The tumors may become large and ulcerate. Though considered biologically benign, the cysts may be locally aggressive. In rare cases, malignant transformation has occurred as demonstrated by distant metastasis. No reliable clinical criteria exist to distinguish a malignant from a benign proliferating pilar tumor. The entire lesion must be evaluated histologically to assess the possibility of malignancy.
Trichilemmal cysts derive 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. 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.