Pilar Cyst (Trichilemmal Cyst) 

  • Author: Anne Elizabeth Laumann, MBChB, MRCP(UK), FAAD; Chief Editor: William D James, MD   more...
 
Updated: Nov 8, 2011
 

Background

Pilar cysts are common, occurring in 5-10% of the population.[1] Greater than 90% occur on the scalp, where pilar cysts are the most common cutaneous cyst. Pilar cysts are the second most frequent type of cyst on the head and neck.[2, 3] Pilar cysts are almost always benign, malignant transformation being extremely rare. Pilar cysts may be sporadic or may be autosomal dominantly inherited.[4] They contain keratin and its breakdown products and are lined by walls resembling the external (outer) root sheath of the hair.[1]

In 2% of pilar cysts, single or multiple foci of proliferating cells lead to proliferating tumors, often called proliferating trichilemmal cysts. Proliferating trichilemmal cysts grow rapidly and may also arise de novo. Although biologically benign, they may be locally aggressive, becoming large and ulcerated. Rarely, malignant transformation leads to distant metastases.[5] No absolute clinical criteria can distinguish malignant from benign proliferating pilar tumors. The entire lesion must undergo histological evaluation to assess for malignancy, and atypia and a high mitotic rate are suggestive of malignancy.

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Pathophysiology

Erroneously but frequently referred to as sebaceous cysts, trichilemmal cysts are derived from the outer root sheath or trichilemma of the hair follicle. The keratinization that occurs in these cysts is similar to that which occurs in the outer root sheath of the hair in the follicular isthmus, where no inner root sheath exists. The squamous epithelium undergoes rapid keratinization, resulting in a cyst wall without a granular layer.[6] Trichilemmal cyst keratin stains with antikeratin antibodies derived from human hair. In contrast, epidermal cyst keratin stains with human callus–derived antikeratin antibodies.[7]

Pilar cysts occur preferentially in areas with dense hair follicle concentrations; therefore, 90% occur on the scalp. Pilar cysts are solitary in 30% of patients and multiple in 70% of patients.[8]

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Epidemiology

Frequency

United States

Pilar cysts are common, occurring in 5-10% of the population. Of people with these cysts, 70% have several and 10% have more than 10 lesions.[8]

Mortality/Morbidity

Pilar cysts are biologically benign, but they may be locally aggressive. Malignant transformation is very rare but may lead to distant metastases.[5]

Case reports have described Merkel cell carcinoma arising from Merkel cells in trichilemmal cysts.[9, 10]

Race

Pilar cysts have no known racial predilection.

Sex

Pilar cysts are more commonly found in women than in men.[8]

Age

Pilar cysts are most common in persons of middle age compared with younger persons.[8]

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Contributor Information and Disclosures
Author

Anne Elizabeth Laumann, MBChB, MRCP(UK), FAAD  Associate Professor, Department of Dermatology, Northwestern University, The Feinberg School of Medicine

Anne Elizabeth Laumann, MBChB, MRCP(UK), FAAD is a member of the following medical societies: American Academy of Dermatology, Association of Professors of Dermatology, British Association of Dermatologists, Chicago Dermatological Society, Chicago Medical Society, Illinois Dermatological Society, Illinois State Medical Society, Illinois State Medical Society, Medical Dermatology Society, and Society for Investigative Dermatology

Disclosure: Abbott Grant/research funds Other; Regeneron Grant/research funds Other; Centocor Grant/research funds Other; OSI Grant/research funds Other; Celgene Grant/research funds Other; Lilly Grant/research funds Other

Coauthor(s)

Jenneé A Rommel, MD  Resident Physician, Combined Internal Medicine/Dermatology Residency Program, Washington Hospital Center, Georgetown University School of Medicine

Disclosure: Nothing to disclose.

Marjan Mirzabeigi, MD  Fellow in Dermatopathology, Department of Dermatology, Northwestern University

Marjan Mirzabeigi, MD is a member of the following medical societies: American Society for Clinical Pathology and College of American Pathologists

Disclosure: Nothing to disclose.

Specialty Editor Board

R Stan Taylor, MD  The JB Howell Professor in Melanoma Education and Detection, Departments of Dermatology and Plastic Surgery, Director, Skin Surgery and Oncology Clinic, University of Texas Southwestern Medical Center

R Stan Taylor, MD is a member of the following medical societies: American Academy of Dermatology, American College of Mohs Surgery, American Dermatological Association, American Medical Association, American Society for Dermatologic Surgery, Christian Medical & Dental Society, and Society for Investigative Dermatology

Disclosure: Nothing to disclose.

David F Butler, MD  Professor of Dermatology, Texas A&M University College of Medicine; Chair, Department of Dermatology, Director, Dermatology Residency Training Program, Scott and White Clinic, Northside Clinic

David F Butler, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Dermatology, American Medical Association, American Society for Dermatologic Surgery, American Society for MOHS Surgery, Association of Military Dermatologists, and Phi Beta Kappa

Disclosure: Nothing to disclose.

Jeffrey Meffert, MD  Assistant Clinical Professor of Dermatology, University of Texas School of Medicine at San Antonio

Jeffrey Meffert, MD is a member of the following medical societies: American Academy of Dermatology, American Medical Association, Association of Military Dermatologists, and Texas Dermatological Society

Disclosure: Nothing to disclose.

