Short answers, pulled from the story.
A trichilemmal cyst, also called a pilar cyst, is a common cyst that forms from a hair follicle and is filled with keratin, a protein found in hair, nails, and skin. It is smooth, mobile, and appears on the scalp in 90% of cases.
Trichilemmal cysts are not sebaceous; they arise from the outer root sheath of the hair follicle rather than from sebaceous glands. True sebaceous cysts contain sebum and are known as steatocystoma simplex or steatocystoma multiplex. Medical professionals recommend avoiding the term "sebaceous cyst" for pilar cysts because it is misleading.
In very rare cases, a trichilemmal cyst can become cancerous. More commonly, it may develop into a proliferating trichilemmal cyst, which is clinically benign but can grow aggressively and display microscopic features that resemble squamous-cell carcinoma.
Treatment involves either minimal excision or formal surgical excision, typically performed under local anesthetic. One conservative technique uses a punch biopsy about one-fourth the diameter of the cyst to empty the contents, then removes the cyst wall with forceps through the small incision.
Trichilemmal cysts are multiple in 70% of cases and solitary in 30% of cases. Their tendency to appear in multiples suggests a genetically determined structural predisposition rather than a random local event.
A proliferating trichilemmal cyst, also called a rapidly multiplying trichilemmal tumor, forms when a trichilemmal cyst develops areas of aggressive cell growth. It is classified as benign but can grow aggressively at the cyst site, and its microscopic features, including nuclear atypia and mitotic figures, can be mistaken for squamous-cell carcinoma.