Abstract

Balloon cell nevus is a rare melanocytic nevus with large polygonal balloon cells with small nuclei and ample clear or foamy cytoplasm. We report a dark, dome-shaped papule over the shoulder in a 15-year-old girl. Histology revealed a predominantly dermal-based, wedge-shaped melanocytic proliferation composed of nevocytes with vesicular nuclei and abundant clear to vacuolated cytoplasm (balloon type cells) with fine pigmentation. The lesional cells show strong nuclear beta-catenin expression. Fluorescence in situ hybridization (FISH) study was negative for copy number aberrations in chromosomes 6, 9, 8 and 11. A diagnosis of a deep penetrating melanocytoma with predominant balloon cell features was rendered. Deep penetrating melanocytoma (DPM) (formerly deep penetrating nevus) is a distinct entity of melanocytic neoplasms of intermediate grade often seen in head, neck and trunk areas with characteristic histology including clusters or loose sheets of enlarged or epithelioid cells with vesicular nuclei and vacuolated pigmented cytoplasm in association with heavily pigmented macrophages. Activating beta-catenin (CTNNB1) mutations or loss of APC function mutation, in addition to a driver MAPK pathway mutation, have been shown as a signature genotype in these DPM lesions. While variation in the cytologic atypia or grade in DPM lesions have been described, to our knowledge, a pure balloon cell type morphology has not been reported. DPM with balloon cell features is a unique and under-recognized variant that may cause diagnostic confusion. More studies are needed to determine if balloon cell morphology is a result of additional mutations and any clinical significance this morphology may carry.

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