Classic IgG4-related sclerosing mastitis is not so classic.

IgG4-related sclerosing mastitis (IgG4-RM) is a rare, benign, mass-forming fibroinflammatory disease of the breast that can mimic a neoplastic process and pose a diagnostic challenge to clinicians and pathologists. We present a case of IgG4-RM in an 84-year-old woman and highlight the characteristic morphologic findings that should allow the pathologist to raise the possibility of this entity. We also briefly discuss pathophysiology and differential diagnosis.