Granulomatous mastitis (GM) is a benign inflammatory disease of the breast that was first reported in 1972 (1). This rare entity commonly affects female patients during their reproductive period. It is associated with an ill-defined etiology (2,3). GM is generally characterized by a non-caseating granulomatous inflammation in close proximity to the ducts and lobules of the breast (2). The granulomas are usually unilateral and appear as a solid mass in the upper outer quadrant of the breast (4). Several clinical findings, including palpable masses, fistulae, nipple retraction, breast skin inflammation, ulcers, and abscess formation have been associated with GM (5,6). The major etiology of GM remains controversial, which may lead to misdiagnosis and complicate treatment (7). Some factors like heredity, bacterial infection, fungal infection, use of contraceptive drugs, high levels of prolactin, alpha-1 antitrypsin deficiency, and smoking have been suggested to cause GM. In addition, triggering autoimmune reactions due to the presence of milk proteins in the interstitial breast tissues is regarded as a favorable hypothesis concerning the occurrence of GM (2,8). With regard to the clinicopathological and radiological findings, GM can mimic and easily be mistaken for breast cancer and other pathologies, including tuberculosis, mycotic infection, and syphilis (9). There are various studies on the pathophysiology and management of GM in the literature; however, numerous controversies have also arisen (2,3,6,10). The present study aimed to represent and evaluate the clinicopathological findings and mark the sensitivity and resistance of isolated bacteria in patients with GM.

A total number of 63 female patients with GM were included in the present study. The age of the patients ranged from 24 to 50 years, with a mean age of 35.7 years. The BMI of the patients was mostly between 25 and 35. The majority of the patients were in the premenopausal or perimenopausal period. The menstrual cycle was abnormal in ~21% of the cases. Of the total number of patients, 82.5% of them had experienced >1 childbirth, and the majority had a history of breastfeeding. In a large part of the patient cohort (87.3%), breastfeeding was unilateral, with a mean duration of 41 months (Table I). The GM was unilateral in 58.7% of the patients. The most common symptom was pain (79.4%), followed by fever and chills (46%), as presented in Table II. The CMV and EBV tests were positive in all the cases. The mean ranges of the following tests were markedly elevated in comparison to the normal ranges: The erythrocyte sedimentation rate, CRP, IL-6, IL-17, C5a, white blood count, neutrophil-to-lymphocyte ratio and prolactin. The laboratory findings and immunological and hormonal profiles are summarized in (Table III). In total, nine different bacterial species were isolated from the bacterial culture of the core biopsy samples. Co-infection occurred in six cases (Table IV). The most common antibiotic resistance was recorded towards oxacillin, while 50% of the isolated bacterial species were sensitive to trimethoprim-sulfamethoxazole (Table V).

The GM, as an inflammatory disease, is commonly described by the presence of non-necrotizing granulomas in the breast. It typically co-exhibits with numerous conditions, including breast cancer, bacterial infection, diabetes mellitus, fat necrosis, trauma, sarcoidosis and Wegener granulomatosis (11,12). The mechanism of the development of GM is considered to begin with the damage of the ductal epithelium of the breast, followed by the spreading of luminal secretions to lobular connective tissue and inducing local inflammation. Finally, lymphocytes and macrophages migrate to the region and cause a granulomatous inflammatory response (6).

GM presents as a palpable mass and can be associated with different types of symptoms, including pain, erythema, sinus formation, tenderness, fever, nipple retraction, nipple discharge, and peau d'orange-like changes (2,9,11). The granulomas are usually unilateral and located in the upper outer quadrant of the breast (13). Bilateral lesions have also been reported in the literature, and they have been associated with a longer recuperation time (5). Azizi et al (14) conducted a study in which skin changes and nipple inversion were observed in ~20% of the patients involved. Furthermore, Kiyak et al (15) reported skin changes in the upper outer quadrant and areolar regions of the breast in 66% of the cases. In the study by Barreto et al (2), the majority of the patients presented with painful, mass-like lesions followed by erythema. In the present study, the most common symptom was pain, followed by fever and chills (46% of the cases). The lesion was unilateral in ~59% of the cases and bilateral in the remaining portion.

