Trichilemmal cyst

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Here is a comprehensive overview of the trichilemmal (pilar) cyst synthesized from Fitzpatrick's Dermatology, Andrews' Diseases of the Skin, and Dermatology (2-Volume Set, 5e):

Trichilemmal Cyst (Pilar Cyst)

Synonyms: Pilar cyst, wen, isthmus-catagen cyst

Origin and Pathogenesis

Trichilemmal cysts arise from the outer root sheath of the hair follicle at the isthmus (the region between the sebaceous gland orifice and the arrector pili muscle insertion). This epithelium undergoes a distinctive pattern of keratinization - cells become swollen and pale as they approach the cyst cavity, then abruptly keratinize without forming a granular cell layer (trichilemmal keratinization). This absence of a granular layer is the histological hallmark of the lesion.

Fitzpatrick's Dermatology, Vol. 1-2
Andrews' Diseases of the Skin

Epidemiology

Prevalence: 5-10% of the population
Predominantly affects middle-aged to older women (women over 60 especially in Andrews')
~90% of lesions occur on the scalp
Lesions are frequently multiple (most patients have more than one); 10% of patients have >10 lesions
Hereditary form (autosomal dominant) - linked to the short arm of chromosome 3 (not to β-catenin or MLH1); germline variant in PLCD1 (phospholipase C delta 1α) has been identified

Clinical Features

Trichilemmal (pilar) cyst on the scalp - a firm, well-circumscribed, dome-shaped nodule

Fig. 1: Trichilemmal (pilar) cyst on the scalp - Fitzpatrick's

Mobile, firm, well-circumscribed dermal/subcutaneous nodule
No overlying punctum (distinguishes it from epidermoid cyst)
Firmer and more mobile than epidermoid cysts
Overwhelmingly located on the scalp; rarely on face, trunk, extremities
Usually asymptomatic, but rupture and secondary infection can occur
Rapid growth is abnormal - raises suspicion for infection or malignant transformation

Histopathology

Fig. 2: H&E showing stratified squamous epithelium with abrupt trichilemmal keratinization and no granular layer - Fitzpatrick's

Key histological features:

Feature	Trichilemmal Cyst	Epidermoid Cyst
Granular layer	Absent	Present
Keratinization	Abrupt (trichilemmal)	Laminated, basket-weave
Cyst contents	Homogeneous eosinophilic keratin	Laminated keratinous material
Calcification	Common	Rare
Punctum	Absent	Usually present
Origin	Outer root sheath	Follicular infundibulum

Cyst contents consist of homogeneous, compact eosinophilic material that frequently calcifies and rarely ossifies. Hybrid cysts with features of both epidermoid and pilar cysts can occur.

Andrews' Diseases of the Skin; Dermatology 5e

Differential Diagnosis

Epidermoid (infundibular) cyst - most common mimic; epidermoid cysts have a granular layer and laminated keratin
Steatocystoma - contains sebaceous material
Alopecic and aseptic nodules of the scalp (if ruptured)
Dermoid cyst
Lymph node (if deep and firm)

Clinically, trichilemmal cysts are 4-5 times less common than epidermoid cysts. At the time of excision, the distinction is often apparent because trichilemmal cysts "deliver" themselves through a small incision more readily without rupture.

Treatment

Surgical excision is the treatment of choice
Because the cyst wall is less adherent to surrounding stroma than in epidermoid cysts, it can often be extracted through a small punch biopsy trephine incision
Pilar cysts are much more easily enucleated - more limited incision is required

Proliferating Trichilemmal Cyst / Proliferating Pilar Tumor

Proliferating trichilemmal cyst - large ulcerated exophytic nodule on the scalp of an elderly woman

Fig. 3: Proliferating pilar cyst - large, ulcerated scalp lesion - Andrews'

This represents a spectrum ranging from typical pilar cysts with focal proliferation to frankly malignant squamous cell carcinoma-like lesions:

Benign proliferating pilar tumor:

Large (up to 25 cm), exophytic neoplasm - almost exclusively on the scalp and back of neck
~5x more common in women; mean age 65 years
Gradually enlarges, may ulcerate
Histology: squamous cell proliferations with trichilemmal differentiation, scroll-like structures; focal atypia, mitoses, and necrosis do not necessarily predict aggressive behavior if an invasive pattern is absent
Vast majority cured by local excision

Malignant proliferating pilar tumor (malignant trichilemmal cyst):

Focal areas of frank SCC: increased cellularity, atypia, frequent mitoses, and stromal invasion
May behave aggressively with metastases
Red flags for aggressive behavior:
- Non-scalp location
- Recent rapid growth
- Size >5 cm
- Infiltrative growth pattern (clinical and histological)
Special context: in KID syndrome (keratosis, ichthyosis, deafness), malignant proliferating pilar tumor may develop in young adulthood and can be fatal

Histological distinction between proliferating pilar tumor and SCC relies on identification of squamous eddies, trichilemmal keratinization, or remnant of a pilar cyst. Immunohistochemistry can also assist.

Andrews' Diseases of the Skin; Fitzpatrick's Dermatology

A 2025 systematic review (PMID 39539041) specifically characterized the clinical and genomic profile of malignant proliferating trichilemmal tumors, confirming its rarity and aggressive potential when frank malignant transformation occurs.

Key Points Summary

Origin: Outer root sheath, isthmus of hair follicle
Site: 90% scalp, predominantly women
Hallmark histology: No granular layer + abrupt trichilemmal keratinization + homogeneous eosinophilic contents ± calcification
No punctum (unlike epidermoid cysts)
Genetics: AD inheritance linked to PLCD1 / chromosome 3p
Treatment: Surgical excision - easy enucleation through small incision
Malignant potential: Rare but real - watch for rapid growth, large size, ulceration, non-scalp location

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