Radiography
Findings
Focal erosion of the calvarial bone may be present adjacent to a dermoid tumor of the scalp or orbit. In particular, epidermoid tumors of the skull can cause local bone expansion or erosion and round, intradiploic, lytic lesions with a thin, sclerotic margin. Deformity of the sella may occur from parasellar dermoid tumors. Spinal dermoid tumors can cause focal vertebral bone erosion, widening of the vertebral spinal canal, and flattening of the pedicles and laminae.
Degree of Confidence
Large, calvarial dermoid tumors can be visualized on plain skull radiographs, but radiographs have low sensitivity in depicting intracranial dermoid tumors. CT scanning and MRI demonstrate the location and imaging features of intracranial lesions.
Computed Tomography
Findings
A dermoid tumor appears as a well-circumscribed, predominantly cystic mass on a CT scan, with decreased attenuation in the range of -20 to -40 HU because of its fat content (see Image 1, Image 5). The tumor may appear slightly heterogeneous due to additional ectodermal elements, including hair follicles, sebaceous glands, and sweat glands. Calcifications are frequent in the wall of the tumor.
Contrast enhancement is uncommon, but some enhancement may be seen in the wall. If enhancement is present in a suprasellar tumor, other diagnoses should be considered, including craniopharyngioma, teratoma, or germinoma. Fat droplets in the ventricular or subarachnoid spaces strongly suggest rupture of a dermoid tumor.
Dermoid tumors located in the fourth ventricle do not often cause obstructive hydrocephalus. The differential diagnoses include tumors that do commonly cause hydrocephalus, such as ependymomas, medulloblastomas, hemangioblastomas, or cystic astrocytomas of the fourth ventricle.
Epidermoid tumors often have attenuation similar to CSF, but they may have hyperattenuation on nonenhanced images because of high tumoral protein content, hemorrhage, or cellular debris. The wall of epidermoid tumors may sometimes enhance after intravenous administration of contrast medium.
Degree of Confidence
CT scanning is useful in the initial diagnosis of CNS dermoid tumors if the typical findings are present. A fat-fluid level in the ventricles or fat droplets in the subarachnoid spaces due to dermoid tumor rupture adds additional confidence to the diagnosis.
MRI may be performed to further localize the lesion and to identify any involvement of adjacent structures.
Magnetic Resonance Imaging
Findings
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 in confirming the presence of fat in the lesion. Centrally, dermoid tumors may appear inhomogeneous due to the presence of hair follicles, calcifications, and cellular debris.
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 fat-fluid interface (see Images 7-8).
Vascular displacement or encasement by the dermoid tumor may be demonstrated by means of magnetic resonance angiography (MRA) or CT angiography (CTA).
Epidermoid tumors are usually hypointense on T1-weighted images and hyperintense on T2-weighted images (ie, epidermoid tumors are slightly more hyperintense with a heterogeneous signal relative to CSF on both T1 and T2 sequences) (see Image 10, Image 12).
Fast fluid-attenuated inversion recovery (FLAIR) sequences demonstrate slightly higher signal intensity than do fast spin-echo T2-weighted sequences in epidermoid tumors. Echo-planar diffusion-weighted imaging shows a hyperintense signal within epidermoid tumors. The calculated apparent diffusion coefficient (ADC) exceeds that of CSF, but it is less than that of the brain substance.
Magnetic resonance spectroscopy in epidermoid tumors has a lactate peak at 1.3 ppm. Additional protein metabolites, when present, can simulate brain abscess. Epidermoid tumors and arachnoid cysts have similar fluid attenuation on CT scans. They often have similar signal intensities on T1- and T2-weighted magnetic resonance images. FLAIR sequences frequently demonstrate a heterogeneously increased signal intensity in epidermoid tumors as compared with that in arachnoid cysts. Diffusion-weighted imaging shows definitive high signal intensity (whiteness) in epidermoid tumors and low signal intensity (blackness) in arachnoid cysts.
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. As of late December 2006, the Food and Drug Administration (FDA) had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.
Degree of Confidence
A unilocular cystic mass located at or near the cranial midline, with signal intensity similar to that of fat, is highly suggestive of a dermoid tumor. Epidermoid tumors are rarely hyperintense on T1-weighted images and may simulate a dermoid tumor. Definitive diagnosis then requires histologic correlation.
Ultrasonography
Findings
Sonography has a limited role in the evaluation of CNS dermoid tumors. In young children, subgaleal dermoid cyst of the anterior fontanelle has been assessed by sonography.
Angiography
Findings
Angiography is seldom used in the study of dermoid and epidermoid tumors. A dermoid tumor is an avascular mass on angiography. Displacement or encasement of blood vessels adjacent to the tumor may be well demonstrated by angiography.
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Further Reading
Keywords
dermoids, dermoid cysts, inclusion cysts, congenital epidermoid tumors, acquired epidermoid tumors, brain tumor, spinal dermoid tumors
