11 Dermoid cysts are rare, nonneoplastic, congenital, ectodermal inclusion cysts. Lipomas are nonneoplastic and result from the maldifferentiation of the primitive meninx. Both lesions are extraaxial and contain fat, although the fat content of dermoid cysts is variable. Differences in natural history, pathology, and treatment of these lesions confers significance to differentiation by imaging (Table 11.1). Ruptured dermoids present with a pathognomonic imaging appearance with scattered speckling of abnormal T1 shortening within the subarachnoid space, sulci, and ventricles with characteristic fat-fluid levels. The characteristic computed tomography (CT) appearance is fat-cerebrospinal fluid (CSF) levels in the ventricles. Other presentations include fat densities within the sulci and subarachnoid spaces, and occasionally an extraaxial or combined extraaxial and intraaxial mass. The only other consideration (for the MRI imaging appearance of fat signal in the CSF space) would be prior intrathecal administration of Pantopaque, which can be confirmed by history or by correlating with hyperdense foci on unenhanced head CT.
Dermoid Cysts and Lipomas
| Lipomas | Dermoid Cysts | |
|---|---|---|
| Location | Usually midline interhemispheric fissure (~50%) Quadrigeminal/superior cerebellar cistern (15–25%) Suprasellar/interpeduncular (15%) Uncommon Cerebellopontine angle, sylvian fissure, surface of hemispheres | Midline Suprasellar Parasellar Posterior fossa Frontonasal Orbital region |
| CT | Homogeneous Hypoattenuating Fat density-40 to -100 HU |
