| Discussion: |
Dermoid tumors are not true neoplasms but inclusion cysts composed of ectodermal
elements. They are uncommon lesions, accounting for approximately 0.3% of all brain
tumors. They are often discussed with epidermoid tumors because of their similar
appearances and developmental origins.
Dermoid and epidermoid tumors both contain stratified squamous epithelium found
in skin, but they 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, which 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 weeks 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.
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 be due
to trauma, frequently from lumbar puncture with epithelial cells deposited within
the lumbar spinal canal. Sites of epithelial deposition can occur anywhere between
the neural tube and 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 and 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 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. 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 may also occur in parasellar areas and
intradiploic in cranial bones. Intracerebral epidermoid occurrence is very rare.
In the US: Dermoid tumors account for approximately 0.3% of brain tumors.
CNS dermoid 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 rarely results in infarction from arterial
There is a slight male predominance of dermoid tumors.
Intracranial dermoid tumors frequently occur in young patients up to 20 years of age.
Dermoid tumors are often located at the cranial midline within the posterior
cranial fossa, suprasellar cistern, and subfrontal areas. Less common sites of
dermoid tumors include the scalp (the commonest location in childhood), skull,
orbit, nasal, oral cavity, or neck.
MRI is the preferred diagnostic procedure because of its high spatial resolution
and multiplanar format for optimal depiction of the location of dermoid tumor
and the involvement of adjacent structures. The fat components that are
characteristic of dermoid tumors are well demonstrated by using MRI. Fat
droplets located in the ventricles or subarachnoid spaces due dermoid tumor
rupture are also better appreciated with MRI than with other studies.
Findings on MRI:
Most dermoid tumors have signal intensity characteristics similar to fat,
that is, they are hyperintense on T1-weighted images and hypointense on T2-
weighted images. Fat-suppression techniques may be helpful to confirm the
presence of fat in the lesion. Centrally, dermoid tumors may appear
inhomogeneous due to the presence of hair follicles, calcifications, and
Rupture of a dermoid tumor can result in fat droplets in the subarachnoid
spaces or ventricles with T1 high signal intensity. Fat-fluid levels may be
found anteriorly in the lateral ventricles. A chemical-shift artifact is often
present on T2-weighted images as a markedly hypointense band posterior at the
Vascular displacement or encasement by the dermoid tumor may be demonstrated
by means of magnetic resonance angiography (MRA) or CT angiography (CTA).
Degree of Confidence:
A unilocular cystic mass located at or near the cranial midline with signal
intensity similar to fat is highly suggestive of a dermoid tumor.
Complete surgical excision is the goal of treatment for dermoid tumors.
Resectability depends on the location of the tumor and the extent of encroachment
and inflammatory attachment to adjacent blood vessels and nerves. Scalp and
intradiploic tumors of the skull are more easily excised completely than
intracranial tumors. Chemical meningitis may occur during removal of the
dermoid tumor if its contents spill into the subarachnoid or ventricular
spaces. Chemotherapy and radiation therapy are not useful in the management
of these tumors
emedicine article by:
Author: Conway Lien, MD, Consulting Staff, Department of Radiology, Santa Clara
Valley Medical Center
Coauthor(s): Mahesh R Patel, MD, Chief of MRI, Department of Radiology, Santa
Clara Valley Medical Center