Background
Trichilemmal (pilar) cysts are common intradermal or subcutaneous cysts, occurring in 5-10% of the population. [1, 2] More than 90% of trichilemma cysts occur on the scalp, where they are the most common type of cutaneous cyst. [3, 4] Trichilemmal cysts are almost always benign. They may be sporadic, or they may be inherited in an autosomal dominant manner. [5] They contain keratin and its breakdown products and are lined by walls resembling the external (outer) root sheath of the hair.
In 2% of trichilemmal cysts, single or multiple foci of proliferating cells lead to tumors called proliferating trichilemmal cysts. [6] Proliferating trichilemmal cysts are gradually enlarging (up to 25 cm in diameter) exophytic nodules that occasionally ulcerate. [3] Although these tumors are biologically benign, they may be locally aggressive. Recurrences and metastases have been observed, with rare malignant transformation. [7, 5, 6, 8, 9]
Pathophysiology
Frequently but erroneously called sebaceous cysts, trichilemmal cysts are lined by stratified squamous epithelium similar to that in the isthmus of the hair follicle. [5] This is the segment between the insertion of the erector pili muscle and the sebaceous gland duct, where no inner root sheath exists. The keratinization is similar to that which occurs in the outer root sheath. [10] The squamous epithelium undergoes so-called trichilemmal keratinization or rapid keratin formation without a granular cell layer, resulting in a cyst wall that lacks a granular cell layer. [3, 10]
Etiology
If the cysts are hereditary, the inheritance pattern is usually autosomal dominant. [5, 11] Hereditary trichilemmal cysts link to the short arm of chromosome 3. [12]
Epidemiology
Trichilemmal cysts are common in the United States, occurring in 5-10% of the population. [1] They are more common in middle-aged persons than in younger persons, and they occur more frequently in women than in men. Trichilemmal cysts have no known racial predilection.
Prognosis
Trichilemmal cysts are biologically benign but may be locally aggressive in some cases. [13] Malignant transformation is very rare but may lead to distant metastases. [5, 6]
Case reports have described Merkel cell carcinoma arising from Merkel cells in trichilemmal cysts. [14, 15]
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Firm, smooth swelling on scalp.
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Surgical removal of intact trichilemmal (pilar) cyst through elliptical excision.
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Closure of defect after surgical removal of trichilemmal (pilar) cyst.
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Trichilemmal (pilar) cyst is lined by squamous epithelium without granular layer and with swelling of cells close to cyst cavity, which is filled with homogenous keratin.
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At higher magnification, trichilemmal (pilar) cyst is lined by squamous epithelium without granular layer and with swelling of cells close to cyst cavity, which is filled with homogenous keratin.
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Numerous cholesterol clefts are identified within homogenous keratin of trichilemmal (pilar) cyst.
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Trichilemmal (pilar) cyst, low power. Trichilemmal cyst is lined by squamous epithelium without granular layer (trichilemmal keratinization) and with swelling of cells close to cyst cavity, which is filled with homogenous keratin. Calcifications are common.
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Trichilemmal (pilar) cyst, medium magnification. Higher magnification shows stratified squamous epithelium without granular layer and shows swelling of cells closest to cyst cavity.
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Ruptured trichilemmal (pilar) cyst. Replacement of squamous lining by histiocytes and rare multinucleated giant cells.
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Ruptured trichilemmal (pilar) cyst, low power. Replacement of epithelial lining by granulomatous reaction with numerous cholesterol clefts and calcifications.
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Ruptured trichilemmal (pilar) cyst, high power. Cholesterol clefts and calcifications are highlighted.
