trichilemmal cyst

Definition: A trichilemmal cystis a common cyst that forms from a hair follicle. They are most often found on the scalp. The cysts are smooth, mobile and filled with keratin, a protein component found in hair, nails, and skin. They may or may not be tender. Trichilemmal cysts may run in families.

Images

 epidermoid cyst (left) VS trichilemmal cyst (right)

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Variants

Rarely, these cysts may grow more extensively and form rapidly multiplying trichilemmal tumors, also called proliferating trichilemmal cysts, which are benign but may grow aggressively at the cyst site. Very rarely, trichilemmal cysts can become cancerous.

Epidemiology

Pilar cysts are common, occurring in 5-10% of the population.

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.

Pilar cysts are almost always benign, malignant transformation being extremely rare.

Pilar cysts may be sporadic or may be autosomal dominantly inherited.

They contain keratin and its breakdown products and are lined by walls resembling the external (outer) root sheath of the hair.

In 2% of pilar cysts, single or multiple foci of proliferating cells lead to proliferating tumors, often called proliferating trichilemmal cysts.

Physiopathology

Trichilemmal 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 high hair follicle concentrations, therefore, 90% of cases occur on the scalp. They are solitary in 30% of cases and multiple in 70% of cases.

Microscopy

Histologically, they are lined by stratified squamous epithelium that lacks a granular cell layer and are filled with compact "wet" keratin.

Areas consistent with proliferation can be found in some cysts.

In rare cases, this leads to formation of a tumor, known as a proliferating trichilemmal cyst.

Subtype

 proliferating trichilemmal cyst

• 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.
• No absolute clinical criteria can distinguish malignant from benign proliferating pilar tumors.
• The entire lesion must undergo histological evaluation to assess for malignancy.
• Atypia and a high mitotic rate are suggestive of malignancy.

Differential diagnosis

 squamous cell carcinoma

• 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.