IgG4-Related Mastitis: An Underrecognized Diagnosis in Breast Core Biopsies

We address the critical diagnostic challenge of IgG4-related mastitis (IgG4-RM), a rare tumor-like condition that may represent an isolated breast involvement or be the initial manifestation of systemic IgG4-related disease. IgG4-RM presents a significant diagnostic pitfall in the context of core needle biopsies (CNBs) because it frequently mimics malignancy, clinically presenting as a firm mass with skin changes or lymphadenopathy. Accurate diagnosis is often hindered by several factors: the nonspecific nature of the histopathology (fibrosis and lymphoplasmacytic infiltrate), significant overlap with inflammatory entities like Granulomatous Lobular Mastitis, limited tissue yield from CNBs, and the under-utilization of definitive IgG4 immunohistochemical staining. Furthermore, the lack of in-house IgG4 immunohistochemistry at many facilities restricts its use. Given that IgG4-RM is a medically managed condition, primarily treated with systemic corticosteroids, the underdiagnosis of this entity leads to unnecessary, costly, and morbid surgical excisions. We advocate for pathologists to maintain a high index of suspicion and routinely employ supplementary IgG4 staining in suspicious CNB cases to ensure correct non-surgical patient management.