The lesions seen in general are mostly ill-defined, tough or hard, a few are soft and hard, a few are brittle, irregularly solid or cystic in section, grayish white or grayish red or grayish yellow, with dark red or yellow corn-like nodules, scattered with multiple pus cavities/pitting foci of necrosis of varying sizes, and some describe the cystic area as decaying (27). The cystic areas contain grayish, grayish-yellow, or grayish-brown secretions, with sinus tract formation visible on the cut surface (25, 28–32). It has been described as a decaying cystic area containing gray, grayish-yellow, or gray-brown secretions, with sinus tract formation on the cut surface.

CNGM is now considered a specific subtype of GLM with distinctive histopathological features, possibly associated with Corynebacterium, microscopically appears as a purulent lipid granuloma, surrounded by a central vacuole (cystic space), usually thought to be formed by lipolysis, with the size of several merged adipocytes (200–800 μm), surrounded by neutrophils, the thickness of the edge of which can be thin or thick (Figure 8). The number of neutrophils varies and the thickness of the rim surrounding the vesicle can be thin or thick, thus forming a microabscess in the granuloma, which is then surrounded by histiocytes, variable numbers of lymphocytes, plasma cells and Langhans giant cells, forming a definite granuloma (37, 55, 56). Sometimes granulomas can be poorly formed (57). However, not all granulomas have cystic vesicles, and neutrophilic inflammation and microabscesses can be seen outside the granuloma, and more often the granulomas fuse with each other to obscure the lobule-centered distribution characteristic of the granuloma (55). There are also cases where there are microorganisms in the lipid vacuoles surrounded by neutrophils but no granulomas. Gram-positive bacilli (GPB) are sometimes seen in the vesicles (Figure 9), apparently rod-shaped, and some of the bacilli are internally beaded or dendritic, arranged in a fenestrated pattern, forming wedge-shaped features (36, 45, 47). The microcystic vacuoles may sometimes be in clusters (38). Although GPBs are not always visible in the vesicles, they are confined to the interior of the lipid vacuoles in identifiable cases (24). Immunohistochemistry is increasingly used in the diagnosis and differential diagnosis of difficult breast lesions and is also the main means of molecular typing of breast cancer and screening of precise therapeutic markers, but in GLM and CNGM, in addition to identifying IgG4-related sclerosing mastitis (IgG4-RSM), its specific role is still unclear and lacks clinical significance (Figures 10–13).

Gram-positive bacilli in lipid vacuoles are currently dominated by Corynebacterium, with the most common isolate being C. kroppenstedtii, followed by C. amycolatum and C. tuberculostearicum (50, 58). Other pathogens such as P. oleovorans, human gammaherpesvirus 4, A. baumannii, T. thermophilus are likely to be closely related to GLM (48). Staphylococcus epidermidis, Staphylococcus aureus, Pseudomonas aeruginosa etc. have also been cultured (59). There are also cases of mixed infections of Corynebacterium with other bacteria, or Corynebacterium is not the predominant pathogen (60, 61). The abundance of Corynebacterium in GLM patients showed inter-individual variability (62). The pus samples are more diverse than their skin samples (63) and are more abundant than in tissue samples. In a study by Wen Chen et al., the samples from 34.1% GLM patients had a Corynebacterium abundance of >1% (1.08–80.8%), with 53.3% displaying an abundance of >10% (64). Pathogen discovery will help guide clinical treatment. Corynebacterium was first isolated in 1998 from a human sputum sample (65). Lipophilic antibiotics may be more effective in treating Corynebacterium-associated breast infections, such as rifampicin, clarithromycin, and methotrexate-sulfamethoxazole (66). Other sensitive drugs such as vancomycin and gentamicin may also be used as adjunctive therapy (67, 68).

Positive bacterial cultures for Corynebacterium are most often seen in the abscess and refractory types (the two types are not statistically different) (35, 37). Patients with CNGM are younger, have larger masses, are more likely to be painful, febrile or with high neutrophils, form sinus tracts, and are more likely to recurrence (24, 69, 70). In the study by Tan QT et al. the risk of recurrence 2.64 times higher in patients with Corynebacterium infection (71). This may confirm the role of Corynebacterium in GLM in promoting abscess formation.

