Granulomatosis with polyangiitis (Wegener's) (GPA) may mimic IgG4-related disease (IgG4-RD) on histologic examination of some biopsies, especially those from head and neck sites. IgG4 immunostain is often performed in this context for differential diagnosis with IgG4-RD. However, the prevalence of IgG4+ cells in GPA has not been explored. We examined the IgG4+ cells in 26 cases confirmed as GPA by a thorough clinical and pathologic assessment. Twenty-six biopsies consisted of 14 sinonasal/oral cavity/nasopharynx, 7 orbit/periorbital, 3 lung/pleura, 1 iliac fossa/kidney, and 1 dura specimens. Eight of 26 (31) biopsies revealed increased IgG4+ cells (30/HPF and >40% in IgG4+/IgG+ ratio). The IgG4+ cells and IgG4+/IgG+ ratio ranged 37-137/hpf and 44-83, respectively. Eight biopsies with increased IgG4+ cells were from sinonasal (n=4) or orbital/periorbital (n=4) sites. In conclusion, increased IgG4+ cells are not uncommonly seen in sinonasal or orbital/periorbital biopsies of GPA, which could pose as a diagnostic pitfall.
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