Teratomas, Dermoids, and Epidermoids of the Head and Neck

Abstract Introduction

Cranial Epidermoid and Dermoid Inclusion Cysts Teratomas Summary

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Cranial Epidermoid and Dermoid Inclusion Cysts

Epidermoid and dermoid lesions are both true ectodermal inclusion cysts, lined by an epithelium, and, unlike teratomas, are not neoplasms (2,3,4,5,10,11).

Origins

The causes of both epidermoids and dermoids include failure of surface ectoderm to separate from underlying structures, sequestration of surface ectoderm, and implantation of surface ectoderm (Figure 1). Most congenital dermoid and epidermoid cysts probably arise due to an embryologic accident during the early stages of development, between 3 and 5 weeks of gestation. Enclosed ectodermal cysts can occur when the surface ectoderm fails to separate completely from the underlying neural tube. Alternately, they may result from abnormal sequestration or invagination of surface ectoderm along the embryologic sites of dermal fusion that form the eyes, ears, and face (11,12). These cysts may also arise, at any age, in the subcutaneous tissues and the spinal canal because of a traumatic implantation that drives skin elements from the surface into the underlying tissues. Implantation can be iatrogenic, caused by a diagnostic or therapeutic lumbar puncture performed with an open needle (without a stylet), or can be brought about by stab and puncture wounds.

Epidermoid inclusion cysts are presumed to arise later in embryologic development than dermoids and are typically lateral in location. They may be extradural, intradiploic masses and usually occur in the temporal or parietal bones. Intradural epidermoid cysts are usually extraaxial lesions, most commonly found in the cerebellopontine angle and less frequently in the middle cranial fossa. Cerebellopontine angle epidermoids are often called congenital cholesteatomas because of their cholesterol content. However, they are quite distinct from the "acquired cholesteatomas," which result from chronic middle ear infection that causes retraction pockets from the pars flaccida of the tympanic membrane.

Intracranial dermoid inclusion cysts are presumed to arise from embryologic accidents that occur earlier than similar events leading to epidermoid cysts. This theory is often offered as an explanation for the almost constant midline location of dermoids. These lesions are more common in the posterior cranial fossa (cisterna magna, fourth ventricle, pre-pontine cistern) and in the suprasellar cistern. Outside the suprasellar region, supratentorial dermoid inclusion cysts are extremely uncommon but, rarely, have been noted in the quadrigeminal plate cistern. Some dermoid inclusion cysts are isolated lesions, as is typical for those in the anterior prepontine and suprasellar locations. However, the more posterior dermoid cysts can be part of a dysraphic malformation that includes a connection to the skin that may be fibrous or may have a patent, fistulous sinus tract.

Pathologic and Histologic Characteristics

Unlike teratomas, dermoid and epidermoid inclusion cysts are mainly unilocular and expand slowly, enlarging over years or decades, by the accumulation (within an enclosed space) of cutaneous products.

Epidermoid inclusion cysts have a thin squamous lining, which only rarely contains calcifications. These cysts contain debris from the desquamation of their squamous epithelial lining. The debris consists of mostly keratin, a proteinaceous material, and some cholesterol, a lipid material derived from the breakdown of cell membranes (Figure 2). Epidermoids are often described as pearly tumors because of the shiny, smooth, waxy character of their "dry keratin" at gross inspection.

Dermoid inclusion cysts have a thicker lining, which may contain dystrophic calcifications. Although dermoids are unilocular, they may show lipid material floating like supernatant "oil on water" over the heavier proteinaceous debris, rather than forming the scattered fatty foci seen in some head and neck teratomas. The lipid material in a dermoid is derived from sebaceous secretions and not from mesodermal (adipose) fat. Dermoids are often described as buttery or cheesy tumors and can contain "wet cholesterol," as well as secretions of apocrine sweat glands and hair.

Clinical Presentation

Epidermoid and dermoid inclusion cysts account for less than 5% of all intracranial masses. There is a slight male predilection, and most patients present in the first 4 decades of life. Epidermoid cysts are frequently identified during the 3rd and 4th decades. Dermoid cysts tend to manifest earlier, usually in the 2nd and 3rd decades, most likely because they are histologically more complex and probably enlarge faster. Dermoid inclusion cysts of the posterior fossa may be noted even earlier, in infants or young children. These latter lesions are commonly associated with a visible occipital dimple or a sinus tract, which, if fistulous, can cause meningitis. Epidermoids can be subcutaneous or deep but are usually covered by intact skin, without a fistula.