In fact, CNGM may be underdiagnosed because (i) Gram staining is usually limited to one or a few sections of formalin-fixed paraffin-embedded. (ii) Not all vesicles contain GPB. (iii) Corynebacterium are difficult to culture, whether the microbiological finding should be part of the diagnostic criteria remains debatable (72).

Based on nanopore sequencing and bacterial culture, Xin-Qian Li et al. find that the bacteria positive rate and vacuoles positive cases in the early stage (only hard mass) was significantly higher than that in late stage (medium/soft mass, with skin inflammation, abscess, fistulas or ulcers), the detection rate of bacteria in the early stage of GLM was over 80% and the dominant bacteria were Corynebacterium species (64%). Thus they speculate that C. kroppenstedtii may be initially present in the breast tissue at the early stage (73). Tariq H considers the early identification of Corynebacterium in deep breast tissue, causing peripheral granulomas and purulent inflammation, to be strong evidence of Corynebacterium pathogenicity (37).

However, the idea of Corynebacterium as the cause of GLM is still being questioned due to the negative microbiological cultures, the poor response to antibiotics, improvement of symptoms with cortisol or immunosuppressive drugs, the concomitant erythema nodosum of the lower limbs in some patients, and the self-limiting nature of GLM, which tends to be an autoimmune disease. And it remains controversial whether CNGM, as a pathological subtype of GLM, belongs to two different diseases from non-CNGM GLM, the relationship between GLM and CNGM needs further clarification.

Additionally, the classification of the period attributed to the patient’s disease in future research needs to be further refined, cause not all patients clinically undergo the process of abscess formation, and all patients with abscesses do not present it at the same period, in addition to the type of mass, the duration of the presence of the mass type, and the impact of treatment methods needs to be taken into account.

As an exclusionary disease, GLM is not associated with infectious diseases, other immune diseases, etc. Wu JM believes that once malignancy is excluded, infectious factors causing granulomatous mastitis should be considered and all cases should be subjected to special staining and microbiological cultures to exclude bacterial, fungal, parasitic and other sources of infection; especially in areas where tuberculosis is endemic, breast tuberculosis should be kept in doubt (72). In addition to the usual Gram stains the main ones are silver hexamine and Schiff periodate stains to exclude fungal infections, antacid stains (Wade-Fite, Ziehl-Neelsen), PCR tests, molecular tests for Mycobacterium tuberculosis to exclude Mycobacterium tuberculosis/Nontuberculosis mycobacteria infection (36, 57, 74, 75).

Corynebacterium is generally detected by Gram staining, but its staining sensitivity is low in paraffin sections, and in a study by Tariq H et al., the Gram staining positivity of paraffin sections of Corynebacterium was significantly lower than that of PCR (17.9% vs. 68.7%) (76). Sangoi AR et al. found that cutting Gram-stained tissue sections of CNGM cases to a thickness of 6 μm instead of the traditional 4 μm increased the detection rate of Gram-positive bacilli (77). However, even with PCR testing, as only one section is selected per case, sampling may be falsely negative, and there is attrition of lesion tissue during processing, and dewaxing and DNA extraction may result in loss of cystic space contents, so Tariq H strongly recommends obtaining fresh biopsies for microbiological studies in suspected GLM cases, not only to improve PCR detection rates, but also to allow for culture. This would not only improve PCR detection but also allow antimicrobial susceptibility testing of positive cases, which would hopefully improve clinical outcomes with targeted antibiotics (76). Zhu Yongze et al. concluded that as long as a single Corynebacterium is isolated from a sterile site specimen in culture, it should be treated as pathogenic, even if no Gram-positive bacilli are seen in the original smear and only a certain number of leukocytes are present (58). The culture of Corynebacterium is now mostly done with blood plate, blood plate +1% Tween 80, extended incubation time and retention of tissue around the pus can increase the positive culture rate of Corynebacterium (59). The rate of positive culture of Corynebacterium can be increased by prolonging the culture time and leaving the tissue around the pus.

The more advanced methods for the identification of pathogenic organisms in CNGM include 16SrRNA gene sequencing, matrix-assisted laser-resolved ionization time-of-flight mass spectrometry (MALDI-TOF MS), rpoB gene sequence amplification by PCR, Sanger sequencing techniques (37, 38, 62, 67).

