Extraaxial Cysts

Extraaxial cysts are between the skull and brain. With few exceptions, most lie within the arachnoid membrane or in the subarachnoid space.

Determining sublocation of an extraaxial cyst (supravs. infratentorial, midline vs. off-midline) is helpful in establishing a meaningful differential diagnosis (Table 28-1). For example, an arachnoid cyst is the only type that commonly occurs in the posterior fossa. Some extraaxial cysts are usually (although not invariably) off-midline. Others—pineal and Rathke cleft cysts—occur only in the midline.

We begin our discussion of extraaxial cysts with the most common type, arachnoid cyst.

Nonneoplastic Cysts

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Arachnoid Cyst

Although arachnoid cysts (ACs) occur throughout the neuraxis, the vast majority are intracranial.

Terminology

An AC, also known as a meningeal cyst, is a CSF-containing cyst lined by a layer of flattened arachnoid cells.

Etiology

General Concepts. The vast majority of ACs arise as anomalies of meningeal development. The embryonic endomeninges fail to merge and remain separated, forming a "duplicated" arachnoid. CSF is secreted by cells in the cyst wall and accumulates between the layers.

Less commonly, arachnoid loculations are acquired as a result of hemorrhage, infection, or surgery. Arachnoid-like cysts also

(28-6A) NECT in a 59y woman with severe headaches and confusion shows a CSF-density mass in the left middle cranial fossa . Note "tight" appearance of brain, with basilar cistern effacement and midbrain compression. (28-6B) More cephalad NECT in the same case shows mixeddensity acute-on-chronic bilateral subdural hematomas (SDHs) .

(28-6C) Sagittal T1WI in the same case shows the middle fossa arachnoid cyst displacing the temporal lobe cortex around it . Note isointense SDH . (286D) Axial T2WI shows the mixed-age SDHs with fluid-fluid levels between the chronic SDH above and the more acute component below. This is arachnoid cyst with associated bilateral SDHs.

(28-7) T2WI (upper L) shows large AC that expands middle cranial fossa , suppresses completely on FLAIR (upper R), does not enhance (lower L), and does not exhibit restricted diffusion (lower R). This is incidental finding in a 53y woman.

sometimes arise adjacent to extraaxial tumors such as meningiomas, schwannomas, and pituitary macroadenomas. These benign fluid-containing tumor-associated cysts are discussed separately.

There is a relationship between subdural hematomas (SDHs) and ACs although whether this is causative or coincidental is unclear. Traumatic SDHs can rupture into ACs. The converse—AC rupture causing a spontaneous SDH—occurs but is rare.

Genetics. Most ACs are sporadic and nonsyndromic. Syndromic ACs have been reported in association with acrocallosal, Aicardi, and Pallister-Hall syndromes.

Pathology

Location. Most ACs are supratentorial. They are usually offmidline and are the most common off-midline extraaxial supratentorial cyst (28-2).

Nearly two-thirds are found in the middle cranial fossa, anteromedial to the temporal lobe (28-3). Fifteen percent of ACs are found over the cerebral convexities, predominantly over the frontal lobes.

Midline ACs are relatively rare in the supratentorial compartment. The most frequent supratentorial midline location for ACs is the suprasellar cistern, followed by the quadrigeminal cistern and velum interpositum. Interhemispheric fissure ACs are typically associated with callosal anomalies.

Between 10-15% of ACs are found in the posterior fossa. The most common location is the cerebellopontine angle cistern, where ACs are the second most common cystic-appearing

(28-8) Large CSF-filled suprasellar AC is shown on T2WI . CSF pulsations in cyst do not suppress completely on FLAIR . CT cisternogram shows dilute contrast in the lateral ventricles , while the noncommunicating cyst does not opacify.

extraaxial mass (after epidermoid). The next most frequent site is retrocerebellar.

Size and Number. ACs vary in size, ranging from small incidental cysts to large space-occupying lesions. ACs are almost always solitary. Multiple meningeal cysts have been reported but are probably acquired, resulting from undetected meningitis.

Gross Pathology. ACs are well-marginated cysts filled with clear colorless fluid that resembles CSF. They are devoid of internal septations and are completely encased by a delicate translucent membrane.

Microscopic Features. ACs consist of a delicate fibrous membrane lined by a single layer of mature, histologically normal arachnoid cells (28-4). Small inflammatory infiltrates occur but are rare.

ARACHNOID CYST: PATHOLOGY

Location

•Supratentorial (90%)

○Middle fossa (67%)

○Convexities (15%)

○Other (5-10%): suprasellar, quadrigeminal cisterns

•Infratentorial (10-12%)

○Mostly CPA cistern (second most common cystic CPA mass)

○Less common = cisterna magna

Gross Pathology

•Thin translucent cyst wall bulging with clear fluid

•Lined by mature arachnoid cells

(28-9) Left cerebellopontine angle arachnoid cyst is isointense with CSF on T1WI and T2WI and suppresses completely on FLAIR . No diffusion restriction is seen on DWI .

Clinical Issues

Epidemiology. ACs are the most common of all congenital intracranial cysts. They account for approximately 1% of all space-occupying intracranial lesions and are identified on imaging studies in 1-2% of patients.

