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Case Reports

Abstract

A 41-year-old male presented with a slowly growing, 0.6cm verrucous papule on the right index finger. Skin biopsy showed an epithelioid melanocytic proliferation with nuclear pleomorphism, abundant eosinophilic cytoplasm, and nuclear pseudo-inclusions. Rare mitoses were also seen. The morphologic features were suggestive of BIMT, but melanoma was also considered. Immunohistochemistry (IHC) showed SOX10 and BRAF V600E positivity, with p16 loss in most melanocytes. PRAME demonstrated patchy positivity. BAP-1 staining was retained. Given the diagnostic ambiguity, next generation sequencing was performed. The tumor tested positive for a BAP1 missense mutation (p.H169Y) in the setting of copy loss of a portion of chromosome 3, without other significant mutations and a low tumor mutational burden, consistent with BIMT. BIMT with retained BAP-1 IHC is rare and likely underdiagnosed. This has potential implications for patients harboring germline BAP-1 mutations, which confer increased cancer risk of ocular melanoma and mesothelioma, among other tumors. BIMTs are typically the first phenotypic manifestations of a germline mutation in BAP-1, and correct diagnosis of these tumors is essential. Pathologists should be aware of the potential limitations of BAP-1 immunohistochemistry in diagnosing BIMT. If BIMT is suspected based on morphology, molecular testing should be considered.