What Is Pigmented Epithelioid Melanocytoma (PEM)? It Is Published by New WHO Classification of Skin Tumors, 2018

Abstract

Pigmented epithelioid melanocytoma (PEM) is a new melanocytic neoplasm composed of heavily pigmented epithelioid and dendritic cells, with metastatic potential limited to regional lymph nodes. It was previously diagnosed as human animal-type melanoma and epithelioid blue nevus. Current experience indicates that it is best considered as a borderline melanocytic neoplasm or a low-grade melanoma. PEM has unique demographic, clinical and histological features. It can occur as a sporadic lesion or in patients with Carney complex. It is a rare tumor with a predilection for young people, including children. All racial groups are affected. It has a generalized distribution, including localization to the extremities, head and neck, and trunk. These lesions occur as a slow-growing pigmented nodule or papule. Histopathologically, it is composed of varying proportions of pigmented dendritic and epithelioid melanocytes admixed with melanophages. The diagnostic epithelioid cells are round, polygonal, or elongated, and they range in size from medium to large. A subset of PEMs show loss of expression of the protein product of PRKAR1A, which is a mutation of families with Carney complex. PEM is an indolent melanocytic tumor with metastatic potential limited to regional lymph nodes and otherwise rare distant metastasis. No deaths from the disease have been reported.