Pigmented Epithelioid Melanocytoma: Morphology and Molecular Drivers

Pigmented epithelioid melanocytoma (PEM) was originally described based on keen morphologic analysis identifying a group of melanocytic tumors sharing heavily pigmented epithelioid melanocytes. It is defined as heavily pigmented epithelioid, spindled, and dendritic melanocytes with characteristic vesicular nuclei, prominent nucleoli, and melanophages. PEM often involves regional lymph nodes. Recent advances in molecular analysis have allowed for subclassification of PEM into more specific subsets of melanocytic tumors. The most common subsets include PRKCA fusions, which result in pure PEMs with sheets of monomorphic epithelioid melanocytes, and PEMs with combined pattern and mutations in both PRKAR1A and BRAF.