Trichilemmal Cyst (Pilar Cyst) Workup

Updated: Jun 11, 2020
  • Author: Anne Elizabeth Laumann, MBChB, MRCP(UK), FAAD; Chief Editor: William D James, MD  more...
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Workup

Imaging Studies

Radiography of the head, CT scanning, and MRI may be needed to differentiate midline scalp lesions that may have a connection to the meninges or the central nervous system. Ultrasonography may be a cheaper, less invasive, and equally diagnostic option. [18]

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Procedures

Excision and histopathologic evaluation can confirm the diagnosis. Trichilemmal cysts can usually be extracted more easily than epidermoid cysts. [3]

If a trichilemmal cyst becomes inflamed and ruptures, the cyst should be excised and submitted for histopathology in order to exclude carcinoma, particularly nodular or nodulocystic basal cell carcinomas. [19] Inflamed, ruptured cysts may have an infectious etiology. Wound culture may elucidate infection and guide therapy if necessary. [1]

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Histologic Findings

Trichilemmal cysts are surrounded by fibrous capsules against which rest layers of small, cuboidal, dark-staining basal epithelial cells in a palisade arrangement with no distinct intercellular bridging. These merge with characteristic squamous epithelium composed of swollen pale keratinocytes, which increase in height as they mature and transform abruptly into solid eosinophilic-staining keratin without the formation of a granular cell layer. They often have foci of calcification. [10] In contrast, epidermoid cysts have a granular cell layer in the lining epithelium. Epidermoid cysts (nonimplantation variant) have laminated keratin, which is believed to be derived from the follicular infundibulum. [10] Note the slides below.

Pilar cyst, low power. The trichilemmal cyst is li Pilar cyst, low power. The trichilemmal cyst is lined by squamous epithelium without a granular layer (trichilemmal keratinization) and with swelling of the cells close to the cyst cavity, which is filled with homogenous keratin. Calcifications are a common feature.
Pilar cyst, medium magnification. Higher magnifica Pilar cyst, medium magnification. Higher magnification shows the stratified squamous epithelium without a granular layer and shows swelling of the cells closest to the cyst cavity.

Tricholemmal cyst keratin stains with antikeratin antibodies derived from human hair. In contrast, epidermal cyst keratin stains with human callus–derived antikeratin antibodies. [20]

Very rarely, sebaceous or apocrine differentiation occurs in a cyst wall. [21] Calcification may be seen, and a few reports have described ossification. [22, 23]

If cyst wall rupture occurs, a foreign body giant cell reaction may surround the cyst. [6, 10]  Note the slides below.

Ruptured pilar cyst. A ruptured trichilemmal cyst Ruptured pilar cyst. A ruptured trichilemmal cyst showing replacement of the squamous lining by histiocytes and rare multinucleated giant cells.
Ruptured, low power. Another ruptured trichilemmal Ruptured, low power. Another ruptured trichilemmal cyst showing replacement of the epithelial lining by granulomatous reaction with numerous cholesterol clefts and calcifications.
Ruptured, high power. Higher-power image highlight Ruptured, high power. Higher-power image highlighting the cholesterol clefts and calcifications.

Lobules of squamous epithelium in the cyst wall suggest a proliferating trichilemmal cyst. [1] Other histologic characteristics of proliferating trichilemmal cysts may include mitoses, cell atypia, and necrosis. Penetration of tumor cells through the cyst lining suggests malignancy. [3]

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