Introduction
Background
Dermoid tumors are not true neoplasms but are inclusion cysts composed of ectodermal elements. They are uncommon lesions, accounting for approximately 0.3% of all brain tumors. Dermoid tumors are often discussed with epidermoid tumors because of their similar appearance and developmental origin.
Pathophysiology
Dermoid and epidermoid tumors contain stratified squamous epithelium found in skin, but they also have histologic differences. Epidermoid tumors are lined with stratified squamous epithelium and have an outer connective tissue capsule. Dermoid tumors have an outer connective tissue capsule and are lined with stratified squamous epithelium that also contains hair follicles, sebaceous glands, and sweat glands. Centrally, both tumors contain desquamated epithelial keratin and some lipid material. The external surface of both tumors commonly has a smooth, lobulated, pearly appearance.
Dermoid tumors are thought to arise from misplaced ectodermal elements during the third to fifth week of embryonic life, when the neural tube closes at the midline. This may explain the frequent midline location of dermoid tumors. In contrast, epidermoid tumors are often located lateral to the midline of the cranium. Dermoid tumors are more commonly associated with dermal sinus tracts and spinal abnormalities than are epidermoid tumors.
Congenital epidermoid tumors may develop from inclusion of ectodermal epithelial elements at the time of neural tube closure or during the formation of the secondary cerebral vesicles. Acquired epidermoid tumors are believed to form due to trauma, frequently lumbar puncture, with epithelial cells deposited within the lumbar spinal canal. Sites of epithelial deposition can occur anywhere between the neural tube and the overlying skin surface. This distribution may account for the presence of dermal sinus tracts or dimples, which are more commonly associated with dermoid tumors.
Dermoid tumors are solitary; they expand slowly over many years due to the central accumulation of epithelial debris and glandular secretions. Common intracranial sites of dermoid tumors include the posterior fossa (within the fourth ventricle or cerebellar vermis) and the suprasellar region.
A congenital lumbar dermal sinus may terminate in an epidermoid or, less frequently, dermoid tumor within or near the conus medullaris or cauda equina and is often associated with spinal dysraphism. A congenital nasal dermal sinus may be associated with dermoid or epidermoid tumors. Other dermoid tumor sites include the scalp, skull, and orbit. Epidermoid tumors are most commonly located near the cerebellopontine angle, but they may also occur in parasellar areas and may be intradiploic in cranial bones. Intracerebral epidermoid occurrence is very rare.
Frequency
United States
Dermoid tumors account for approximately 0.3% of brain tumors and occur 4-10 times less frequently than do epidermoid tumors.
Mortality/Morbidity
- Central nervous system (CNS) dermoid and epidermoid tumors are usually benign, slow-growing lesions that rarely undergo malignant transformation.
- Morbidity depends on the location of the tumor and on the involvement of adjacent structures. The rupture of a dermoid tumor can cause a granulomatous chemical meningitis that, in rare cases, produces infarction from arterial vasospasm.
Race
No known racial predilection exists.
Sex
There is a slight male predominance of dermoid tumors. Epidermoid tumors occur with similar frequency in male and female patients.
Age
Intracranial dermoid tumors are seen most frequently in patients up to 20 years of age. In contrast, epidermoid tumors are most often first diagnosed in patients aged 40-50 years.
Anatomy
Dermoid tumors are often located at the cranial midline within the posterior cranial fossa, suprasellar cistern, and subfrontal areas. Epidermoid tumors are typically lateral and are most frequently located in the cerebellopontine angle; in the suprasellar and parasellar regions; in choroidal, sylvian, and interhemispheric fissures; or intraventricularly.
Spinal dermoid tumors are most commonly situated near the thoracolumbar junction and tend to involve the conus medullaris and cauda equina. About 50% are intradural intramedullary, and 50% are intradural extramedullary. Extradural location is least common. Less common sites of dermoid tumors include the scalp (the most common location in childhood), skull, orbit, nasal and oral cavities, and neck.
Presentation
Dermoid tumors grow slowly. Symptoms and signs are associated with the location of the tumor and the mass/pressure effect on adjacent tissues. Suprasellar tumors can cause visual abnormalities from compression of the optic chiasm. Diabetes insipidus and hypopituitarism may occur. Parasellar tumors may be associated with seizures from mass effect or extension to the temporal lobe and sylvian fissure.
Intraventricular dermoid tumors are most frequently located in the fourth ventricle and sometimes cause hydrocephalus. It has been suggested that the cerebrospinal fluid (CSF) flow may occur through interstices on the surface of the tumor.
Dermoid tumors in the spinal canal may cause back or leg pain due to mass effect. Headache and meningitis may occur if an associated dermal sinus tract becomes infected. Vertebral abnormalities, such as diastematomyelia, hemivertebra, and scoliosis, are frequently associated with dermal sinuses, dermoid tumors, or epidermoid tumors.
Dermoid tumors can rupture, releasing lipid contents into the ventricular or subarachnoid spaces (see Image 7). This causes a chemical meningitis that can lead to recurrent symptoms, most commonly headache. The subsequent meningeal inflammation may result in arterial vasospasm and, rarely, stroke and death.
Preferred Examination
Dermoid tumors are often first detected on computed tomography (CT) scans. Low attenuation values consistent with fat are suggestive of the diagnosis of dermoid tumor. Calcifications are frequent in dermoid tumors and are best seen through CT scanning.
Magnetic resonance imaging (MRI) is the preferred diagnostic procedure — not only because of its high spatial resolution, but also because of its multiplanar format — for optimal depiction of the location of dermoid tumors and the involvement of adjacent structures. The fat components that are characteristic of dermoid tumors are well demonstrated through MRI. Fat droplets located in the ventricles or subarachnoid spaces due to dermoid tumor rupture also are better appreciated with MRI than through other studies.
Differential Diagnoses
Arachnoid Cyst
Craniopharyngioma
Epidermoid, Brain
Other Problems to Be Considered
Ependymoma
Hemangioblastoma
Cystic astrocytoma
Germinoma
Cysticercosis
Teratoma
Lipoma
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References
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Further Reading
Keywords
dermoids, dermoid cysts, inclusion cysts, congenital epidermoid tumors, acquired epidermoid tumors, brain tumor, spinal dermoid tumors
