using FLAIR and DWI. ACs suppress completely on FLAIR and

do not show diffusion restriction.

Extraaxial tumors, such as meningioma, schwannoma,

pituitary macroadenoma, and craniopharyngioma, may be

associated with prominent extratumoral cysts. These

nonneoplastic TACs occur in both the supra- and infratentorial

compartments.

TACs are benign collections of fluid that vary from clear and

CSF-like to proteinaceous. TACs are typically positioned

between at the tumor-brain interface, between the mass and

adjacent cortex. Whether TACs are true ACs, obstructed PVSs

(Virchow-Robin), or fluid collections mostly lined by gliotic

brain is debatable.

Scalp and skull cysts are less common than intracranial cysts.

Sebaceous cysts [more accurately termed trichilemmal cysts

(TCs)] are a common scalp mass in middle-aged and older

patients. Most are identified incidentally on MR and CT scans.

TCs can be solitary or multiple, are well delineated, and vary in

size from a few millimeters to several centimeters. The classic

finding is a subepidermal scalp tumor in a woman over the age

of 60 years.

Leptomeningeal cysts, also known as "growing fractures," are

a rare but important extraaxial cyst that is most commonly

found in the parietal bone. An enlarging calvarial fracture

adjacent to posttraumatic encephalomalacia is typical. The

vast majority of patients are under 3 years of age. They

present with an enlarging, palpable soft tissue mass. Fluid and

encephalomalacic brain extrude through torn dura and

arachnoid and then through the enlarging linear calvarial

fracture. Leptomeningeal cysts are seen as linear lucent skull

lesions with rounded, scalloped margins.

Infratentorial extraaxial cysts: Most nonneoplastic cysts in

the posterior fossa occur off midline. The 2 major cyst types

found in this location are ECs and ACs.

The cerebellopontine angle (CPA) is by far the most common

posterior fossa sublocation of an EC. Occasionally, an EC

occurs in the 4th ventricle. A 4th ventricular EC can mimic a

trapped, dilated 4th ventricle, but ECs do not suppress on

FLAIR and usually exhibit some degree of restricted diffusion.

The next most common posterior fossa cyst is AC. Although

ACs can also occur in the midline cisterna magna, the CPS is

the most common site. TACs sometimes occur in the CPA

cistern. Most are associated with vestibular schwannoma, but

a CPA meningioma may also cause formation of a TAC.

NEs are congenital endodermal cysts that are much more

commonly found in the spinal canal. Intracranial NE cysts occur

in the cerebromedullary cistern and are usually midline or

slightly off midline, lying just anterior to the pontomedullary

junction. Sometimes NE cysts occur off midline, in the lower

CPA (cerebromedullary) cistern. Bony skull defects can occur

but are rare.

An anatomic variant that can be confused with a posterior

fossa NE cyst is retroclival ecchordosis physaliphora (EP),

found in about 2% of autopsies. EP is a gelatinous notochordal

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Primary Nonneoplastic Cysts Overview

remnant that can occur anywhere from the dorsum sellae to

the sacrococcygeal region. Intracranial EPs typically occur in

the prepontine cistern and are attached to a defect in the

dorsal clivus by a thin, stalk-like pedicle. NE cysts and EPs are

both hyperintense on T2WI. Chordomas are the malignant

counterparts of ecchordosis.

Intraaxial Cysts

Supratentorial intraaxial cysts: Here, anatomic sublocation is

key to the differential diagnosis. Parenchymal cysts represent

a completely different group than intraventricular cysts. The

most common parenchymal cysts in the brain are enlarged

PVSs (Virchow-Robin). PVSs have a distinct predilection for the

basal ganglia, where they tend to cluster around the anterior

commissure. The midbrain is another common site. When they