We report a very unusual case of melanocytic neoplasm appearing clinically as a 0.5-cm dome-shaped pigmented papule on the chest of a 63-year-old man. Microscopically, it was an asymmetric, entirely dermally based neoplasm characterized by a multinodular, vaguely plexiform architecture composed of moderately pleomorphic spindled melanocytes with ample, dusty pigmented cytoplasm and scattered multinucleated cells. The tumor cells were strongly positive for Melan-A, HMB45, S100, and PRAME, whereas p16 showed diffuse nuclear loss. β-catenin presented a strong and diffuse cytoplasmic staining, while nuclei were negative. Despite an increased cellularity, mitotic count was low (1/mm 2 ). Fluorescence in situ hybridization revealed no copy number alteration in melanoma-related genes ( CDKN2A, MYB, MYC, CCND1 and RREB1 ). DNA and RNA sequencing identified KIT c.2458G>T and APC c.6709C>T mutations. No further genetic alteration was detected including TERT-promoter (TERT-p ) hot-spot mutation. A re-excision was performed. A sentinel lymph node biopsy was negative. Clinical investigations revealed no extracutaneous involvement. The patient is disease-free after a follow-up period of 8 months. Given the peculiar morphologic and molecular findings, we hypothesize the lesion may represent a novel subtype of an intermediate grade melanocytic tumor (melanocytoma).

Department of Pathology Fondazione Policlinico Universitario Campus Bio Medico Rome Italy

IDP Institut für Dermatohistopathologie Pathologie Institut Enge Zürich Switzerland

Ospedale Israelitico di Roma Roma Italy

Sikl's Department of Pathology Medical Faculty in Pilsen Charles University Prague Pilsen Czech Republic

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