SOX-10 and Melan-A Immunostaining in Areas of Focal Acantholytic Dyskeratosis and Epidermolytic Hyperkeratosis Within Dysplastic Nevi Biopsies: An Observational Study.

Background: Focal acantholytic dyskeratosis (FAD) and epidermolytic hyperkeratosis (EHK) are common incidental epidermal histologic findings within dysplastic nevi biopsies. We evaluate whether areas of FAD and EHK within dysplastic nevi biopsies stain with immunostains used to characterize melanocytic neoplasms.

Methods: In this case series, a natural language search of histopathology reports from our institution in the past year (2020-2021) identified dysplastic nevus biopsies with concurrent FAD and/or EHK. Tissue samples were examined for positive melanocytic immunostaining with SOX-10 and Melan-A in areas of FAD and EHK.

Results: Out of 32 biopsies, 20 of 26 FAD specimens (76.9%) and 2 of 6 EHK specimens (33.3%) showed unexpected suprabasal layer staining with a melanocytic marker that did not correspond to definitively identified melanocytes on the H&E-stained sections. The immunohistochemical staining of FAD and EHK was observed in 2 forms: nonspecific background staining or "true" staining (ie, seemed nuclear on SOX-10 or cytoplasmic on Melan-A).

Conclusions: This pilot examination provides evidence that areas of incidental FAD within dysplastic nevi biopsies demonstrate unexpected suprabasal layer staining with melanocytic markers. When dermatopathologists evaluate melanocytic neoplasms with melanocytic markers, it is possible the presence of incidental FAD could lead to over diagnosing pagetoid scatter within these lesions. This study is a proof of concept with mild to moderately dysplastic nevi that do not typically incur the use of melanocytic stains; however, the implication of this unexpected staining pattern would be important when using melanocytic markers on borderline melanocytic neoplasms that have incidental FAD. Close correlation with H&E is imperative to prevent misinterpretation in these cases.