Dermoid Cyst

Clinical Information: A five-year-old white female presents with complaints of headache, nausea, vomiting, blurred vision and hemiparesis for approximately three days. No complaints of fever or chills are noted.

T1WT2WT1+Gd

Imaging Findings: CT scan (not shown): There is a fairly well-defined mass in the region of the left thalamus and hypothalamus which has areas of both low and intermediate attenuation. No definite calcifications are seen within the mass. Some lateral ventricle dilatation is seen presumably secondary to compression of the foramen of Monro. MR: There is again a well-circumscribed left thalamic hypothalamic mass measuring 2.8 X 2.8 X 3.2 cm. The lateral ventricle dilatation is again seen. Mass effect is also noted upon the cerebral peduncles particularly on the left. It is mostly low in signal on T1 images and non-enhancing. It is high in signal on T2.

The patient was referred for excision of the mass and ventriculostomy placement. During surgery, virtually all the mass was removed. It was thought to be extra-axial, and it was noted to have a firm white capsule. It was felt that most of the mass was removed, although portions may remain since it was rather adherent to the lateral wall of the third ventricle, portions of the optic chiasm, and nearby vessels/nerve.

Pathology: Excisional biopsy: sections through the suprasellar lesion show a fragmented, thin-walled cyst lined by true epidermis with numerous cutaneous adnexal appendages. Minimal focal capsular ossification is present (a lamina of ossification is noted on low power microscopy). The lumen of the cyst contains abundant keratinous debris in the form of squames and occasional hair shafts.

Discussion: CNS Dermoid Cyst.

CNS dermoid cysts are considered a non-neoplastic ectodermal inclusion cyst. Most are thought to represent a congenital abnormality resulting from the inclusion of ectodermal elements at the time of neural tube closure or during the formation of secondary cerebral vesicles. Some may occasionally be formed secondary to traumatic or iatrogenic events resulting in the implantation of skin elements such as with a lumbar puncture.

Dermoid cysts are associated with several misconceptions. First, they are not true neoplasms, but rather simply inclusion cysts. They are also not mesodermal in origin as many people believe. This misconception has resulted from the presence of hair, sebaceous glands, and sweat glands within dermoid cysts. Many incorrectly believe these are mesodermal structures when they are actually ectodermal in origin.

Dermoid cyst pathology typically demonstrates a well-defined lobulated cystic mass containing lipids and cholesterol. A dermal sinus may be present, especially if it is located in the region of the spine or posterior fossa. Cyst rupture may occasionally occur with scattering of its fatty content throughout the ventricles and subarachnoid space. Therefore, a secondary chemical meningitis may occasionally be seen. Unlike epidermoids, dermoid cysts are typically in or near the midline. Microscopically, it demonstrates an outer, dense, fibrous capsule with an inner, squamous, epithelial lining. This lining includes hair and dermal appendages not seen in epidermoids such as hair follicles, sebaceous glands and sweat glands. Calcifications are common and may be dystrophic or dental enamel (also ectodermal).

Epidemiologically, this is a rare tumor which comprises less than 1% of primary intercranial tumors. It is much less common (four to nine times) than epidermoid cysts. It typically presents in patients under thirty years of age, which is a slightly younger presentation than for epidermoid cysts. There is a mild, male predominance according to some references, although this is disputed.

Clinically, the most common symptoms are seizures and headaches. This is probably largely related to secondary mass effect. The patient may also demonstrate an occipital/cutaneous dimple or sinus tract. If there has been rupture of the cyst, the patient may present with a chemical meningitis, seizures, and/or vasospasm. The vasospasm may, in fact, lead to infarction and even death.

Imaging shows the most frequent location to be the spinal canal in the lumbosacral area. This is followed by the parasellar, frontal basilar, posterior fossa, and orbital regions. This is, in fact, the most common orbital tumor in children.

On CT scans, it is typically a well-defined round hypodense mass. It typically has an attenuation consistent with fat (-20 to -40 hus). If there has been rupture, then scattered low density fatty droplets may be scattered throughout the ventricles and subarachnoid space. A fat/CFS fluid level may also be present. Calcifications, particularly in the capsule are particularly common. There is no enhancement after contrast administration.

MR imaging typically demonstrates high signal on T1 and variable signal on T2. This is consistent with the lipid and cholesterol which typically collects within the dermoid cyst. Our case is, in fact, atypical due to the low signal on T1. Hair may be noted as fine, low signal structures within the cyst. If the cyst ruptures, then high signal droplets on T1 images may be seen scattered throughout the CSF. Again, a fat/CSF fluid level may also be identified. As with other fatty masses, chemical shift artifact may also be present.

Angiographically, dermoid cysts typically show no significant enhancement. However, vasospasm and secondary infarction may be noted, particularly in cases with secondary meningitis from rupture of the cyst.

The differential diagnosis in a child of this age for an extra-axial mass near the suprasellar region would typically include dermoid cyst, epidermoids, teratoma, glioma, and craniopharyngioma.

References:

1. Smirniototopoulos and Chiechi: "Archives of the AFIP, Teratomas, Dermoids and Epidermoids of the Head and Neck". Radiographics. November 1995. Vol. 15, No. 6:1447-1454.
2. Diagnostic Neuroradiology. Osborn AG. Mosby Publishing, 1994. Pp. 635-636.

Submitted by Dr. Michael Smith
Radiology Resident, Baptist Health System
Children's Hospital
Birmingham, Alabama