Small (<1-2-cm) epidermoid and dermoid inclusion cysts are among the most common masses of the head, especially in and about the orbit. However, many of these small but palpable superficial lesions are embedded in the skin and subcutaneous tissue and only rarely require radiologic evaluation.

Dermoid inclusion cysts are the most common orbital tumors of childhood (13). They are typically located at sites of embryologic fusion (eg, zygomaticofrontal and frontoethmoidal sutures) (12,13). The vast majority of orbital dermoids are superficial, in the lateral third of the eyebrow or the supraorbital ridge (14). Intraorbital dermoids are in the upper outer quadrant or lacrimal fossa in more than 80% of cases (14). Diagnostic imaging is usually not requested for small orbital rim masses that are mobile and subcutaneous.

Rupture of either dermoid or epidermoid cysts can produce a severe chemical meningitis, usually attributed to the irritating effects of the cholesterol in the cellular debris. Dermoids with a sinus tract can become infected or can produce infectious meningitis. When large enough, either lesion can cause hydrocephalus due to ventricular or cisternal obstruction, but this occurs more commonly with dermoids of the posterior fossa or fourth ventricle.

Radiologic Appearance

Although they are slow growing, dermoid and epidermoid inclusion cysts produce pressure changes on surrounding structures that are visible at radiography. Intraosseous (diploic) cysts cause expansile bone erosion with sclerotic remodeling (Figure 3a).

At computed tomography (CT) and magnetic resonance (MR) imaging, epidermoid cysts usually have attenuation and signal intensity values that roughly parallel those of water (cerebrospinal fluid) (Figure 3b-3e). Occasionally, the cyst contents may have slightly negative attenuation values at CT and signal intensity slightly greater than that of water at T1-weighted MR imaging. Unlike dermoids, epidermoid cysts do not usually have the low negative attenuation values (-60 HU to -90 HU) of true (adipose) fat, although the actual characteristics of an epidermoid may vary with its ratio of keratin (protein) to cholesterol (lipid). The squamous epithelial lining of an epidermoid cyst is usually too thin to be seen radiologically and only rarely shows either calcification or enhancement with contrast material.

Dermoid inclusion cysts may have a more complex imaging appearance, but they are still unilocular lesions. Their imaging characteristics are more variable because their lining and thus their contents are more complex: apocrine glands, sweat glands, sebaceous glands, and hair follicles. Because of these dermal adnexa, the cyst wall is thicker and may be radiologically visible. This thicker lining can calcify and may enhance with contrast material. The sebaceous, lipid material in a dermoid has attenuation and signal intensity characteristics that simulate those of fat on both CT and MR images (Figure 4): hypoattenuating relative to water at CT and hyperintense with T1-weighted pulse sequences (T1 shortening). Sometimes, a dermoid does not have these radiologic characteristics and thus may be indistinguishable from an epidermoid (Figure 5). The converse situation radiologic signs of a dermoid (prominent lipid) with a histologic diagnosis of an epidermoid must be accepted cautiously, since failure to see the dermal appendage structures (indicative of a dermoid) may be caused by incomplete wall sampling or inadequate pathologic review.

The appearance of a unilocular, midline, "cystic" mass, especially one with attenuation and signal intensity characteristics similar to those of fat or with a fat-fluid level, is very suggestive of a dermoid inclusion cyst (Figure 4, Figure 5). A suspected dermoid inclusion cyst should be evaluated carefully for any associated skeletal dysraphism, fibrous band, or sinus tract leading to the skin surface. Accidental or iatrogenic rupture of a dermoid can be diagnosed when characteristic lipid droplets are seen in the subarachnoid spaces of the sulci and cisterns (Figure 4).

Not all intracranial lipid or fatty masses are dermoid inclusion cysts. In the pineal region, heterogeneous, multiloculated, fat-containing masses are most likely to be teratomas. Homogeneous fatty masses are usually not dermoids nor teratomas, but rather are maldevelopmental lipomas (15,16) arising from abnormal differentiation of the meninx primitiva.

Orbital dermoids are typically unilocular masses containing soft-tissue or lipid attenuation in the upper outer quadrant of the orbit.

Abstract Introduction

Cranial Epidermoid and Dermoid Inclusion Cysts Teratomas Summary

Source Information References