
-frontal lobe: 30% of cases
-caudate nucleus
-cerebellum: presentation is then more commonly with ataxia rather than seizures
-pons

Radiographic Features
CT
if cortical may scallop the inner table of of the skull vault (44 - 60%)
calcification in 20 - 40% (more common histologically)
low density
no enhancement
MRI
high T2WI signal with high signal 'bubbly appearance'
FLAIR mixed signal intensity with bright rim sign.
low on T1WI
haemosiderin staining uncommon on Gradient Echo as bleeding into DNETs only occasionally occurs, although as calcification is not infrequent
no enhancement
no restriction on DWI
non-specific MRS although lactate may be present

Differential diagnosis
The differential diagnosis will depend on the location of the tumour.
Mesial temporal lobe
see also temporal lobe tumours
tumours (in order of decreasing frequency)
ganglioglioma
DNET
pilocytic astrocytoma
diffuse astrocytoma
oligodendroglioma
pleomorphic xanthoastrocytoma
cysts
neuroepithelial cyst
choriod fissure cyst
other
herpes simplex encephalitis: usually some bilateral changes, and different presentation
limbic encephalitis: usually some bilateral changes, and different presentation
mesial temporal sclerosis
Cortical
low grade astrocytoma
ganglioglioma
pleomorphic xanthoastrocytoma
oligoastrocytoma/oligodendroglioma
taylor dysplasia