As the patient’s presentation is similar to breast cancer both clinically and on imaging, breast cancer becomes the primary diagnosis to be differentiated. The use of core needle biopsy or surgery to obtain pathological tissue is recommended. As the incidence of GLM has been increasing in recent years, cases of GLM in combination with breast cancer have gradually emerged. The diagnosis of breast cancer could be delayed because GLM has a long course and varied clinical presentation, its inflammatory features could have masked the presence of breast cancer until the clinical and imaging manifestations continued to suggest the risk of breast cancer or a new lump was found, which was then taken seriously by the clinician, lead to a new round of pathological biopsy (78–80).

Inflammation has now been progressively shown to be an important factor in tumor progression, chronic inflammatory responses promote cell division and repair, creating an environment that stimulates cancer growth and progression (81). Mammary ductal epithelial cells of GLM and plasma cell mastitis (PCM) showed injury and apoptosis, and MAC (C5b-9n) was mainly located on their cell membrane (82). Compared to healthy tissues, IGM tissues have elevated levels of both immune system and cancer-related proteins (83).

Currently, some large samples have shown that patients with mastitis have an increased risk of developing breast cancer and that mastitis can be considered a risk factor for breast cancer (84, 85). Further, respectively, observational research is needed on the relationship between lactational mastitis, GLM, and breast cancer, and specific mechanisms need to be explored.

There is a lot of histological overlap between GLM and tuberculosis, with granulomas and giant cells present, and it is particularly easy to misdiagnose on the basis of cytologic features alone, especially with a small number of fine needle punctures. Unlike the granulomas of GLM, which are centered on the lobules of the breast, the granulomas of TB distribution are irregular. The granulomas of GLM can be septic and necrotic, but TB is more prone to necrosis, especially the characteristic caseous necrosis (26). Ail DA et al. noted that Langhans giant cells are common in tuberculosis of the breast, whereas foreign body giant cells are common in GLM (41). However, most scholars have observed that Langhans cells are in fact not a minority in GLM (23, 40, 55). The predominance of neutrophils in the inflammatory infiltrate also contributes to the diagnosis of GLM (86). neutrophils and vesicles in TB are uncommon. Positive bacterial culture and Z-N staining is the gold standard for the diagnosis of TB, but the sensitivity is low and can be found by PCR for Mycobacterium tuberculosis (26, 41).

Usually the patient has diabetes mellitus (especially type I) or other autoimmune deficiency, and the mass is firm and irregular, with a white, homogeneous solid section, and microscopically shows a lymphocytic infiltrate of mainly B lymphocytes in the lobules, periductal and perivascular areas (87–89). Lobular inflammation is more common in women, with widespread ductal inflammation, atrophy of the ducts and thickening of the basal lamina, and vasculitis involving mainly small and medium-sized vessels. It is associated with marked interlobular fibrosis, scar vitrification, epithelial fibroblasts, and lymphatic nodule formation with or without germinal centers (34, 90). Tomaszewski JE et al. suggest that epithelioid fibroblasts are unique to diabetic patient (91).

IgG4-related sclerosing mastitis is characterized by the formation of dense lymphoplasmacytic infiltrates, lymphoid follicle formation, occlusive phlebitis, extensive sclerosis, or fibrosis with at least a localized stellate distribution, with marked stromal sclerosis and loss of breast lobules (47, 72, 87). Immunohistochemical staining reveals large numbers of IgG4+ plasma cells and elevated IgG4 serum concentrations, but Goulabchand R et al. argue that in fact increased numbers of IgG4+ plasma cells are only part of the classical histological presentation and are not necessary for the diagnosis, nor does the diagnosis of GLM require consideration of the number of IgG4+ plasma cells or the IgG4:IgG ratio or the presence of other histological features of IgG4 -RSM (47, 92). Ogura K et al. have classified GLM into IgG4-associated and non-IgG4-associated subtypes (92). Interestingly Kong C et al. found that nipple invagination is a necessary basis for differentiating between IgG4-related and non-IgG4-related GLM, suggesting that the pathogenesis and immune mechanisms of the two diseases may be different and that the required treatment may be different (93).

