Use of p16 immunohistochemical stain to help differentiate inflamed melanocytic nevi from metastatic melanoma in the setting of immunotherapy

Abstract

A 45 year-old woman with a history of metastatic melanoma (left supraclavicular lymph node with unknown primary) undergoing treatment with nivolumab presented with a suspicious new pink papule on her right upper arm. Histopathology revealed an intradermal proliferation of nonpigmented oval-to-spindled melanocytes admixed with numerous lymphocytes (Fig 1, A and B). Cytologic detail was somewhat obscured by the infiltrating lymphocytes. The differential diagnosis included metastatic melanoma vs an atypical inflamed melanocytic nevus in the setting of immunotherapy.