There is a lack of consensus on the exact etiology of GM. At present, three hypotheses have been proposed on the pathogenesis of this disease: autoimmunity response, infection disease, and hormone disorders. Some other factors, including smoking, pregnancy, breastfeeding, contraceptive drugs, and childbirth, are considered risk factors (13,16). It has been reported that women in the reproductive age or those who consume contraceptive drugs are more susceptible to being infected by GM (9). In several studies, the mean age of patients with GM was reported to be around 35 yearsold (17,18). Galactostasis, an atypical milk accumulation in the breast, can generate an autoimmune response (13). In a previous study, 59% of the cases were smokers, and almost 8% of the patients presented with BMI values >30 kg/m² (2). In addition, the history of passive smoking, breastfeeding duration, and the number of parturitions have also been associated with this entity (19). Some other studies have revealed the association of breastfeeding with GM, in the manner that around 83% of the cases were breastfeeding mothers (13,14,20). In this study, most of the patients were in the reproductive period, with a median age of 36 years. Most of them (81%) experienced more than one parturition. The breastfeeding history was significant in >96% of the cases. In total, 25% of the patients had consumed hormonal contraceptives, followed by anti-prolactin (4.8%) and psychiatric (3.2%) drugs. All these findings were in agreement with the previous studies; however, none of the cases were smokers, which was mentioned as a risk factor in some studies (13,16,17,19,20).

Several studies have been conducted to determine the roles of immunological patterns and hormonal imbalance in the development of GM (16,21,22). Cytokines are important proteins that regulate the response of the immune system to foreign bodies and antigens. Furthermore, they can prompt local and systemic inflammatory responses. Several interleukins (IL-17, IL-22, and IL-23) can be involved in triggering autoimmune diseases by taking advantage of two factors: Their significant role in inflammatory processes and their capability to induce the accumulation of proinflammatory cells (16). In a study on IL-33 as an inflammatory marker in the differential diagnosis of breast cancer and GM, it was claimed that patients with GM have higher levels of IL-33(21). Another study by Huang et al (5) revealed that serum CRP and IL-6 can be depended on as biomarkers for the severity of GM. Hormonal imbalances, including the elevation of estrogen, progesterone, and prolactin have been mentioned to be associated with the pathogenesis of GM (2). Another study demonstrated that elevated prolactin prolongs the duration of the disease and also increases the rate of recurrence (22). A hypothesis was proposed, stating that prolactin activates the nuclear factor-κB pathway of mammary epithelial cells and inflammatory factors like IL-1, IL-6, interferon-c, tumor necrosis factor-a. Inflammatory factors can induce an inflammatory response of the interlobular stroma and result in abscess formation (23,24). In the present study, CRP levels were increased, with a mean level of 17.18 IU. The present study supports the findings of the previously mentioned studies (23,24) with regard to the effects of immunological patterns and prolactin hormone, since IL-6, IL-17, and prolactin were significantly increased.

Even though the role of pathogenic bacteria in the occurrence of GM is not yet well established, it has been reported that Gram-positive bacteria may be involved in the progression of GM. Several types of bacterial flora, including Corynebacterium, Streptococci and Propionibacterium, have been isolated from the discharge of GM (13). In the study by Taylor et al (25), Corynebacterium was isolated in the breast tissue of ~55% of patients. Another study reported that Corynebacterium kroppenstedtii was the most commonly isolated species in patients with GM (26). In the present study, bacterial culture was conducted for the tissue biopsies of 14 patients, and Burkholderia cepacia was the most common isolated bacterium, followed by Staphylococcus epidermidis. A previous study revealed that bacterial infections, particularly those caused by Mycobacterium tuberculosis, can cause granulomas and chronic inflammatory responses. These factors can be involved in the occurrence of GM or deteriorate the disease into being increasingly aggressive. Therefore, prolonged antibiotic courses have been recommended to prevent the further progression of the disease and provide more effective treatment (27).

One of the major perspectives in the present study was the testing of 46 types of antibiotics to determine which of the isolated bacteria has the highest level of sensitivity and resistance and is better to control the growth of the isolates. This can aid clinicians in the selection of antibiotics to restrict the growth of those bacteria that may prolong the period and severity of the disease. The use of oxacillin exhibited the lowest sensitivity, while antibiotic sensitivity was at its highest for trimethoprim-sulfamethoxazole.

Core needle biopsy plays a crucial role in the definitive diagnosis of GM and in ruling out other diagnoses, including ductal ectasia, fungal infection and sarcoidosis (2,12). All the patients involved in the present study were diagnosed with GM, according to the corresponding histopathological evaluation.

The small sample size, the short duration of the study, and the lack of a proper statistical comparison due to the aggressiveness of the condition are the main limitations of the present study.

In conclusion, there is no consensus on the etiology of GM. Any studies related to this aspect expand our current understanding of this mysterious condition. Conducting bacterial cultures and sensitivity tests aids in the determination of the best antibiotic to decrease the duration and morbidity of this disease.