Demographics. ACs can be seen at any age. Most (nearly 75%) are found in children and young adults. There is no sex difference.

Presentation. Most ACs are asymptomatic and found incidentally. Symptoms vary with size and location. Headaches are common in symptomatic ACs.

Some suprasellar ACs become very large and cause obstructive hydrocephalus.

Natural History. ACs are not associated with increased mortality. Most incidentally discovered ACs remain stable over many years. Enlargement—if any—is very gradual. Enlargement is strongly associated with younger age and rarely occurs in children older than 4 years at the time of initial diagnosis.

Hemorrhage—either traumatic or spontaneous—into an intracranial AC is rare but may cause sudden enlargement. The presence of an AC increases shear force on impact and may be a risk factor for intracystic bleeding. Recent studies have shown no significant role of location or shape of ACs on hemorrhage risk.

Treatment Options. Asymptomatic ACs are usually "leave me alone" lesions. Surgical options for symptomatic ACs include endoscopic resection or fenestration, open fenestration/marsupialization, or cystoperitoneal shunting

(28-10) Large midline posterior fossa AC is isodense with CSF on NECT . The unilocular AC compresses and displaces vermis and brainstem anteriorly. The cyst suppresses completely on FLAIR and does not restrict on DWI .

with a programmable valve. Following shunting, 60% of ACs disappear completely; in half of these patients, it is possible to remove the shunt without shunt dependence.

Imaging

General Features. Uncomplicated ACs behave exactly like CSF on CT and MR (28-5). FLAIR and DWI are the best sequences to distinguish cystic-appearing intracranial masses from one another.

CT Findings. Uncomplicated ACs are CSF density (28-6A). If intracystic hemorrhage has occurred, the cyst fluid may be moderately hyperdense compared with CSF (28-6B). Large middle cranial fossa ACs expand the fossa and cause temporal lobe hypoplasia or displacement (28-6A).

With moderately large ACs, bone CT may show pressure remodeling of the adjacent calvaria. ACs do not cause frank bone invasion.

ACs do not enhance. Installation of intrathecal contrast ("CT cisternography") may be helpful in demonstrating communication with the subarachnoid space (SAS) (28-8). Most symptomatic ACs do not demonstrate direct, free communication with the SAS and may require microsurgical decompression. Patients with completely communicating ACs may not need surgical intervention.

MR Findings. ACs are sharply marginated, somewhat scalloped-appearing lesions that parallel CSF signal intensity on all sequences. They are therefore isointense with CSF on T1and T2-weighted images (28-9). ACs cause moderate focal mass effect, displacing but not engulfing adjacent brain, vessels, and cranial nerves.

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The internal appearance of an AC is intrinsically featureless, containing neither septations nor vessels.

ACs suppress completely with FLAIR (28-10). Occasionally, CSF pulsations within large lesions may cause spin dephasing, producing heterogeneous signal intensity and significant propagation of phase artifact across the scan (28-8).

ACs do not restrict on DWI and do not enhance. CSF flow imaging such as 2D cine PC may demonstrate communication between cyst and adjacent subarachnoid space.

Differential Diagnosis

The major differential diagnosis of AC is epidermoid cyst (EC). ECs are often almost—but not quite—exactly like CSF. They have a cauliflower-like, lobulated configuration instead of the sharply marginated borders of an AC. ECs engulf vessels and nerves, insinuating themselves along CSF cisterns. ECs do not

suppress completely on FLAIR and typically show moderate to marked hyperintensity on DWI.

Enlarged subarachnoid spaces caused by brain volume loss are usually more diffuse CSF collections and do not cause mass effect on adjacent structures.

A subdural hygroma or chronic subdural hematoma (cSDH) is not precisely like CSF and is usually crescentic, not round or scalloped. cSDHs usually show evidence of prior hemorrhage, especially on T2* sequences, and may have enhancing encasing membranes.

A porencephalic cyst looks just like CSF, but it is intraaxial and lined by gliotic white matter that is often hyperintense on FLAIR. Rarely, neurenteric cysts can resemble ACs although they are usually hyperintense compared with CSF. Supratentorial neurenteric cysts are rare.

ARACHNOID CYST: CLINICAL ISSUES, IMAGING, AND

DDx

Clinical Issues

•Most common nonneoplastic intracranial cyst

○1% of all intracranial masses

○All ages; children + young adults (75%)

○Prevalence = 2% on imaging studies

•Most do not communicate freely with subarachnoid space

Imaging

•Behaves exactly like CSF

•FLAIR/DWI best to distinguish from other cysts

Differential Diagnosis

•Most common = epidermoid cyst

•Less common

○Enlarged subarachnoid spaces

○Loculated subdural hygroma/hematoma

○Porencephalic cyst

○Neoplasm-associated cyst

•Rare = neurenteric cyst

Choroid Fissure Cyst

The choroid fissure is an infolding of CSF between the fornix and thalamus. It is normally a shallow, inconspicuous, C- shaped cleft that curves posterosuperiorly from the anterior temporal lobe all the way to the atrium of the lateral ventricle. The choroidal arteries and choroid plexus lie just medial to the choroid fissure.

