Idiopathic granulomatous mastitis (IGM) is a rare relapsing benign, chronic inflammatory breast disease characterized by infiltration of inflammatory cells including multinucleated giant cells, which was first described in 1972 by Kessler et al. as a mastitis presenting with multiple granulomas and abscess formation. 1 Although various causes have been proposed for granulomatous mastitis, its etiology is not clearly identified and both intrinsic and extrinsic factors are thought to be involved; therefore, it is considered to be "idiopathic". 2345 The imaging and fine needle aspiration findings cannot completely distinguish between IGM and the breast malignancy; thus, histopathological diagnosis by core needle biopsy is essential for definitive diagnosis. 26789 The following five criteria have been proposed by Carmalt et al. as diagnostic criteria for granulomatous mastitis. 10 1) The women of childbearing age within 5 years of the last childbirth, 2) Infiltration of neutrophils and epithelioid histiocytes or lymphocytes, and granuloma formation with multinucleated giant cells, 3) Abscess formation, 4) lobular lesions, and 5) No evidence of caseous necrosis and Mycobacterium or fungal infections. Moreover, this disease was classified by Kaviani et al. 11 IGM is considered to be a special form of galactophoritis. A hormonal imbalance such as hyperprolactinemia can be one of the causes of the breast inflammation. 11 Hyperprolactinemia can be induced and maintained by high doses of estrogen therapy, the use of oral contraceptives and antidepressants, thyroid dysfunction, or prolactinoma. 4111213 Therefore, even in the non-puerperal period, high blood levels of prolactin are thought to cause milk production followed by ductal ectasia leading to the rupture of the ducts and a persisting inflammation of stromal cells, resulting galactophoritis. 11 IGM is considered as the final step of a pathophysiological process of these retention syndrome. 14 The most common physical finding is a hard masslike lesion with indistinct borders on palpation with pain. 46 Other symptoms including tenderness, erythema, skin thickening, sinus formation or axillary adenopathy may also be present. 2346 Several imaging findings are reported as follows. 3689 The most common mammographic finding showed a focal asymmetric density. The most common ultrasonographic findings showed a heterogeneous hypoechoic mass with irregular shape and an ill-defined margin. On dynamic contrast-enhanced MRI, the most frequent enhancement patterns were rim enhancement in masses. However, these imaging findings are similar to breast abscess and cancer; therefore, histopathological diagnosis is necessary. 369 If IGM is determined as a definite diagnosis, appropriate treatment should be given with caution to its associated diseases.

It has been suggested that infection with Corynebacterium species (spp.) may be involved in the onset of the disease since the isolation of the bacteria was first reported by 15. 15 Therefore, the aim of this article is to summarize the previous IGM reports related to Corynebacterium spp.

The articles examined in this research were collected using the terms "granulomatous mastitis Corynebacterium" in PubMed and "granulomatous mastitis Corynebacterium" with Japanese in Google Scholar, which resulted in 63 English articles and 71 Japanese articles. I read all the abstracts and summarized the recent articles with Corynebacterium in the title.

We found 16 English articles, 151617181920212223242526272829[30] and 4 Japanese ones (Table 1). 31-34 Corynebacterium (C.) kroppenstedtii was detected most frequently, followed by C. tuberculostearicum. C. accolens and C. jeikeium were reported only one article each. 1621

Corynebacterium spp. is a Gram-positive rod characterized by fat affinity. Hence, this organism is thought to cause infection in fatty mammary tissue. Therefore, it is necessary for appropriate methods of aseptic sampling of specimens and culture in a medium supplemented with Tween 80. 31 Although Corynebacterium spp. is susceptible to many antimicrobial agents, the usage of high fat-soluble antimicrobial agents such as macrolides, tetracyclines, and new quinolones is recommended because the mammary gland is a lipid-rich tissue. Doxycycline, amoxicillin, ciprofloxacin, and cefuroxime were reported for most common antimicrobials and favorable outcomes have been associated with longterm use. Moreover, there are no mortality rate in the clinical case of C. kroppenstedtii infection. 26 However, it has been suggested that fatty tissues in the mammary gland make it difficult for neutrophils to approach the organism and this may be the cause of the refractoriness of IGM. 31 On the other hand, it is difficult to exclude completely the possibility of the contaminations because some species of Corynebacterium spp. are normal, endogenous, bacterial flora of the skin and the breast. 19 Therefore, recent articles indicated not only bacterial culture but also real-time polymerase chain reaction (PCR) and formalin-fixed, paraffin-embedded (FFPE) biopsy specimens analysis for detecting pathogenically Corynebacterium spp. 1825 It is reported that real-time PCR of C. kroppenstedtii from FFPE sections was useful (39-69% positive) in IGM cases. 1825 The histopathological feature is granulomatous inflammation with dense exudation of lymphocytes, plasma cells, epithelioid histiocytes, multinucleated giant cells and neutrophils. The inflammation is accentuated in the lobule and/or around the mammary duct. 18192325 Non-caseating granulomas are often associated with neutrophilic infiltration to form suppurative granulomas. 1518 Recently, cystic neutrophilic granulomatous mastitis with characteristic granulomas with central cystic spaces, in particular, has been considered to have Corynebacterium spp. infection. 2829[30] Surgery may be indicated for some IGM patients with Corynebacterium infection that are resistant to common antibiotic therapies. 22

Corynebacterium infection is widely detected with IGM. In addition to bacterial culture, real-time PCR and FFPE biopsy specimens analysis can be used to detect Corynebacterium spp.