Track: Case Reports

Abstract:

ALK expression can be found in a variety of cutaneous and soft tissue lesions such as Spitz tumors, inflammatory myofibroblastic tumors, histiocytomas, lymphomas, leiomyosarcoma, and some mesenchymal tumors. Immunohistochemistry (IHC), fluorescence in situ hybridization (FISH), and next generation sequencing (NGS) are common techniques used to detect ALK status. Occasional inconsistencies have been observed between IHC, FISH, and NGS for neoplasms including inflammatory myofibroblastic tumors, angiomatoid fibrous histiocytomas, and non-small cell lung cancer. However, no reports have identified inconsistencies in melanocytic lesions. In this case report, we present a 74-year-old man with an atypical dermal-based densely cellular spindled melanocytic proliferation with gray cytoplasm, pigmented dendritic cells, prominent nucleoli, and rare mitotic figures, originally concerning for an atypical Spitz tumor. Immunohistochemical studies were positive for ALK, SOX10, and Melan-A, and negative for CD163, CD34, PRAME, and BRAF, with mosaic p16 expression. Oncoplex NGS studies revealed negative ALK mutations, BAP1 inactivation, negative microsatellite instability with a low tumor mutation burden, and a GNAQ mutation. Correlation of the histopathologic findings with the molecular findings of BAP1 inactivation and GNAQ mutation led to classification as a malignant blue nevus. This is a unique case highlighting the first reported false ALK IHC positivity in a melanocytic lesion and underscoring the value of molecular testing in challenging melanocytic lesions.

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Stephne Rasiah