A CSF-containing cyst can form anywhere along the choroid fissure. These "choroid fissure cysts" are probably caused by maldevelopment of the embryonic tela choroidea, a double layer of pia that invaginates through the choroid fissure to reach the lateral ventricles.

Choroid fissure cysts can therefore be regarded as a subtype of arachnoid cyst. We consider them separately because of their unique location and imaging appearance.

Imaging

Most choroid fissure cysts are discovered incidentally on imaging studies. They lie just medial to the temporal horn of the lateral ventricle between the hippocampus and diencephalon. Choroid fissure cysts follow CSF density/signal intensity on all sequences (28-11). On axial and coronal images they are round to oval but on sagittal images have a distinctive, somewhat elongated "spindle" shape (28-11D).

Epidermoid Cyst

Both congenital and acquired epidermoid cysts are found in the CNS. Although spinal epidermoid cysts (ECs) are often acquired lesions, intracranial ECs are always congenital in origin.

Nonneoplastic Cysts

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Terminology

An intracranial EC is an inclusion cyst that is derived from embryonic ectodermal elements. ECs cysts have incorrectly been called "tumors," but they are not neoplastic. The term "cholesteatoma" should be reserved for an acquired lesion arising as a complication of chronic otitis media.

Etiology

General Concepts. ECs arise during the third to fifth gestational weeks. Ectodermal cellular remnants caused by incomplete cleavage of neural from cutaneous ectoderm result in the inclusion of epiblasts in the neural tube. Congenital CPA epidermoids are derived from cells of the first branchial groove.

Pathology

Location. Extracranial ECs commonly involve the scalp, face, and neck. Over 90% of intracranial ECs are intradural and are almost always extraaxial (28-12). ECs are more often offor paramidline and have a predilection for the basal cisterns, where they insinuate themselves around cranial nerves and vessels.

The CPA cistern is the single most common site, accounting for nearly half of all intracranial ECs. The middle cranial fossa (sylvian fissure) and parasellar region together account for 1015% of ECs. Less common locations are the cerebral ventricles, usually the fourth ventricle. Purely extradural intradiploic ECs account for 5-10% of cases. Parenchymal ECs do occur but are rare.

Gross Pathology. The outer surface of an EC is often shiny, resembling mother of pearl (28-13). Multiple "cauliflower" excrescences are typical (28-14). The cyst is filled with soft, waxy, creamy, or flaky material.

Microscopic Features. The cyst wall consists of an outer fibrous capsule lined by stratified squamous epithelium. The cyst contains concentric lamellae of keratinaceous debris and solid crystalline cholesterol. Dermal appendages (a characteristic of dermoid cysts) are absent.

Clinical Issues

Epidemiology. ECs represent 0.2-1.8% of primary intracranial tumors and tumor-like lesions. They are the most common intracranial developmental cyst and are four to nine times more common than dermoid cysts. Overall, EC is the third most common CPA mass (after vestibular schwannoma and meningioma) and the most common cystic mass in this location.

Demographics. Peak age of presentation is 20-60 years. Symptomatic ECs are rare in children. There is no sex predominance.

Presentation. ECs may remain clinically silent for many years. Symptoms are location dependent. Headache and cranial neuropathy (especially involving CNs V, VII, and VIII) are common features.

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(28-12) Graphic shows a multilobulated epidermoid cyst within prepontine cistern encasing the basilar artery , displacing pons.

(28-13) Autopsy shows epidermoid cyst as a whitish, "pearly" tumor . Note the encased basilar artery , oculomotor nerves .

(28-14) Close-up view of a surgical specimen shows classic "cauliflower" appearance of the external surface of an epidermoid cyst.

Natural History. ECs grow very slowly via progressive accumulation of normally dividing epidermal cells and accretion of desquamated keratin. ECs often reach considerable size before becoming symptomatic. In contrast to dermoid cysts, rupture of an EC is rare. Malignant transformation occurs but is extremely rare.

Treatment Options. The insinuating characteristics of ECs make them difficult to resect. Although total resection minimizes the risk of postoperative aseptic meningitis, hydrocephalus, and tumor recurrence, aggressive surgery may be associated with cranial nerve or ischemic deficits.

EPIDERMOID CYST: ETIOLOGY, PATHOLOGY, AND EPIDEMIOLOGY

Etiology

•Congenital inclusion cyst

•Epithelial remnants in neural tube

Pathology

•Gross pathology

○Insinuates in/around CSF cisterns

○Encases vessels/cranial nerves

○"Cauliflower-like" excrescences

○"Pearly" whitish surface

○Waxy, creamy, or flaky contents

•Microscopic pathology

○Squamous epithelium + keratin debris, solid cholesterol

○NO dermal appendages!

Epidemiology

•0.2-1.8% of primary intracranial tumors

•Peak age 20-60 years

•Symptomatic ECs rare in children

Imaging

General Features. Epidermoid cysts resemble CSF on imaging. Irregular frond-like excrescences and an insinuating growth pattern in CSF cisterns are characteristic.

