Their benign and malignant transformation is extremely rare. Inheritance may occur in an autosomal dominant pattern.
Most pilar cysts occur on the scalp.
Proliferating trichilemmal cysts are rapidly growing tumors that often occur in trichilemmal cysts but also can occur de novo without a preexisting lesion. 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.