Trichilemmal cysts, also known as pilar cysts, are benign accumulations of keratin along the outer hair root sheath, most commonly on the scalp. They are the most common subcutaneous nodule incidentally found on head imaging and are of no clinical relevance when asymptomatic 1. Uncommonly, they can develop into the neoplastic form known as proliferating trichilemmal cysts, which can be locally aggressive.
Epidemiology
They are found in 2.5% of adult patients undergoing head imaging 1.
Pathology
The name trichilemmal relates to its presumed origin from the trichilemma (the outer root sheath) of the hair follicle. This part undergoes a specific form of keratinization called trichilemmal keratinization which is diffuse and homogenous unlike the lamellated keratin of epidermal cysts.
Grossly, these lesions are firm cystic structures that contain white semisolid cheese like material. They frequently contain areas of dystrophic calcification as well as areas of high density on CT due to presence of abundant cholesterol crystals within compact keratin. Due to the nature of their content, these lesions are typically avascular on imaging.
Radiographic features
These nodules are small, ovoid, circumscribed subcutaneous masses, without overlying skin ulceration or underlying bone erosion 1.
Ultrasound
Trichilemmal cysts usually have a heterogenous echotexture, mostly hypoechoic. They should not have complete internal echogenic foci and should be avascular 3. There is often posterior acoustic enhancement 3.
CT
Trichilemmal cysts are soft tissue attenuation and may contain speckled calcifications as well as areas of high density due to cholesterol crystals and compact keratin 1.
MRI
T1: isointense 1 or hyperintense 4 to brain parenchyma
T1 C+: no internal contrast enhancement
T2: intermediate signal intensity usually with linear or reticular hypointensity related to calcification 4
Differential diagnosis
proliferating trichilemmal cyst: consider this entity when the mass is large (>5 cm), recently growing, or shows heterogeneous high signal on T2-weighted images or contrast enhancement 1
epidermal inclusion cysts (also known as epidermoid or sebaceous cysts): uncommon on the scalp, more often have high or mixed signal intensities on T1 and T2-weighted images 1
pilomatricoma (also known as calcifying epithelioma): usually in children or adolescents
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