CT Findings. Over 95% of ECs are hypodense and appear almost identical to CSF on NECT scans. Calcification is present in 10-25%. Hemorrhage is very rare. Hyperdense "white" epidermoids are uncommon, representing 3% of reported lesions. Enhancement is rare.

MR Findings. ECs are isoor slightly hyperintense compared with CSF on both T1and T2-weighted sequences. Slight heterogeneity in signal intensity is often present (28-15A).

"Atypical" epidermoids account for only 5-6% of cases. A "white" epidermoid is a rare type of EC that has a high protein content and may be hyperintense on T1WI and hypointense on T2WI. Enhancement is generally absent although mild peripheral enhancement can be seen in 25% of cases.

ECs either do not suppress at all or suppress incompletely on FLAIR (2815B). They restrict on DWI and are therefore moderately to strikingly hyperintense (28-15C).

Differential Diagnosis

The major differential diagnosis is arachnoid cyst (AC). ACs are smoothly marginated, behave exactly like CSF on all sequences, suppress completely on FLAIR, and do not restrict on DWI. The rare "white" epidermoid can mimic neurenteric cyst.

Parasitic cysts such as neurocysticercosis (NCC) are usually multiple and small and often contain a discernible scolex. Most NCC cysts are located within the depths of cerebral sulci or in the brain parenchyma. Cystic neoplasms are rarely mistaken for ECs, as the cyst wall and/or nodule enhances.

Dermoid cysts should not be confused with ECs. Dermoid cysts contain fat and dermal appendages and do not resemble CSF on imaging studies.

EPIDERMOID CYST: FEATURES, IMAGING, AND DDx

General Features

•Resembles CSF (vs. fat-like dermoid)

•Insinuates around/along CSF cisterns

•Encases, displaces vessels and cranial nerves

Location

•Intradural (90%), intradiploic (10%)

•CPA cistern most common site (~ 50%)

CT

• Hypodense (> 95%)

MR

•Slightly hyperintense to CSF on T1WI

•Does not suppress on FLAIR

•Restricts ("bright") on DWI

Differential Diagnosis

•Arachnoid cyst

○Suppresses on FLAIR, restricts on DWI

•Other

○Inflammatory cyst (e.g., neurocysticercosis)

○Neurenteric cyst (not exactly like CSF)

Dermoid Cyst

Terminology

A dermoid cyst (DC) is a histologically benign cystic mass with mature squamous epithelium, keratinous material, and adnexal structures (hair follicles and sebaceous and sweat glands).

Etiology

General Concepts. Like epidermoid cysts, DCs are thought to arise from the inclusion of ectodermally committed cells at the time of neural tube closure (third to fifth week of embryogenesis). DCs grow slowly secondary to the production of hair and oils from the internal dermal elements.

Genetics. Most DCs are sporadic although there is a reported association with Goldenhar and Klippel-Feil syndromes.

Pathology

Location. DCs are usually extraaxial lesions that are most often found in the midline (28-16). The suprasellar cistern is the most common site, followed by the posterior fossa and frontonasal region.

Gross Pathology. A dermoid cyst is a thick-walled unilocular cyst lined by stratified squamous epithelium. Sectioned DCs typically contain thick, greasy sebaceous material, keratin debris, and skin adnexa such as hair follicles (2817). Lipid and cholesterol elements floating on proteinaceous material may be present.

Nonneoplastic Cysts

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(28-15A) T2WI shows a lobulated, irregular hyperintense mass in the right CPA and basilarcisterns. The CSF looks "dirty."

(28-15B) FLAIR demonstrates that the lobulated, cauliflower-like mass does not suppress.

(28-15C) The mass restricts on DWI. This is a classic epidermoid cyst.

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(28-16) Ruptured DC is heterogeneous fatcontaining midline mass with ventricular fat-fluid level and fat droplets in SASs .

(28-17) Dermoid cyst contains thick, greasy sebaceous material, keratin debris, and hair . (Courtesy R. Hewlett, MD.)

Microscopic Features. The outer wall of a DC consists of squamous epithelium. The inner lining contains multiple sebaceous and apocrine glands, fat, and hair follicles (28-18).

Clinical Issues

Epidemiology. DCs are much less common than epidermoid cysts, representing less than 0.5% of intracranial masses.

Demographics. Presentation occurs at significantly younger ages compared with epidermoids, peaking in the second to third decades.

Presentation. DCs often remain asymptomatic until they rupture. Although cyst rupture is usually not fatal, chemical meningitis with seizure, coma, vasospasm, infarction, and even death may ensue as a consequence.

Natural History. DCs typically grow very slowly although rapid enlargement with rupture has been reported. Cyst rupture is usually spontaneous but has also been associated with head trauma. Fat from a ruptured DC may persist for years. DCs occasionally degenerate into squamous cell carcinomas.

Treatment Options. Complete surgical resection is the goal, but residual tumor adherent to neurovascular structures is often left behind to minimize postoperative complications. Unlike epidermoid cysts, the recurrence rate after DC resection is very low.

Imaging

General Features. DCs resemble fat. A round, well-circumscribed lipidcontaining mass is the usual appearance.

