Epidermoid cyst (pearly tumor) is a slow-growing, benign, congenital intracranial lesion that arises from inclusion of ectodermal remnants during neural tube closure in embryological development. It most commonly occurs in the cerebellopontine angle (CPA) and parasellar region. It is an encapsulated cystic structure containing keratin laminations, cholesterol crystals, and debris. Despite showing CSF-like CT density and T1/T2 signal characteristics, the marked diffusion restriction on DWI is a pathognomonic finding for differentiation from arachnoid cyst. It accounts for approximately 1% of all intracranial tumors and typically becomes symptomatic between ages 20-40.
Age Range
20-60
Peak Age
40
Gender
Equal
Prevalence
Uncommon
Epidermoid cyst forms when surface ectoderm cells become sequestered during neural tube closure at 3-5 weeks of gestation during embryogenesis. These ectopic ectoderm cells maintain a keratinization cycle like normal skin; continuous desquamation leads to accumulation of keratin laminations, cholesterol crystals, and cellular debris within the lumen. This accumulation creates a tightly packed layered structure (lamellar keratin) within the cyst that restricts free diffusion of water — forming the basis of the pathognomonic diffusion restriction on DWI. The cyst grows approximately 0.2-0.5 mm linearly per year and causes slowly progressive neurological deficits through mass effect on surrounding structures. In case of cyst rupture, dissemination of keratin and cholesterol crystals into the subarachnoid space can cause severe chemical/aseptic meningitis.
The combination of marked diffusion restriction on DWI (bright signal, low ADC) and non-suppression on FLAIR (hyperintense signal relative to CSF) is pathognomonic for epidermoid cyst. Together, these two findings provide definitive differentiation from the most important differential diagnoses including arachnoid cyst (DWI: no restriction, FLAIR: complete suppression) and abscess (DWI: restriction present, but accompanied by enhancement and perilesional edema). The behavior of keratin laminations and cholesterol crystals on both sequences creates this pathognomonic appearance.
On non-contrast CT, a lobulated lesion near CSF density (0-20 HU) or slightly hypodense is observed. Margins are irregular with insinuative extension between surrounding structures. Calcification is uncommon but peripheral calcification may be seen (10-25%). Bone remodeling due to chronic pressure may be observed in adjacent osseous structures. Differentiation from arachnoid cyst on CT is difficult; both may appear at CSF density.
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On CT, an extra-axial lobulated lesion near CSF density is identified, without calcification or enhancement.
On T1-weighted images, epidermoid cyst generally appears isointense or slightly hyperintense relative to CSF. T1 signal varies depending on the amount of cholesterol crystals and proteinaceous debris within the cyst. In some cases, marked T1 hyperintensity may be seen ('white epidermoid'); this reflects high cholesterol/triglyceride content. Lobulated margins and insinuative growth pattern between cisterns are observed. Thin capsule appears isointense to gray matter.
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On T1-weighted sequence, an extra-axial lobulated lesion near CSF signal but slightly hyperintense is observed.
On T2-weighted images, epidermoid cyst shows markedly hyperintense signal and may be confused with CSF at first glance. However, careful examination reveals slightly different signal intensity from CSF ('dirty CSF' appearance) and internal heterogeneity. Fine laminations or debris within the cyst may cause slight signal variations on T2. Lobulated margins, insinuative extension within cisterns, and relationship with surrounding structures are best evaluated on T2 sequences. Perilesional edema is typically absent.
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On T2-weighted sequence, a markedly hyperintense lobulated extra-axial lesion near CSF signal but showing internal heterogeneity ('dirty CSF' appearance) is observed.
On FLAIR, epidermoid cyst signal does not fully suppress and appears markedly hyperintense compared to CSF — this finding is one of the most valuable MRI findings for differentiation from arachnoid cyst which completely suppresses on FLAIR. The hyperintensity results from the T1 shortening effect of keratin debris, cholesterol crystals, and proteinaceous material within the cyst. Internal heterogeneity may be more pronounced on FLAIR. A thin hyperintense rim at the cyst margin may reflect reactive changes due to chemical irritation.
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On FLAIR, lesion signal does not fully suppress and appears markedly hyperintense compared to CSF, supporting differentiation from arachnoid cyst and favoring epidermoid cyst.
On DWI, epidermoid cyst shows markedly high signal (bright) with low signal on ADC map — true diffusion restriction is present. This is the most critical and pathognomonic finding for epidermoid cyst diagnosis and provides definitive differentiation from arachnoid cyst (no diffusion restriction on DWI). Diffusion restriction results from tightly packed keratin laminations and cholesterol crystals within the cyst impeding free movement of water molecules. ADC values are typically 0.6-1.0 × 10⁻³ mm²/s (CSF: ~3.0 × 10⁻³ mm²/s).
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On diffusion-weighted imaging, the lesion shows markedly high signal with low signal on ADC map — true diffusion restriction is detected, which is pathognomonic for epidermoid cyst diagnosis.
On contrast-enhanced MRI, epidermoid cyst contents show no enhancement. Minimal thin linear enhancement of the capsule may rarely be seen, but no solid enhancing component is present. The absence of enhancement reflects the avascular nature of the tumor and intact cyst wall. If significant enhancement is present, malignant transformation (squamous cell carcinoma — very rare, <1%) or inflammatory changes should be considered.
