intracranial dermoid cyst
Intracranial dermoid cysts are uncommon lesions with characteristic imaging appearances. They can be thought of as along the spectrum: from epidermoid cysts at one end (containing only desquamated squamous epithelium) and teratomas at the other (containing essentially any kind of tissue from all three embryonic tissue layers).
On imaging, they are usually well-defined lobulated midline masses that have low attenuation (fat density) on CT and high signal intensity on T1-weighted MR images. Typically they do not enhance after contrast administration.
Dermoid cysts account for ~0.5% of all primary intracranial tumors and are slightly more common in females . Typically, dermoid cysts present in the first three decades of life .
Many intracranial dermoid cysts are asymptomatic and are only found incidentally. Often there is a long history of vague symptoms, with headache being a prominent feature . Symptomatic clinical presentation usually occurs in one of two ways:
- mass effect
- compression of adjacent structures, e.g. optic chiasm
- rupture (spontaneous, traumatic, or iatrogenic (at resection))
- leakage of sebum into the subarachnoid space results in aseptic chemical meningitis
- the presentation is variable, ranging from a headache to seizures, vasospasm, and even death
There may be an association with Goldenhar syndrome (oculo-auriculo-vertebral dysplasia). This combination of both entities may not be coincidental but due to their related embryological origin . An association with Klippel-Feil syndrome has also been reported.
Dermoid cysts are thought to occur as a developmental anomaly in which embryonic ectoderm is trapped in the closing neural tube between the 5-6 weeks of gestation .
Dermoid cysts, like epidermoid cysts, are lined by stratified squamous epithelium. Unlike epidermoid cysts, however, they contain epidermal appendages as well, such as hair follicles, sweat glands and sebaceous glands. The latter secrete the sebum that gives the characteristic appearance of these lesions on CT and MRI.
A common misconception is that dermoid cysts contain adipose tissue. This is not the case, as lipocytes are mesodermal in origin, and dermoid cysts by definition are purely ectodermal. A dermoid cyst with adipose tissue would be a teratoma.
Intracranial dermoid cysts are typically located in the midline, although they can grow towards one side or the other. Locations include:
- midline sellar and suprasellar
- frontonasal region
- posterior fossa/vermis
It is interesting to note that publications vary regarding the most common location of dermoid cysts .
Historically, when skull radiographs were routinely used in the assessment of suspected intracranial pathology, a focal lucency due to the low density of fatty sebum, would suggest the diagnosis .
Typically dermoid cysts appear as well defined low attenuating (fat density) lobulated masses. Calcification may be present in the wall. Enhancement is uncommon, and if present should at most be a thin peripheral rim.
Very rarely they demonstrate hyperdensity, thought to be due to a combination of saponification, microcalcification and blood products. This most often occurs when present in the posterior fossa, although the reason is uncertain .
Ruptured dermoid cysts are characteristically associated with fat-attenuation material in the subarachnoid spaces, such as intraventricular fat-fluid levels or fat globules in the sulci.
Unlike intracranial lipoma that follows fat density on all sequences, intracranial dermoid has more variable signal characteristics :
- typically hyperintense (due to cholesterol components)
- hyperintense droplets in the subarachnoid spaces may be visible if rupture has occurred
- T1 C+ (Gd): generally do not enhance
- extensive pial enhancement may be present in chemical meningitis caused by ruptured cysts
- T2: variable signal ranging from hypo- to hyperintense
Cyst rupture is the most common complication and often responsible for the patient's symptoms at presentation. The cyst contents results in chemical meningitis. The best diagnostic clue for cyst rupture is fat-like droplets in cortical sulci, subarachonoid cisterns and ventricles. Liquid fat content inside the ventricles gives a fat-fluid level.
Treatment and prognosis
Dermoid cysts, when symptomatic, can be surgically excised. Recurrence is uncommon, provided that complete excision is achieved. Sometimes, due to local adhesion of the capsule to vital structures, incomplete excision must be performed. In either case, recurrent growth is slow .
Extremely rare malignant transformation into squamous cell carcinoma has been reported .
The differential is limited to pituitary region lesions with intrinsic high T1 signal or to an even smaller list (as CT and fat-suppressed sequences are often available) of lesions that contain fat:
- intracranial lipoma
- homogeneous fat attenuation/signal intensity
- chemical shift artifact
- intracranial epidermoid cyst
- attenuation similar to CSF
- restricted diffusion on DWI
- intracranial teratoma: immature
- usually occur in the pineal region
- most are strikingly hyperintense on T2
- most enhance strongly
- contains epidermal appendages such as hair follicles, sweat glands and sebaceous glands; does not contain fat tissue
- low attenuation (fat density) on CT and high signal intensity on T1-weighted MR images
- may rupture: droplets in the subarachnoid space and chemical meningitis