Proliferating trichilemmal tumor: morphological considerations on its biological behaviour

Abstract

Abstract: 

Proliferating trichilemmal tumor (PTT) is an adnexal neoplasm of the skin derived from the outer sheath of the hair follicle that frequently occurs on the scalp of elderly women. Although usually described as benign, cases with malignant transformation have been reported but there are no clear morphological criteria to define this change except for clinical evidence of metastasis.
 
A 60-year-old woman consulted for a 20-year evolution of a scalp tumor. Several pieces of tissue were received, the largest being 4.5 x 3.5 cm. and the rest, measured as a whole 3.7 x 3.5 cm. Among the latter were two pieces of hairy skin and a cyst with compact content. It was processed with routine techniques and stained with hematoxylin/eosin. Microscopy: histological sections showed a trichilemmal cyst with marked proliferation of the epithelium in the form of polypoid projections towards the lumen and wide and anastomosed tongues that formed wide nests towards the surrounding dermis, with evident atypia and frequent mitosis. In the center of the nests there was trichilemmal keratinization. The stroma was fibrous with a dense mononuclear inflammatory infiltrate. Surgical limits could not be assessed. Diagnosis: Proliferating trichilemmal tumor. A comment was added about the limitation to define biological behaviour only with the morphological study, also highlighting that the fragmentation of the material added difficulty to this interpretation. We do not have clinical information about the follow-up of the patient.

According to the bibliographic consultation, morphological considerations such as tumor size, atypia, mitosis and/or infiltrating borders are not sufficient to clearly define the biological behaviour of this tumor. In our case, the fragmentation of the material we received added difficulties in interpretation. PTT could be considered as a low-grade adnexal carcinoma of the skin rather than suggesting the existence of benign PTT. Therefore, complete surgical excision and strict monitoring of all cases are recommended because relapses and metastases are possible events.