CT Findings. DCs are quite hypointense on NECT scans (28-19A). Calcification of the capsule is seen in 20% of cases. With rupture, hypodense fatty "droplets" disseminate in the CSF cisterns and may cause discernible fat-fluid levels in the ventricles.

In infants with frontal DC, bone CT usually discloses a bifid crista galli with a large foramen cecum and sinus tract.

MR Findings. Signal intensity varies with fat content in the cyst. Most DCs are heterogeneously hyperintense on T1WI (28-19B). T1WI is also the most sensitive sequence to detect disseminated fat "droplets" in the subarachnoid space, diagnostic of ruptured dermoid (28-20). Fat suppression is helpful to confirm the presence of lipid elements.

Standard PD and T2 scans show increasingly more pronounced "chemical shift" artifact in the frequency-encoding direction as the time of repetition is lengthened. Fat is very hypointense on standard T2WI but is "bright" (hyperintense) on fast spin echo T2-weighted sequences. DCs demonstrate heterogeneous hyperintensity with linear or striated laminations if hair is present within the cyst.

Uncomplicated DCs are heterogeneously hyperintense on FLAIR. Ruptured DCs demonstrate subtle FLAIR sulcal hyperintensity (28-20C) and "bloom" on T2* GRE or SWI (28-20D).

Most DCs do not enhance although ruptured DCs may cause significant chemical meningitis with extensive leptomeningeal reaction and enhancement.

Spectroscopy may show an elevated lipid peak at 0.9-1.3 ppm.

DERMOID CYST

Pathology

•Location

○Usually extraaxial

○Midline > off-midline

○Suprasellar > posterior fossa > frontonasal

•Wall of squamous epithelium

○Cyst contains fatty sebaceous material, keratin, skin adnexa

Clinical Findings

•Grow slowly

•Usually asymptomatic until rupture

Imaging

•NECT

○Hypodense, Ca++ in 20%

○"Fatty" droplets in cisterns if ruptured

•MR

○Heterogeneously hyperintense on T1WI/FSE T2

○Heterogeneously hyperintense on FLAIR

○Ruptured DCs "bloom" on T2*GRE

Differential Diagnosis

The major differential diagnosis of DC is epidermoid cyst. Epidermoid cysts behave more like CSF on both CT and MR, whereas DCs resemble fat on imaging studies. Dermoids often rupture, spilling fatty droplets into the subarachnoid space. Epidermoid cyst rupture does occur but is much less common and may demonstrate high serum carbohydrate antigen CA 199.

Lipoma may resemble a DC but is generally much more homogeneous on MR and is often associated with other congenital malformations such as callosal dysgenesis.

Craniopharyngioma is often multicystic, extends into the sella, calcifies, and enhances. Teratoma may resemble a DC but most commonly occurs in the pineal gland and is much more heterogeneous on imaging than the typical DC.

DERMOID vs. EPIDERMOID CYST

Pathology

•Both dermoid, epidermoid contain squamous epithelium + keratin debris

•Only dermoid also contains fat, dermal appendages

Clinical Issues

•Dermoid cysts less common than epidermoids

•Dermoid cysts more common in children/young adults

•Dermoid cysts commonly rupture

Imaging

•DC behaves mostly like fat

○Most often midline, supraor juxtasellar

•Epidermoid cyst more like CSF

○Most often midline

○Most common site = posterior fossa (cerebellopontine angle cistern)

Nonneoplastic Cysts

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(28-19A) Axial NECT in a 20y man with severe headaches shows a very hypodense mass in the prepontine cistern.

(28-19B) T1WI in the same case demonstrates that the mass is heterogeneously hyperintense.

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Neurenteric Cyst

Neurenteric (NE) cysts are rare endodermal-derived developmental CNS lesions. They are significantly more common in the spine than in the brain.

Terminology

NE cyst is also called enterogenous cyst, enteric cyst, endodermal cyst, and neuroendodermal cyst. Other less common terms, primarily applied to intraspinal NE cysts, are gastrogenic and archenteric cysts.

Etiology

NE cysts, along with Rathke cleft and colloid cysts, are endodermally derived developmental lesions of the CNS. Immunohistochemical studies have demonstrated that NE cysts are most likely derived from embryonic multipotent endodermal cells. These cells freely migrate along the

(28-20A) (L) Axial T1WI shows dermoid cyst as heterogeneously hyperintense paramedian mass . (R) Cyst rupture has spilled T1 hyperintense fatty droplets into the sulci along the sylvian fissure. (28-20B) Sagittal T1WI in the same case nicely shows hyperintense droplets from the ruptured dermoid along the sylvian fissure sulci.

(28-20C) Droplets from the ruptured dermoid in the left sylvian fissure are hyperintense on FLAIR (compare to the normalappearing right sylvian fissure). (28-20D) T2* SWI shows that the droplets exhibit "blooming" and appear as punctate hypointensities in the left sylvian fissure.

embryonic neuroectoderm, which lies just dorsal to the developing notochord. If the neuroectoderm fails to separate from the notochord, displaced nests of respiratory or alimentary tissue may ultimately form an NE cyst.

