Epidermoid cysts are benign ectodermal inclusion cysts. They can be congenital or acquired.
They account for approximately 1% of all intracranial tumors. Although these lesions are congenital, patients are usually not symptomatic until they are aged 20-40 years.
Congenital epidermoid cysts probably arise from inclusion of ectodermal epithelial elements at the time of neural tube closure or during formation of the secondary cerebral vescicals.
Acquired epidermoid cysts develop as a result of trauma. Here,epidermis is implanted into deeper underlying tissues and forms a cyst that continues to desquamate keratin.Intracranial implantation epidermoid cyst are uncommon,most eidermal inclusion cyts are found in lumbosacarl spine following non-stylet needle puncture.
Pathology:-
Gross pathology: Intracranial epidermoid tumours are well delineated cystic lesions that insinuate along the CSF cisterns.They have an irregular lobulated, or "cauliflower -like" outer surface that often has a shiny "mother of pearl" appearance.The cyst interior is filled with soft,waxy,or flaky keratohyaline material that results from progressive desquamation of the cyst wall.Epidermoid cysts encase vessels and engulf cranial nerves.
Microscopy: The cyst wall is composed of simple stratified cuboidal squamous epithelium and the centre is filled with keratanaceous debris and solid crystalline cholesterol
Location:-
90% cranial epidermoid tumours are intradural.Intradural epidermoid tumours occur primarily in the basal subarachnoid spaces : off-midline sites are the common locations.Between 40-50% are found in CP angle cisterns making epidermoid the 3rd most common CP angle tumour( after acoustic schwannoma and meningioma)
Intra axial epidermoid tumours are uncommon.The 4th ventricle is the most common site.
10% epidermoid cysts are intradural mostly intradiploic.
Cranial epidermoid and dermoid cysts are the most common scalp lesions in children.
Imaging :-
Plain ragiographs: Diploic space tumours can affect any part of the skull;the scalp,diploie(inner or outer table or both) ,and sometimes the epidural space as well.Diploic epidermoid cysts typically have round or lobulated ,well delineated focal bone erosions with sclerotic margins.
Angiography: Cerebral angiography is usually normal or discloses mass effect.
CT: NECT scans shows a well defined lucent-appearing lobulated masses with attenuation similar to CSF.Calcification is seen in 10-20%. Occasionaly they may appear hyperdense on NECT ,possibly due to haemorrhage,high protein content ,saponification cyst debrisor deposition of iron pigments.Most do not show any contrast enhancement but sometimes enhancement can be seen at the tumour margins.
MR: Epidermoid tumours typically have long T1 and T2 relaxation times and signal characteristics similar to CSF.Sometimes they appear iso or even hyperintense on T1WI.These so called "white epidermoids" have high lipid content on MR Spectroscopy,whereas epidermoids with long T1 values (black epidermoids) have reduced lipid content on spectroscopy.
Differential diagnosis:-
Diffusion weighted MR can help in distinguishing between arachnoid cysts and epidermoid tumours.The ADC of arachnoid cysts is similar to stationary water, whereas ADC of epidermoid tumours is similar to CSF.
Dermoid tumours appear hyperintense on T1 and hypointense on T2