Very rarely, sebaceous and apocrine differentiation may occur in the cyst wall. Epidermoid cysts can be histologically differentiated from pilar cysts by presence of granular layer in the lining epithelium. Acute inflammation may be due to bacteria, which results in rupture of the cyst wall and intense foreign body giant cell reaction. In patients with Gardner's syndrome, the epidermoid cyst lining occasionally shows focal basaloid cell proliferation with ghost cell change, as seen in pilomatrixoma. The wall can rupture with formation of granulation tissue, which at times may resemble well-differentiated squamous cell carcinoma clinically and histologically.
Malignant transformation is extremely rare.
The therapeutic approach in cases of rare proliferating trichilemmal cyst is surgical removal with clear margins. Infection - Cysts may become tender and infected. Cutaneous horn - Cysts may exude contents and form a cutaneous horn. Cysts may be traumatized when combing or brushing hair. Cysts are benign and malignant transformation is exceedingly rare.