The most cephalad aspect of the notochord forms the clivus, which is why most intracranial NE cysts are found in the midline posterior fossa. The rare supratentorial NE cysts probably form when the traveling multipotent endodermal cells proceed over the notochord into the supratentorial area.

Pathology

Location. The most common CNS site is the spine. Intracranial NE cysts are rare, accounting for less than 25% of all cases. Almost 75% of these occur in the posterior fossa, and almost all are extraaxial. The typical location is midline or slightly offmidline, just anterior to the pontomedullary junction (28-21). The lower CPA cistern is also a common site.

(28-21) Sagittal graphic shows a classic neurenteric cyst . Intracranial neurenteric cysts are most often found near the midline, anterior to the brainstem.

(28-22) Neurenteric (endodermal) cysts are mostly lined with pseudostratified ciliated epithelium and contain variable numbers of goblet cells . (Courtesy P. Burger, MD.)

Between 25-30% of intracranial NE cysts are supratentorial. Almost all reported cases are off-midline, located adjacent to the frontal lobes.

Size and Number. NE cysts vary in size. Most are relatively small (1-3 cm in diameter). Occasionally, NE cysts can become very large (up to 9 cm), especially when they occur in the supratentorial compartment.

NE cysts are almost always solitary. "Seeding" or wide dissemination with multiple cysts spreading throughout the spinal canal is reported but rare.

Gross Pathology. NE cysts are typically well-delineated, smoothly marginated cysts. The wall is thin and translucent. Cyst contents range from clear colorless fluid that resembles CSF to thick viscous mucoid secretions.

Microscopic Features. NE cysts are lined by three cell types that resemble the respiratory tract (pseudostratified-ciliated), stomach (goblet-columnar), or respiratory bronchioles (simple cuboidal) (28-22). A few cases of squamous metaplasia and even mucinous adenocarcinomas arising in an NE cyst have been reported.

Clinical Issues

Demographics. NE cysts occur in patients of all ages. Age distribution is bimodal, with a large peak in the third and fourth decades and a smaller peak in the first decade. Average age at presentation is 34 years. There is a slight male predominance.

Presentation. Posterior fossa NE cysts typically present with waxing and waning neck pain or occipital headaches.

Headaches, behavior changes, and seizures have been reported with supratentorial lesions.

Natural History. NE cysts grow very slowly and are often stable for years.

Treatment Options. Small NE cysts are sometimes monitored with periodic imaging. Symptomatic cysts are excised. Total surgical removal is the treatment goal but may be difficult due to adhesion of the cyst membrane to critical neurovascular structures.

NEURENTERIC CYST: PATHOLOGY AND CLINICAL ISSUES

Location

•Posterior fossa (75%)

○Extraaxial, midline/slightly off-midline

○Anterior to pontomedullary junction

•Supratentorial (25%)

○Extraaxial, adjacent to frontal lobes

Pathology

•Endodermal-derived congenital inclusion cyst

•Size varies from 1-9 cm

•Round/ovoid shape, smoothly marginated

•Wall contains mucin-secreting goblet cells

Clinical Issues

•Spinal NE cyst 3-4x more common than intracranial

•Asymptomatic or headache, neck pain

Imaging

General Features. NE cysts are all well delineated and round to ovoid. Density and signal intensity vary according to protein

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content of the cyst fluid. Most NE cysts are moderately proteinaceous and therefore often do not precisely parallel CSF.

CT Findings. Most NE cysts are isoto slightly hyperdense compared with CSF. Hyperdense ("white") NE cysts are seen in about 25% of cases. Calcification and intracystic hemorrhage are absent. In contrast to spinal NE cysts, bony anomalies are rare. NE cysts do not enhance following contrast administration.

MR Findings. NE cysts are sharply demarcated lesions that displace but do not engulf adjacent neurovascular structures. The cyst wall itself is often inapparent. Signal intensity of cyst contents varies widely, depending on imaging sequence and protein content.

Cyst fluid is almost always isoto hyperintense compared with CSF on T1WI (28-23). Over 90% are hyperintense to CSF on PD and T2WI. Between 5-10%—typically NE cysts with

(28-23A) Sagittal T1WI of a neurenteric cyst shows a hyperintense ovoid midline mass in front of the medulla. (28-23B) Axial T2WI shows that the mass is well delineated. It is heterogeneously hypointense to CSF, suggesting inspissated contents. More typically NE cysts are hyperintense to CSF.

(28-23C) Coronal T1 C+ scan shows that the ovoid mass is well demarcated and hyperintense. (28-23D) Axial T1 C+ FS shows that the mass extends inferiorly to the lower medulla. The center of the mass is just slightly offmidline, a typical solution for a posterior fossa neurenteric cyst.

inspissated, significantly dehydrated contents—are hypointense.

NE cysts do not suppress on FLAIR and are almost always hyperintense relative to CSF. As NE cysts almost never calcify or hemorrhage, T2* (GRE, SWI) sequences do not demonstrate "blooming."

Only a few reports of NE cysts have included DWI findings. In those cases, diffusion restriction was mild or absent.

Most NE cysts do not enhance following contrast administration. A few cases of mild posterior rim enhancement at the cyst-brain interface have been reported.

