You are in: eMedicine Specialties > Dermatology > BENIGN NEOPLASMS Pilar CystArticle Last Updated: Sep 24, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 8
  Author: Mohsin Ali, MBBS, FRCP, MRCP, MRCPI, Consulting Staff, Department of Dermatology, Amersham General Hospital, UK Editors: R Stan Taylor, MD, Professor of Dermatology, University of Texas Southwestern Medical School; Director of Skin Surgery and Oncology Clinic, Department of Dermatology, University of Texas Southwestern Medical Center; 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; Jeffrey Meffert, MD, Assistant Clinical Professor of Dermatology, University of Texas Health Science Center-San Antonio; Catherine Quirk, MD, Clinical Assistant Professor, Department of Dermatology, Brown University; William D James, MD, Paul R Gross Professor of Dermatology, University of Pennsylvania School of Medicine; Vice-Chair, Program Director, Department of Dermatology, University of Pennsylvania Health System Author and Editor Disclosure Synonyms and related keywords: pilar cysts, trichilemmal cysts, scalp cysts, benign cysts, keratinizing cyst INTRODUCTIONSection 2 of 8
  BackgroundPilar cysts are common, occurring in 5-10% of population. They are the second most common keratinizing cyst on the scalp. Their benign and malignant transformation is extremely rare. Inheritance may occur in an autosomal dominant pattern. Pilar or trichilemmal cysts contain keratin and its breakdown products, lined by a wall resembling the external (outer) hair root sheath. Most pilar cysts occur on the scalp.1 In 2% of pilar cysts, single or multiple foci of proliferating cells that can lead to rare cases of proliferating pilar tumors, more commonly called proliferating trichilemmal cysts, have been observed. They were originally considered malignant; however, Wilson Jones suggested that they are pseudocarcinomatous. Proliferating trichilemmal cysts are rapidly growing tumors that often occur in trichilemmal cysts, but they can occur de novo without a preexisting lesion. The tumors may become large and ulcerated. Although considered biologically benign, the cysts may be locally aggressive. In rare cases, malignant transformation has occurred as demonstrated by distant metastasis.2 No reliable clinical criteria exist to distinguish a malignant tumor from a benign proliferating pilar tumor. The entire lesion must be histologically evaluated to assess the possibility of malignancy. PathophysiologyTrichilemmal cysts are derived from the outer root sheath of the hair follicle. Their origin is unknown, but it has been suggested that they are produced by budding from the external root sheath as a genetically determined structural aberration. They arise preferentially in areas of dense hair follicle concentrations; therefore, 90% of cases occur on the scalp. They are solitary in 30% of cases and multiple in 70% of cases. Histologically, areas with signs of proliferation can be found in some cysts. In rare cases, this leads to formation of a tumor, known as a proliferating trichilemmal cyst. The tumor is clinically benign, although it may display nuclear atypia, dyskeratotic cells, and mitotic figures. These features can be misleading, and a diagnosis of squamous cell carcinoma may be mistakenly rendered. FrequencyUnited StatesPilar cysts are common, occurring in 5-10% of the population. Of patients with these cysts, 70% have several lesions and 10% have more than 10 lesions. Mortality/MorbidityPilar cysts are considered biologically benign, but they may be locally aggressive. Malignant transformation has occurred in rare cases, as demonstrated by distant metastasis. SexPilar cysts are more common in middle-aged women. CLINICALSection 3 of 8
History
Physical
CausesIf the cysts are inherited, the pattern is autosomal dominant. DIFFERENTIALSSection 4 of 8
Acne Keloidalis Nuchae Dermoid Cyst Epidermal Inclusion Cyst Favre-Racouchot Syndrome (Nodular Elastosis with Cysts and Comedones) Pilomatrixoma WORKUPSection 5 of 8
Imaging Studies
Procedures
Histologic FindingsPilar cysts are surrounded by a fibrous capsule against which rests layers of small, cuboidal, dark staining basal cells. These merge with characteristic squamous epithelium composed of pale keratinocytes, which increase in height as they mature and transform abruptly into solid eosinophilic-staining keratin without forming a granular cell layer. Calcification occurs in 25% of the lesions, regardless of the age or the size of the cysts. Cholesterol clefts occur in 90%. Very rarely, sebaceous and apocrine differentiation may occur in the cyst wall. Epidermoid cysts can be histologically differentiated from pilar cysts by the presence of a granular layer in the lining epithelium. Epidermoid cysts have laminated keratin, which is believed to be derived from the follicular infundibulum (the nonimplantation variant). Acute inflammation, which results in rupture of the cyst wall and an intense foreign body giant cell reaction, may be due to bacteria. In patients with Gardner syndrome, the epidermoid cyst lining occasionally shows focal basaloid cell proliferation with ghost cell change, as seen in pilomatrixoma. The wall can rupture with the formation of granulation tissue, which, at times, may clinically and histologically resemble well-differentiated squamous cell carcinoma. Malignant transformation is extremely rare. TREATMENTSection 6 of 8
Surgical CareThe therapeutic approach in cases of a rare proliferating trichilemmal cyst is surgical removal with clear margins. The Medscape Dermatologic Surgery Resource Center may be helpful. FOLLOW-UPSection 7 of 8
Complications
Prognosis
REFERENCESSection 8 of 8
Article Last Updated: Sep 24, 2008 |
