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A proliferating trichilemmal cyst, also known as proliferating pilar tumour of the scalp, is a rare benign hair follicle tumor. Most proliferating trichilemmal cysts arise from within a preexisting trichilemmal or pilar cyst. Whilst trichilemmal cysts are common intradermal or subcutaneous cysts occurring in 5-10% of the population, 2% of these cases will develop into proliferating trichilemmal cysts. These proliferating cysts may gradually grow up to 25cm in diameter.
Proliferating trichilemmal cysts are usually benign tumours but in rare instances one may transform into a malignant proliferating trichilemmal cyst.
Proliferating trichilemmal cysts occur most commonly in women aged between 50-75 years in age, and in 90% of cases appear on the scalp. Other locations where cysts have been reported include the back, chest, groin, thigh, vulva, face and eyelid.
Proliferating trichilemmal cyst arises from a benign trichilemmal cyst, which may have been present as a symptomless nodule for months to years. Growth may be provoked by an unknown trigger, such as trauma, irritation or inflammation. Rapid growth is unusual and may be a sign of infection or malignancy.
The lesion is usually a single, firm-to-soft, painless nodule that can vary in size from 1-10 cm. In some cases, lesions have grown as large as 25 cm and may cause pressure necrosis (destruction) on underlying tissues. Ulceration and foul-smelling discharge may also occur.
Specific features on histology of a proliferating trichilemmal cyst allow the diagnosis to be made on skin biopsy. Ideally, the entire lesion should be excised for examination. Specialised staining techniques help to distinguish it from trichilemmal carcinoma.
Proliferating trichilemmal cyst should be differentiated from squamous cell carcinoma, dermoid cysts and cylindroma.
The recommended treatment is complete surgical excision with a margin of normal tissue. Mohs micrographic surgery may be used to ensure better margin control.
Patients should be followed up looking for local recurrences and metastases.
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