[Importance of the proliferating trichilemmal cyst].

The biological behavior of proliferating trichilemmal cysts is commented on, based on two cases. Two female patients, 71 and 70 years of age, developed solitary monstrous tumours of the scalp in the course of repeatedly recurring trichilemmal cysts. The biological behaviour of both was marked by infiltration of the galea aponeurotica, and in the latter case, invasion of the calvaria and the venous sinuses of the brain with a lethal outcome. Histologically, the differentiation of the cyst walls ranged from pseudoepitheliomatous hyperplasia to areas largely lacking differentiation, indicating a trichilemmal carcinoma. Therefore, a more cautious evaluation of proliferating trichilemmal cysts, commonly underestimated as pseudocarcinomatous, is indicated. The second case in particular shows that on occasion an invasive carcinoma can arise in a trichilemmal cyst. Radical surgery can not only prevent recurrence with loss of differentiation, but also enable a better histological evaluation of the deep-lying tumour tissue.