Catherine M Quirk, MD  Clinical Assistant Professor, Department of Dermatology, University of Pennsylvania

Catherine M Quirk, MD is a member of the following medical societies: Alpha Omega Alpha and American Academy of Dermatology

Disclosure: Nothing to disclose.

Chief Editor

William D James, MD  Paul R Gross Professor of Dermatology, Vice-Chairman, Residency Program Director, Department of Dermatology, University of Pennsylvania School of Medicine

William D James, MD is a member of the following medical societies: American Academy of Dermatology and Society for Investigative Dermatology

Disclosure: Elsevier Royalty Other

Additional Contributors

The authors and editors of Medscape Reference gratefully acknowledge the contributions of previous author, Mohsin Ali, MBBS, FRCP, MRCP, to the development and writing of this article.

References

  1. Casas JG, Woscoff A. Giant pilar tumor of the scalp. Arch Dermatol. Dec 1980;116(12):1395. [Medline].

  2. Al-Khateeb TH, Al-Masri NM, Al-Zoubi F. Cutaneous cysts of the head and neck. J Oral Maxillofac Surg. Jan 2009;67(1):52-7. [Medline].

  3. Wolff K, Johnson RA, Suurmond D. Miscellaneous Cysts and Pseudocysts. In: Seils A, Englis MR, eds. Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology. 5th ed. New York, NY: McGraw-Hill; 2005:200.

  4. Leppard BJ, Sanderson KV, Wells RS. Hereditary trichilemmal cysts. Hereditary pilar cysts. Clin Exp Dermatol. Mar 1977;2(1):23-32. [Medline].

  5. Weiss J, Heine M, Grimmel M, Jung EG. Malignant proliferating trichilemmal cyst. J Am Acad Dermatol. May 1995;32(5 Pt 2):870-3. [Medline].

  6. Kirkham N. Tumors and Cysts of the Epidermis. In: Elder DE, Elenitsas R, Johnson BL, Murphy GF, Xu X, eds. Lever's Histopathology of the Skin. 10th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2004:694-7.

  7. Cotton DW, Kirkham N, Young BJ. Immunoperoxidase anti-keratin staining of epidermal and pilar cysts. Br J Dermatol. Jul 1984;111(1):63-8. [Medline].

  8. Valencia TD, Swanson NA, Lee KK. Benign Epithelial Tumors, Hamartomas, and Hyperplasias. In: Wolff K, Goldsmith LA, Katz SI, Gilchrest B, Paller AS, Leffell DJ, eds. Fitzpatrick's Dermatology in General Medicine. Vol 1. 7th ed. New York, NY: McGraw-Hill; 2008:1064-5.

  9. Ivan D, Bengana C, Lazar AJ, Diwan AH, Prieto VG. Merkel cell tumor in a trichilemmal cyst: collision or association?. Am J Dermatopathol. Apr 2007;29(2):180-3. [Medline].

  10. Su W, Kheir SM, Berberian B, Cockerell CJ. Merkel cell carcinoma in situ arising in a trichilemmal cyst: a case report and literature review. Am J Dermatopathol. Oct 2008;30(5):458-61. [Medline].

  11. Eiberg H, Hansen L, Hansen C, Mohr J, Teglbjaerg PS, Kjaer KW. Mapping of hereditary trichilemmal cyst (TRICY1) to chromosome 3p24-p21.2 and exclusion of beta-CATENIN and MLH1. Am J Med Genet A. Feb 15 2005;133A(1):44-7. [Medline].

  12. Hanau D, Grosshans E. Trichilemmal cyst with intrinsic parietal sebaceous and apocrine structures. Clin Exp Dermatol. Sep 1980;5(3):351-5. [Medline].

  13. Mehrabi D, Leonhardt JM, Brodell RT. Removal of keratinous and pilar cysts with the punch incision technique: analysis of surgical outcomes. Dermatol Surg. Aug 2002;28(8):673-7. [Medline].

  14. Satyaprakash AK, Sheehan DJ, Sangueza OP. Proliferating trichilemmal tumors: a review of the literature. Dermatol Surg. Sep 2007;33(9):1102-8. [Medline].

  15. McGavran MH, Binnington B. Keratinous cysts of the skin. Identification and differentiation of pilar cysts from epidermal cysts. Arch Dermatol. Oct 1966;94(4):499-508. [Medline].

  16. Pinkus H. "Sebaceous cysts" are trichilemmal cysts. Arch Dermatol. May 1969;99(5):544-55. [Medline].

  17. Rook A, Wilkinson DS, Ebling FJG, et al, eds. Trichilemmal cyst. In: Textbook of Dermatology. Vol 2. 6th ed. London, England: Blackwell Science; 1998:1667-8.

 
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A firm, smooth swelling on the scalp.
Surgical removal of an intact pilar cyst through an elliptical excision.
Closure of the defect after surgical removal of the pilar cyst.
The trichilemmal cyst is lined by a squamous epithelium without a granular layer and with swelling of the cells close to the cyst cavity, which is filled with homogenous keratin.
At higher magnification, the cyst is lined by a squamous epithelium without a granular layer and with swelling of the cells close to the cyst cavity, which is filled with homogenous keratin.
Numerous cholesterol clefts are identified within the homogenous keratin of the trichilemmal cyst.
 
 
 
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