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On post-contrast T1-weighted sequence, no enhancement of the lesion contents is observed, consistent with the avascular nature of epidermoid cyst.
On SWI, epidermoid cyst typically shows no significant blooming artifact. No hemosiderin deposition or calcification is present (unlike cavernous malformation). However, in rare cases, minimal calcification in the cyst wall may create foci of low signal on SWI. No flow voids or vascular structures are seen (unlike hypervascular tumors). SWI is useful for differentiating epidermoid cyst from cavernous malformation and hemorrhagic lesions.
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On SWI, no significant blooming artifact, hemosiderin deposition, or vascular structures are identified within the lesion.
On MR spectroscopy, epidermoid cyst shows a prominent lactate peak (1.33 ppm — doublet) and variable lipid content may be present. N-acetylaspartate (NAA) and choline peaks are absent or very low because the lesion contains no neuronal or proliferative tissue. Creatine peak is also absent. This metabolite profile — lactate dominant, other metabolites absent — reflects a cystic/debris-containing lesion and is valuable for differentiation from solid tumors showing elevated choline.
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On MR spectroscopy, a prominent lactate peak is detected within the lesion with absent choline, NAA, and creatine peaks; this metabolite profile is consistent with a cystic/debris-containing lesion.
Criteria
Most common type. Isointense or slightly hyperintense to CSF on T1, hyperintense on T2. Marked restriction on DWI. Typical CPA, suprasellar, or sylvian fissure location. Keratin lamellae and cholesterol crystals predominant.
Distinct Features
Standard epidermoid cyst imaging features. CT and T1/T2 signal that may be confused with CSF but definitive differentiation possible with DWI and FLAIR. No enhancement.
Criteria
Rare variant (2-5%). Markedly hyperintense (white) signal on T1 — reflecting high cholesterol/triglyceride/protein content. Variable signal on T2. Lipid peaks may be prominent on MRS. 'White epidermoid' name comes from T1 appearance.
Distinct Features
T1 hyperintensity may be confused with dermoid cyst, but dermoid shows signal loss on fat-suppressed sequences while white epidermoid shows variable response to fat suppression. DWI restriction supports diagnosis. Chemical shift artifact may provide a clue.
Criteria
Rare variant located in the diploic space of skull bones. Usually in temporal or occipital bones. Seen as lytic lesion with sclerotic margins on CT. May constitute 25% of all epidermoids.
Distinct Features
Expansile lytic lesion with sclerotic margins on bone window CT. May show intra- and extracranial extension. DWI features are same as intracranial epidermoid. Differential diagnosis with dermoid cyst, eosinophilic granuloma, and hemangioma is required.
Distinguishing Feature
Arachnoid cyst follows pure CSF signal on all sequences: complete suppression on FLAIR, no diffusion restriction on DWI (high ADC). Epidermoid cyst does not suppress on FLAIR and shows marked restriction on DWI — these two sequences provide definitive differentiation.
Distinguishing Feature
Dermoid cyst is markedly hyperintense on T1 (due to fat content) and shows signal loss on fat-suppressed sequences. Epidermoid cyst is generally CSF-like on T1 with minimal change on fat suppression. Subarachnoid fat droplets are pathognomonic when dermoid cyst ruptures.
Distinguishing Feature
Brain abscess also shows diffusion restriction on DWI (viscous purulent content), but ring enhancement on contrast MRI, perilesional vasogenic edema, and clinical infection findings (fever, leukocytosis) are present. Epidermoid cyst shows no enhancement or edema.
Distinguishing Feature
Neurocysticercosis cystic lesion shows scolex (eccentric nodule) within the cyst, and enhancing wall and perilesional edema may accompany. Endemic region history is important. Epidermoid cyst shows no scolex, enhancement, or edema.
Distinguishing Feature
Primary CNS lymphoma is a solid, homogeneously enhancing periventricular lesion showing DWI restriction due to cellularity. Epidermoid cyst shows no enhancement, is cystic, and has extra-axial location; lymphoma's intratumoral enhancement and periventricular predilection are distinguishing.
Urgency
electiveManagement
surgicalBiopsy
Not NeededFollow-up
Annual MRI if asymptomatic; post-surgical MRI at 3 months then annually for 5 years to detect residual/recurrenceEpidermoid cyst is a benign lesion and treatment is surgical in symptomatic cases. Surgical goal is evacuation of cyst contents and removal of as much capsule as possible. When capsule is adherent to brainstem or cranial nerves, subtotal resection is preferred — aggressive capsule dissection may cause neurological deficits. Recurrence from residual capsule is reported at 10-36%, and DWI follow-up is the most sensitive method. Aseptic meningitis may develop post-surgically or with spontaneous rupture and requires steroid treatment. Malignant transformation (squamous cell carcinoma) is very rare, reported at <1%.
Epidermoid cysts are slow-growing benign lesions. Symptoms usually arise from cranial nerve compression (trigeminal neuralgia, facial palsy) or obstructive hydrocephalus. Treatment is surgical resection. Spillage of cyst contents into subarachnoid space can cause aseptic meningitis (Mollaret meningitis). Total resection may not always be possible — recurrence after subtotal resection is slow.