Characteristic for idiopathic granulomatous mastitis are multinucleated giant cells and epithelioid histiocytes around lobules. Often minor ductal and periductal inflammation is present. The lesion is in some cases very difficult to distinguish from breast cancer.
Patients usually present with a distinct firm mass mostly in the subareolar region. It occurs on average 2 years and almost exclusively up to 6 years after pregnancy, usual age range is 17 to 42 years. Use of hormonal contraceptives, prolactin raising medications and hyperprolactinemia have been implicated in the pathogenesis or as predisposing factors.
Other diseases that may cause granulomatous inflammatory lesions of the breast are tuberculosis, sarcoidosis and Wegener's granulomatosis and must be included in a differential diagnosis.
Granulomatous mastitis is most often completely aseptic and has been frequently associated with elevated prolactin levels but infectious causes must be considered as well.
Idiopathic granulomatous mastitis is thought to be an autoimmune reaction to extravasated fat and protein rich luminal fluid (denaturized milk). This form is often associated with increased prolactin levels while some infectious forms are often associated with diabetes.
Granulomatous mastitis can be associated with corynebacteria infection, particularly infection by C. kroppenstedtii.(Taylor GB. Paviour SD. Musaad S. Jones WO. Holland DJ.Pathology. 35(2):109-19, 2003 Apr)
This disease is still very rare and therefore optimum treatment protocol is still being established. Treatments include antinflammatory drugs, (prednisone, methotrexate), and very often full mastectomy is the most successful treatment. Wide excisions are often complicated due to the disease characteristics.