IgG4 related sclerosing mastitis: expanding the morphological spectrum of IgG4 related diseases.

IgG4 related disease (IgG4RD) is a recently recognised condition characterised by mass forming lesions associated with storiform fibrosis, obliterative phlebitis, lymphoplasmacytic infiltrate rich in IgG4 positive plasma cells and elevated serum IgG4 levels. Although rare, mammary involvement has been reported as IgG4 related sclerosing mastitis, the morphological counterpart of a growing family of IgG4 related diseases. A total of 17 cases belonging to mass forming benign inflammatory breast lesions such as plasma cell mastitis, granulomatous lobular mastitis, non-specific mastitis and inflammatory pseudotumour were investigated as a possible member of IgG4 related sclerosing mastitis. Clinical, radiological, histopathological and immunohistochemistry findings were noted in all cases. Cases diagnosed as inflammatory pseudotumour showed all the histopathological features of IgG4RD along with increased number of IgG4 positive plasma cells and IgG4/IgG ratio >40%. However, only a few IgG4 positive cells were seen in plasma cell mastitis, granulomatous lobular mastitis and non-specific mastitis cases. These cases also did not fulfill the morphological criteria for the diagnosis of IgG4 related diseases. IgG4RD should be excluded in plasma cell rich lesions diagnosed on core biopsies by IgG4 immunostaining. This can avoid unnecessary surgery as IgG4 related diseases respond to simple and effective steroid treatment.