Proliferating trichilemmal cyst.

Fourteen cases (9 female, 5 male) of proliferating trichilemmal cysts are reported. The patients age-range was 40-80, 12 being over 60. The cysts wee present for 2-50 years and enlarged in the months or years before the observation. Seven were ulcerated. In 4 with multiple lesions both proliferating and ordinary trichilemmal cysts were present. The histological features were typical in all but one patient (evolution towards squamous cell carcinoma?) with some variation from lesion to lesion and from area to area in the same tumour. The microscopic features suggest that the proliferating trichilemmal cyst must be considered an hamartoma of the terminal hair follicle, not only related to the isthmic segment of the outer root sheath but with some differentiation (smaller and more variable) towards the infundibular and matricial segments. The tumour is likely to be a proliferating trichilemmal cyst from the beginning and not a peculiar evolution of an ordinary, pre-existant, trichilemmal cyst.