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Background: IgG4‐related disease (IgG4‐RD) represents a recently characterized multisystemic fibroinflammatory condition that can manifest a spectrum of skin findings (IgG4‐related skin disease; IgG4‐RSD). Histopathologic and immunohistochemical criteria have been proposed; however, the specificity of these criteria merits scrutiny given the potential histopathologic overlap of IgG4‐RSD and both neoplastic and inflammatory skin conditions featuring lymphoplasmacytic infiltrates (IgG4‐RSD mimics). This study sought to assess the specificity of the criteria by quantifying the frequency by which an expanded spectrum of IgG4‐RSD mimics meet proposed thresholds. Methods: Following IRB approval, a total of 69 cases of IgG4‐RD mimics, representing 14 different diagnoses featuring plasma cells, were reviewed and analyzed for the following histopathologic and immunohistochemical features: (i) maximum IgG4+ count/high‐powered field (hpf) >200; (ii) IgG4/IgG ratio >0.4 averaged over 3 hpfs; (iii) IgG4+ count >10 per hpf. Results: Screening for IgG4‐RSD by histopathologic criteria demonstrated the high frequency of lymphoplasmacytic infiltrates, contrasted with the rarity of storiform fibrosis (only one case of erythema elevatum diutinum [EED]) and obliterative phlebitis (0 cases). By immunohistochemical criteria, the analysis revealed that no cases exceeded 200 IgG4+ cells; 13% (9/69) cases demonstrated an IgG4/IgG ratio of >0.4 averaged over 3 hpfs; and 23% (16/69) cases demonstrated a mean IgG4+ count of >10 per hpf. Conclusion: Application of proposed IgG4‐RSD histopathologic criteria to an expanded spectrum of potential IgG4‐RSD mimics (to include cutaneous marginal zone lymphoma, syphilis, necrobiosis lipoidica, lichen sclerosus, ALHE, psoriasis, lymphoplasmacytic plaque, EED, and erosive pustular dermatosis), highlights the relative nonspecificity of lymphoplasmacytic infiltrates contrasted with the stringency of storiform fibrosis and obliterative fibrosis. Furthermore, an IgG4+ cell count of >10 per hpf and an IgG4/IgG ratio of >0.4 are not specific to IgG4‐RSD alone. In the appropriate clinical context for IgG4‐RSD, histopathologic features still represent the entry threshold for diagnosis consideration, which then allows for further screening by immunohistochemical criteria. [ABSTRACT FROM AUTHOR]

Syringocystadenoma papilliferum (SCAP), tubular apocrine adenoma (TAA), and eccrine nevus are rare benign sweat gland tumors with varied clinical presentations but generally distinctive histomorphologic profiles. TAA and SCAP have been associated with other cutaneous hamartomas, most commonly with nevus sebaceus. Additionally, TAA and SCAP have uncommonly co-occurred in the same lesion. In contrast to nevus sebaceus, eccrine nevus is considerably less common and is rarely associated with other benign adnexal lesions. Here we present an unusual case of a complex sweat gland hamartoma containing features of syringocystadenoma papilliferum, tubular apocrine adenoma, and eccrine nevus in a 7-year-old female. [ABSTRACT FROM AUTHOR]