IgG4-related tumour-forming mastitis with histological appearances of granulomatous lobular mastitis: comparison with other types of tumour-forming mastitis

Sometimes, mastitis needs to be differentiated from carcinoma because of its association with induration and with ultrasound findings (such as low-echo lesions) that resemble those in carcinoma. The aim was to define this type of mastitis and to examine 18 cases to clarify its clinicopathological features.All cases were categorized into three types: non-specific mastitis with neutrophilic infiltration (n = 7); non-specific mastitis with lymphoplasmacytic infiltration (n = 9); and granulomatous lobular mastitis (n = 2). The three types of mastitis presented similar ultrasound findings and shared certain histological features including fibrosis and diffuse or lobulocentric inflammation. Granulomatous lobular mastitis showed specific clinicopathological features including lobulocentric inflammation with giant cells, diffuse IgG4+ plasma cells, and also a high level of serum IgG4.Granulomatous lobular mastitis could be categorized into IgG4-related and non-IgG4-related granulomatous lobular mastitis. IgG4 immunohistochemistry serum IgG4 might be useful for diagnosis of IgG4-related granulomatous lobular mastitis and could help to avoid overtreatment such as wide excision.