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Case Reports

Abstract

A 33-year-old woman presented with the abrupt-onset growth of numerous pruritic, pigmented papules involving the abdomen, back, buttocks, and scalp that began six weeks prior. Skin biopsy showed a biphasic dermal process composed of mildly atypical nevomelanocytes in poorly formed nests closely admixed with a dense infiltrate of CD1a/S100/langerin-positive Langerhans cells and abundant eosinophils. Next-generation sequencing identified a BRAF V600E mutation (c.1799T>A) with variant allele frequency (VAF) of 15%, and several other variants of uncertain significance, with VAFs ranging from 43%-52%, all likely representing benign single nucleotide polymorphisms. Over the following month, the patient developed additional similar papules on the extremities and scalp, and repeat biopsies mirrored the original findings. This case presents an interesting diagnostic challenge, as both melanocytic nevi and Langerhans cell histiocytosis (LCH) commonly have BRAF V600E mutations. While BRAF mutated eruptive nevi with reactive Langerhans cell infiltrates is favored, the progressive, multifocal eruption and eosinophil-rich inflammatory background are unusual. Alternatively, the findings could reflect LCH harboring a BRAF V600E mutation occurring with eruptive nevi. Though eruptive nevi have been described in children with LCH, to our knowledge, no such cases have been reported in adults. Additionally, this case could be a true biphasic clonal proliferation with both components arising from BRAF-mutant progenitors. However, a VAF of greater than 15% would be expected in this circumstance. This case illustrates the difficulty that can arise when evaluating biphasic neoplasms with overlapping genetic landscapes, and highlights the importance of clinicopathologic and molecular correlation in evaluating these unusual lesions.