SOX-10 and S100 Negative Desmoplastic Melanoma: Apropos a Diagnostically Challenging Case.

An 83-year-old man presented with a tumor of the neck, clinically consistent with an epidermal inclusion cyst. Excisional biopsy revealed a deeply infiltrating spindled cell tumor. Immunohistochemical markers for S100, SOX-10, Melan-A, HMB-45, and NK1/C3 were negative. Based on the presence of an area of lentigo maligna and the histologic pattern of the spindle cell component, a diagnosis of desmoplastic melanoma was made despite the absence of immunophenotypic evidence for melanocytic differentiation. To the best of our knowledge, the complete lack of both S100 and SOX-10 makes this tumor an unprecedented case. To avoid ruling out the diagnosis of desmoplastic melanoma prematurely, physicians should be made aware of this possible immunohistochemical profile.