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. Awareness of clinical and morphologic features suggestive of this rare condition and its variable presentation in the breast can prevent unnecessary surgeries as well as undue patient anxiety.