Sebaceous cyst (trichilemmal cyst / pilar cyst) is a benign cystic lesion originating from the outer root sheath epithelium (trichilemma), showing abrupt keratinization without a granular layer, unlike epidermoid cyst. It most commonly involves the scalp (90%) and is more common in females. The cyst wall is filled with compact keratinous material that is more homogeneous and dense than the laminated keratin of epidermoid cyst. Clinically, it presents as a firm, mobile, slow-growing nodule on the scalp — typically without a punctum (central pore), unlike epidermoid cyst. Multiple trichilemmal cysts may show autosomal dominant inheritance. Rare transformation to proliferating trichilemmal cyst/tumor has been reported. Ultrasonography is the primary diagnostic modality, with homogeneous hypoechoic internal structure and punctate echogenic foci (microcalcifications) being characteristic findings.
Age Range
20-70
Peak Age
40
Gender
Female predominant
Prevalence
Common
Trichilemmal cyst originates from the outer root sheath (trichilemma) epithelium of the hair follicle. In the normal hair follicle, trichilemmal keratinization at the isthmus region occurs without a granular layer, unlike the epidermis — cells transform directly into compact keratin material. This abrupt keratinization ensures the keratin produced by the cyst wall is homogeneous, dense, and compact — structurally different from the concentric laminated keratin of epidermoid cyst. On ultrasonography, this homogeneous keratin creates a homogeneous hypoechoic appearance with low-level internal echoes: keratinous material contains uniformly distributed small particles that diffusely scatter sound waves (Rayleigh scattering) → homogeneous low-level echoes. Calcifications within the cyst (in 25% of cases) result from dystrophic calcification of keratin material — calcium salt accumulation creates strong acoustic impedance mismatch → punctate hyperechoic foci. Scalp predominance is explained by the dense hair follicle density in this region. Skin surface connection (punctum) is typically absent because trichilemmal cysts originate from deeper portions of the follicle.
A well-defined cystic lesion with homogeneous hypoechoic internal structure containing punctate echogenic foci (microcalcifications) on the scalp is a highly characteristic combination for trichilemmal (sebaceous) cyst. The absence of laminated pattern seen in epidermoid cyst and presence of calcifications form the basis of differential diagnosis.
On B-mode ultrasonography, homogeneous hypoechoic internal structure is seen — distinctly different from the laminated (onion-skin) pattern of epidermoid cyst. Low-level internal echoes are uniformly distributed within the cyst, distinguishing from anechoic fluid. This homogeneous pattern reflects the compact and uniform keratin production of trichilemmal keratinization. Best evaluated with high-frequency (12-18 MHz) linear probes.
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The cystic lesion on the scalp shows homogeneous hypoechoic internal structure; no laminated pattern is seen, consistent with trichilemmal (sebaceous) cyst.
Scattered punctate echogenic foci may be seen within the cyst — corresponding to dystrophic microcalcifications. Observed in approximately 25% of cases and is a highly specific finding for trichilemmal cyst. Calcifications are typically 1-3 mm and may or may not produce minimal posterior acoustic shadowing — very small calcifications show no shadowing. Calcifications are randomly distributed within the cyst, showing irregular arrangement unlike the laminated pattern of epidermoid cyst.
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Scattered punctate echogenic foci (microcalcifications) are seen within the cyst; characteristic finding for trichilemmal cyst.
Posterior acoustic enhancement is observed behind the trichilemmal cyst — reflecting lower acoustic attenuation of cyst content compared to surrounding soft tissue. Enhancement degree is similar to epidermoid cyst but may be less prominent in some cases due to dense compact keratin content. Presence of calcifications may create focal shadowing foci, causing heterogeneity in the posterior enhancement pattern.
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Posterior acoustic enhancement is observed behind the lesion; consistent with cystic content.
No vascular signal is detected within the cyst on Color and Power Doppler — avascular internal structure confirms cystic nature. Minimal vascularity may be seen in the cyst wall but the interior is completely avascular. Pericystic hypervascularity may develop during inflammation. Doppler evaluation is critical for differentiation from solid thyroid nodule or soft tissue tumor.
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No vascular signal detected within the cyst on Color Doppler; avascular internal structure consistent with cystic nature.
On T1-weighted MRI, trichilemmal cyst shows intermediate-to-high signal — reflecting short T1 relaxation time of dense proteinaceous keratinous content. This T1 signal elevation can distinguish from simple cysts with pure fluid content (low T1) and epidermoid cysts (variable T1). The cyst wall shows low T1 signal. No enhancement of cyst content on contrast-enhanced sequences.
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The lesion shows intermediate-to-high signal on T1-weighted images; consistent with dense proteinaceous keratinous content of trichilemmal cyst.
On T2-weighted images, trichilemmal cyst shows variable signal — fluid-dominant cysts show high T2, compact keratin-dominant cysts show intermediate T2 signal. Calcified foci show signal loss on all sequences. Variable degree of diffusion restriction may be seen on DWI — dense keratin material restricts water molecule movement.
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The lesion shows heterogeneous signal on T2-weighted images with focal signal voids consistent with calcifications.
Criteria
Solitary or multiple, well-defined cystic lesion on the scalp. Compact keratinous content.
Distinct Features
Homogeneous hypoechoic, posterior enhancement, avascular. Clinically firm, mobile, painless nodule. In multiple cases, autosomal dominant inheritance should be questioned.
Criteria
Rare neoplastic transformation. Rapid growth, large size (>5 cm), lobulated contour, solid components.
Distinct Features
Heterogeneous internal structure on US, increased vascularity, irregular wall, solid components. Shows enhancement on MRI. Wide excision needed due to malignancy potential.
Criteria
Inflammatory changes following cyst wall rupture or secondary infection. Pain, erythema, swelling.
Distinct Features
Thickened irregular wall, pericystic edema (hypoechoic halo), pericystic hypervascularity, heterogeneous content. May mimic abscess.
Distinguishing Feature
Epidermoid cyst shows laminated (onion-skin) pattern — trichilemmal cyst is homogeneous hypoechoic. Punctum (skin connection) is common in epidermoid, absent in trichilemmal. Epidermoid is common outside scalp while trichilemmal is 90% on scalp.
Distinguishing Feature
Lipoma is isoechoic/hyperechoic with parallel echogenic lines (fat lobules), compressible, no posterior enhancement. Trichilemmal cyst is hypoechoic, non-compressible, with posterior enhancement.
Distinguishing Feature
Pilomatrixoma is a solid hypoechoic nodule at dermis/subcutis junction with prominent calcification and shadowing — common in children. Trichilemmal cyst is cystic with posterior enhancement.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upTrichilemmal cyst is a completely benign lesion. Diagnosis can be confidently made when typical ultrasonographic findings are present. No treatment needed for asymptomatic cysts. Surgical excision is curative for symptomatic lesions. Gardner syndrome screening should be considered for multiple cysts. Histopathology is required for lesions showing rapid growth, solid components, or increased vascularity to exclude proliferating trichilemmal tumor.
Trichilemmal cysts are benign lesions. Surgical excision is curative. Proliferating trichilemmal tumor (malignant transformation) is extremely rare. Multiple cysts may show autosomal dominant inheritance. Infected cysts are treated with incisional drainage and antibiotic therapy.