Differential Diagnosis

Other endodermal-derived cysts such as Rathke cleft (sella) and colloid cyst (foramen of Monro) are very location specific, and their anatomic sites do not overlap with those of NE.

(28-24) Sagittal graphic shows a small cystic lesion within the pineal gland . Small benign pineal cysts are often found incidentally at autopsy or imaging.

(28-25) Axial (L), sagittal (R) autopsy views of pineal cyst show the typical location behind the tectal plate. (Courtesy E. T. Hedley-Whyte, MD.)

The major differential diagnosis of NE cyst is epidermoid cyst (EC). ECs are insinuating lesions that have lobulated frond-like surfaces. Most restrict strongly on DWI. Posterior fossa ECs are usually more lateral than NE cysts, occurring more commonly in the CPA cistern than at the pontomedullary junction. Some reported cases of "white" epidermoids may actually have been NE cysts.

Arachnoid cyst follows CSF signal intensity on all sequences (e.g., suppresses completely on FLAIR) and does not restrict on DWI. Schwannoma is the most common extraaxial posterior fossa mass in adults. It typically enhances strongly and rarely occurs in the midline.

Ecchordosis physaliphora (EP) is a gelatinous-appearing notochordal remnant that typically occurs in the prepontine cistern and is attached to a visible defect in the dorsal clivus by a thin stalk-like pedicle (see Figures 26-19-22).

NEURENTERIC CYST: IMAGING AND DDx

CT

•NECT: Iso-/slightly hyperintense to CSF

•CECT: No enhancement

MR

•T1WI: Almost always iso-/slightly hyperintense to CSF

•PD, T2WI: Hyperintense to CSF (> 90%)

•FLAIR: Does not suppress

•DWI: Mild/no restriction

Differential Diagnosis

•Most common = epidermoid, arachnoid cysts

•Less common = schwannoma (cystic)

•Rare = ecchordosis physaliphora

Pineal Cyst

Modern imaging has resulted in a plethora of pineal "things" found on CT and especially MR. These lesions, often seen in patients with vague complaints and no symptoms referable to the pineal region, can be troublesome to both radiologists and referring clinicians.

Terminology

A pineal cyst (PC) is a benign glia-lined, fluid-containing cyst within the pineal gland parenchyma.

Etiology

The precise etiology of PCs is unknown. Theories include persistent coalescing embryonic pineal cavities and glial degeneration with cavitation.

Pathology

Location. An easy way to remember the normal midline anatomic structures in the pineal region—from top to bottom—is "famous V.I.P." for fornix, velum interpositum, internal cerebral veins, and pineal gland (see Chapter 20). As expected, any pineal gland mass—including PCs—therefore lies below the fornix, velum interpositum, and internal cerebral veins, displacing them superiorly. Although PCs may compress the posterior third ventricle, most PCs exert little or no mass effect on the tectum and aqueduct, so hydrocephalus is rare except with very large cysts.

Size and Number. PCs are well-demarcated round or ovoid expansions within an otherwise normal-appearing pineal gland (28-24). Most PCs are less than 10 mm in diameter. The

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largest reported PC is 4.5 cm. PCs are usually unilocular, but lesions containing multiple smaller cysts do occur.

Gross Pathology. The general appearance is that of a smooth, soft, tan-yellow pineal gland that contains a unior multilocular cyst (28-25). Cyst fluid is clear to yellowish.

Microscopic Features. PCs are cavities of various sizes surrounded by an outer layer of attenuated pineal parenchyma. The inner layer is a sharply defined zone of finely fibrillar glial tissue with Rosenthal fibers. PCs have no ependymal or epithelial lining.

The inner surface of a pineal cyst cavity is often hemosiderin stained as the result of intralesional hemorrhage. There are no gross pathologic or histologic features that distinguish symptomatic from asymptomatic PCs.

(28-26A) Coronal NECT in a 49y woman with chronic headaches shows a large cystic pineal gland with thick rim calcification . (28-26B) Axial FLAIR in the same case shows that the fluid in the pineal cystdoes not compress completely.

(28-26C) T2WI in the same case shows that the cyst fluid appears slightly hyperintense compared with CSF in the ventricles. (28-26D) T1 C+ shows that the cyst fluid is slightly hyperintense compared with CSF in the adjacent third ventricle. Minimal enhancement of the cyst wall is present. This is nonneoplastic pineal cyst.

Clinical Issues

Epidemiology. Cystic-appearing lesions in the pineal gland are common on MR scans, seen in one-half of children and over one-quarter of healthy adults. Between 25-40% of autopsied pineal glands contain cysts.

Demographics. PCs can occur at any age although they are more often discovered in middle-aged and older adults.

The overall F:M ratio is 2:1. The incidence among women ages 21-30 years is significantly higher than in any other group.

Presentation. Most PCs are clinically benign and asymptomatic, discovered incidentally at imaging or autopsy. Large PCs may obstruct the cerebral aqueduct, resulting in hydrocephalus and headache. Parinaud syndrome (tectal compression) is less common.