Lupus mastitis is characterized by lymphocytic lobular lipofuscinosis with plasma cell and hyaline fat necrosis (34). Lymphocytic infiltration may be nodular, diffuse, periductal and/or perifollicular and germinal center, and lymphocytic vasculitis is also common, involving mostly small and medium-sized vessels. Immunohistochemistry shows a mixed population of T and B lymphocytes, mainly CD3+ and CD4+ T cells mixed with CD20-positive B cells and polyclonal plasma cells (94).

When granuloma is present in conjunction with lymphocytic vasculitis, it needs to be differentiated from Wegener’s disease and nodular disease. Both nodular disease and Wegener’s granulomatosis affect mainly small and medium-sized vessels, the lesions are not lobularly centered, and both have necrotizing vasculitis and thrombosis, with a few cases of necrosis. Wegener’s disease is characterized by necrotizing vasculitis and granulomatous inflammation, mainly affecting the upper and lower respiratory tract and the kidneys (47). Nodular disease results in well-defined epithelioid nodules with microscopic lymphocytic infiltration in the vessel wall, but without neutrophil infiltration, and rarely vasculitis and fatty necrosis (25). Breast nodular disease is often secondary to generalized nodular disease, invading the dermis, with microscopic clusters of epithelioid cells of variable size, rarely extending into the subcutaneous tissue, and without neutrophil infiltration (39).

Plasma cell mastitis is a late stage in the development of ductal dilatation of the breast or accompanies it, but is not an inevitable part of it. It mainly invades the large ducts behind the areola (especially within the 2 cm ring of the areola) (95). The affected ducts are highly dilated and the duct lumen contains secretions, exfoliated epithelium and foamy histiocytes. The duct wall is fibrotic and thickened, with atrophy of the epithelial cells of the duct wall. The duct is surrounded by a large infiltrate of diffuse lymphocytic plasma cells and other inflammatory cells, with a predominance of plasma cell infiltration. The ductal or lobular structures are frequently obscured or distorted (29, 34, 96). Necrosis may occur in the later stages and the masses are often interspersed with lipid-like (pimple-like) material after they have broken down (25). The mass is often interspersed with lipid-like material after rupture. Although GLM and ductal dilatation are pathologically distinct, they can occur together, and PDM can also present with granulomas and chronic purulent inflammation (75). The two can be difficult to distinguish when GLM is associated with dilated ducts and high plasma cell infiltration. The fusion lesions are mainly between GLM lesions, but fusion between ductal dilatation and GLM is rare (51). Cholesterol crystals and calcifications may also be present in MDE and are not statistically different from GLM (46).

Granulomatous lobular mastitis is a non-caseating granulomatous inflammatory disease occurring in the lobules of the breast. Granulomas are composed of epithelioid cells and multinucleated giant cells with an infiltration of inflammatory cells such as lymphocytes, neutrophils, plasma cells, and eosinophils, and as their features can be masked by focal fusion and abscess formation, as much breast tissue as possible should be selected to aid in the diagnosis. CNGM in the center of a granuloma containing neutrophils surrounded by lipid vesicles with Gram-positive bacilli visible in the vesicles is currently considered a subtype of GLM and its association with Corynebacterium is a current hot topic of research. However, recent studies have also tended to suggest that CNGM is a later stage of the GLM, a view that still needs to be confirmed. Our next step will be to conduct clinical studies based on this. We believe that the significance of pathology, in addition to diagnostic and differential diagnosis, can also contribute to the understanding of the disease by explaining its pathogenesis at the cytological level. Granulomatous lobular mastitis has been reported for the first time since 1972, and its incidence has gradually increased in recent years. Although there are many hypothese, the pathogenesis of granulomatous mastitis has not yet been fully established. In addition, GLM not only needs to be differentiated from breast cancer, but may also be combined with breast cancer and is a risk factor for breast cancer, which requires clinicians to spend effort on the diagnostic step of the disease. The relationship between inflammatory diseases of the breast, such as granulomatous lobular mastitis, and breast cancer needs to be further investigated. At present, animal studies of granulomatous mastitis have a high failure rate due to modeling difficulties, and are mostly clinical studies. In the future, there is a need not only for prospective clinical trials incorporating more samples, but also for collaborative imaging, testing and pathology contracts.