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 (as seen in the images below), with decreased attenuation in the range of -20 to -40 HU because of its fat content. 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.
Elderly woman with chronic headaches. A nonenhanced computed tomography (CT) scan of the head demonstrates a well-circumscribed, cystic, low-attenuating lesion located at the cranial midline in the suprasellar region, posterior to the third ventricle. A small focus of calcification is noted at the posterior margin of the tumor.
Coronal computed tomography (CT) scan of the paranasal sinuses in a 26-year-old man with sinusitis. A large, well-circumscribed, low-attenuating, cystic lesion is present in the right temporal lobe, lateral to the cranial midline. Note the peripheral, marginal calcification in the lesion. No erosion is seen in the adjacent bone of the sella.
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 (as seen in the images below) 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.
Axial gadolinium-enhanced T1-weighted image of a lesion demonstrating a hyperintense signal. In addition, multiple small foci of hyperintense signal are present along the sulci of the right temporal lobe. These represent fat droplets in the subarachnoid space, left by the focal rupture of the dermoid tumor.
The dermoid tumor is hyperintense on this gadolinium-enhanced coronal T1-weighted image. Note the heterogeneity of the lesion posteriorly. The hyperintensity results not from the gadolinium contrast enhancement but instead from the short T1 of fat. Multiple hyperintense foci consistent with fat droplets are demonstrated in the subarachnoid spaces. A mild midline septal shift to the left is noted; this is due to compression of the right lateral ventricle by the dermoid tumor. A chemical-shift artifact is also noted at the superior marginal surface of the lesion.
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 T1 and T2 sequences). (See the images below.)
This epidermoid tumor is isointense relative to cerebrospinal fluid (CSF) on a nonenhanced T1-weighted axial image. The lobulated extra-axial mass surrounds the right cerebral peduncle and extends to the suprasellar region and the interpeduncular cistern. In contrast, a dermoid tumor will demonstrate a high T1 signal intensity because it contains fat constituents.
Epidermoid tumor (same as in the image above) demonstrating no enhancement in this contrast-enhanced T1-weighted image.
T2-weighted axial image demonstrating a hyperintense signal (in the same epidermoid tumor as in the above 2 images) that is similar to the signal intensity of cerebrospinal fluid (CSF). Compression by the epidermoid tumor mass has widened the interpeduncular cistern and narrowed the right cerebral peduncle.
Coronal T1-weighted image demonstrating a hypointense epidermoid tumor (same tumor as in the above 3 images). Note that the epidermoid has higher signal intensity than the adjacent CSF. There is a compressive mass effect on the third ventricle, but hydrocephalus is minimal.
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 been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis. 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.
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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 Imaging: Dermoid Tumor, CNS |
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Keywords
dermoid tumor, dermoid, dermoid cyst, epidermoid, brain cyst, inclusion cyst, epidermoid cyst, dermoid cysts, dermoid cyst pictures, epidermoid tumor, inclusion cysts
