The T2/FLAIR mismatch sign describes the MRI appearance considered a highly specific radiogenomic signature for astrocytomas (IDH-mutant, 1p19q non-codeleted), as opposed to other low-grade gliomas 4,5.
Indeed, a 2021 meta-analysis showed a pooled specificity of 100% for identifying tumours with IDH-mutant, 1p19q non-codeleted status, though pooled sensitivity was 42% 6.
It is particularly helpful in distinguishing an astrocytoma from an oligodendroglioma that will not demonstrate T2/FLAIR mismatch.
Radiographic features
MRI
On T2 weighted images, these tumours have extensive areas of fairly homogeneous and strikingly high signal. On T2/FLAIR, instead, the majority of these areas become relatively hypointense in signal due to incomplete suppression. At the margins of the tumour, a rim of hyperintensity is usually seen.
This appearance is typical of the entity previously known as protoplasmic astrocytoma which is, however, no longer recognised as a distinct entity in the current WHO classification of CNS tumours.
Differential diagnosis
It is important to note that similar signal characteristics can be seen in the bright rim sign of dysembryoplastic neuroepithelial tumours (DNET), although DNETs are almost invariably smaller 3.
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