Pineal "apoplexy" occurs with sudden intracystic hemorrhage. Acute worsening of headaches combined with visual

(28-27) Sagittal T2WI in a 17y girl with sudden onset of severe headache, visual difficulties shows a large pineal cyst with blood-fluid level indicating hemorrhage (cyst "apoplexy").

symptoms can occur. A "thunderclap" headache may mimic symptoms of aneurysmal subarachnoid hemorrhage. Pineal "apoplexy" can result in acute intraventricular obstructive hydrocephalus. Rare cases result in sudden death.

Natural History. Serial follow-up of indeterminate cystic lesions of the pineal region shows, in most lesions, no significant change in size or character over time intervals from months to years. Most investigators recommend that incidentally identified PCs be followed clinically and do not require serial imaging. Patients with growing lesions, atypical contrast enhancement, or hemorrhage on MR are more likely to develop hydrocephalus and have malignant pathology, so follow-up serial MRs are recommended.

Treatment Options. Most PCs are "leave me alone" lesions. Symptomatic cysts may require stereotactic aspiration or biopsy/resection.

Imaging

CT Findings. At least 25% of PCs show calcification within the cyst wall (28-26A). The cyst fluid is isoto slightly hyperdense compared with CSF. A very hyperdense PC in a patient with severe headache should raise suspicion of hemorrhage with cyst "apoplexy."

The ventricles are usually normal. Large ventricles with "blurred" margins indicate acute obstructive hydrocephalus.

Enhancement is typical. Rim, crescentic, or nodular patterns have all been described with PCs.

MR Findings. Thin-section high-resolution sagittal and axial T2 scans are especially helpful for detecting and characterizing lesions in the anatomically complex pineal region.

(28-28) Small pineal cyst is seen with hemorrhage , "blooming" on T2* , with rim and nodular enhancement on T1 C+ FS .

As with other cysts, PC signal intensity varies with imaging sequence and cyst contents (28-26).

Most PCs are small and cause minimal or no mass effect. Large cysts—or PCs with acute intracystic hemorrhage—may cause obstructive hydrocephalus. In such cases, PD and T2/FLAIR scans show "fingers" of hyperintensity extending into the periventricular white matter due to subependymal accumulation of brain interstitial fluid. These are especially well demonstrated on sagittal scans.

Between 50 and 60% of PCs are slightly hyperintense compared with CSF on T1WI. Approximately 40% are isointense with CSF. Approximately 1-2% are very hyperintense, which may indicate intracystic hemorrhage. A blood-fluid level may be present (28-27).

The vast majority of PCs are hyperintense to CSF on intermediate (PD) sequences and isoto slightly hyperintense on T2WI. Internal septations are visible in 20-25% of cases, and 10% are multicystic. If acute hemorrhage has occurred, intracystic blood may appear very hypointense.

PCs do not suppress completely on FLAIR and are moderately hyperintense relative to brain parenchyma. If intracystic hemorrhage has occurred, cyst fluid "blooms" on T2* (GRE, SWI) (28-28). Rim calcifications may show mild "blooming."

Between oneand two-thirds of PCs enhance. The most common pattern is a thin circumferential rim of enhancement. Less common patterns include nodular, crescentic, or irregular enhancement.

PCs typically do not restrict on DWI. Neuronal markers are absent on MRS.

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Differential Diagnosis

The most common differential diagnosis is normal pineal gland. Normal pineal glands often contain one or more small cysts and can have nodular, crescentic, or ring-like enhancement.

The most important pathologic entity to be differentiated from a PC is pineocytoma. Pineocytoma is a WHO grade I pineal parenchymal tumor that is usually solid or at least partially solid/cystic. Purely cystic pineocytomas are much less common and can be indistinguishable from PC on imaging. Pineocytomas can remain stable for many years without significant change on serial imaging.

Atypical imaging findings, focal invasion, or significant interval change in a presumed PC or pineocytoma should raise suspicion for the more aggressive pineal parenchymal tumor of intermediate differentiation (PPTID).

(28-29) Autopsy specimen shows a frontal meningioma with CSFvascular "cleft" and a large tumor-associated cyst . (28-30) Coronal T2WI shows a typical sphenoid wing meningioma with hyperintense pools of trapped fluid between the tumor and brain. (Courtesy M. Thurnher, MD.)

(28-31) FIESTA shows a large left vestibular schwannoma with small intratumoral , larger marginal cysts with blood-fluid levels , and very prominent extratumoral cysts in the cerebellopontine angle cistern that are different signal intensity from the adjacent normal CSF. (28-32) Axial T2WI shows a pituitary macroadenoma with suprasellar extension , a prominent tumorassociated cyst with a blood-fluid level .

PINEAL CYST

Pathology

•Usually < 1 cm, unilocular > multicystic

•Wall contains attenuated pineal parenchyma

•Fluid clear to yellowish

Clinical Issues

•Common

○23% of normal MRs, 25-40% of autopsies

•Occur at any age; more common in adults

•Usually asymptomatic, found incidentally

Imaging

•Ca++ (25%)

•Fluid slightly hyperintense to CSF on MR

•Rim, nodular, or crescentic enhancement

Differential Diagnosis

• Normal pineal gland, pineocytoma