Malignant proliferating trichilemmal cyst, also known as malignant proliferating pilar tumour, is a very rare malignant hair follicle tumor. The cause is unknown but it appears that a malignant proliferating trichilemmal cyst may have started out as a benign trichilemmal cyst which then progressed to a proliferating trichilemmal cyst before becoming malignant. It is thought that some form of trauma, inflammation, and irritation may provide the stimulus for the transformation from a benign to malignant lesion.
To date there have been less than fifty documented cases of malignant proliferating trichilemmal cysts.
In most cases, the first sign of malignancy is the rapid growth of a long-standing nodular scalp lesion that may or may not have been diagnosed as a proliferating trichilemmal cyst. The malignant cyst may be a recurrence of a previously diagnosed and excised proliferating trichilemmal cyst.
Being the malignant counterpart of proliferating trichilemmal cysts, most lesions have been found on the scalp and in women aged between 50-75 years of age. Most lesions are greater than 5 cm, with some growing to as large as 25 cm. These may cause pressure and necrosis on underlying tissues. Regional or distant metastases may also be present in advanced cases.
The diagnosis of a malignant proliferating trichilemmal cyst must be based on histological findings from skin biopsy. Ideally, the entire lesion should be excised and submitted for histological examination.
Specialised staining techniques help to differentiate between benign and malignant proliferating trichilemmal tumours. Histological evidence of abnormal mitosis, marked cellular pleomorphism, infiltrative growth, and aneuploidy may be indicators of malignant transformation. Immunohistochemical studies have also shown that expression of CD34 may be an additional feature to distinguish malignant proliferating trichilemmal cyst from squamous cell carcinoma.
Malignant proliferating trichilemmal carcinoma should be differentiated from other skin tumours such as squamous cell carcinoma, dermoid cysts and cylindroma. Due to its rarity and also to inconsistencies in nomenclature and misdiagnosis as squamous cell carcinoma, the real incidence of malignant proliferating trichilemmal cysts is unknown.
The recommended treatment is complete surgical excision with a margin of normal tissue. Mohs micrographic surgery may be used to ensure better margin control. Currently there is no consensus regarding the margin size of normal tissue. Additional radiotherapy and/or chemotherapy may be used to treat metastases and for lesions with high metastatic potential.
Patients should be followed-up closely after surgery for recurring lesions and any metastases. It has been found that malignant proliferating trichilemmal cysts tend to recur and metastasize in up to 30% of cases.
