Update on Peripheral Nervous System Tumors: Lesions with Epithelioid and Melanotic Features

While the majority of peripheral nerve sheath tumors (PNSTs) are histomorphologically composed of non-pigmented and slender spindle cells, unusual PNST lesions can present as epithelioid or pigmented cell tumors, a reflection of the inherent plasticity of neural-crest derived cells. These lesions deviating from the classic PNST histomorphologic mainstream can potentially trigger confusion and misdiagnosis. Herein, we review the salient characteristics of 2 of these unusual PNST subgroups: (1) tumors with predominant epithelioid cell morphology (granular cell tumor, epithelioid schwannoma and epithelioid malignant peripheral nerve sheath tumor) and (2) lesions with melanic pigment production (pigmented neurofibroma and malignant melanotic nerve sheath tumor, previously known as melanotic/pigmented schwannoma). The clinical features, syndromic associations, gross and microscopic appearances, immunohistochemical profiles, differential diagnoses and genomic/mutational landscapes characterizing these entities will be discussed in an effort to improve diagnostic accuracy by becoming familiarized with these unusual groups of lesions.