The imaging findings of GM are nonspecific and often overlap with those of malignant lesions. Ultrasonography findings often show an ill-defined hypoechoic mass, and in advanced cases, fluid collection and cavities in association with skin fistulas. Ipsilateral reactive axillary lymph node enlargement may also be present. GM generally has abundant blood flow that can be detected by Doppler imaging within and around the lesion [6]. MRI imaging may show an ill-defined mass or non-mass-enhancement. Often, progressive or plateau enhancement patterns predominate with interspersed areas of rapid enhancement and washout. Advanced cases demonstrate T2 hyperintense, peripherally enhancing masses with central areas of non-enhancement representing abscess formation [12]. In our case, an internal high-signal fluid collection was observed on T1-weighted images, suggesting the possibility of hemorrhage rather than an abscess. In addition, the lesion was a solitary mass, with no surrounding microabscesses, so an intra-cystic tumor rather than GM was the primary concern. As mentioned above, GM generally presents as an irregular shaped mass with ill-defined margins. However, advanced GM cases sometimes demonstrate fluid retention and cavities [5]. Therefore, clinicians should bear in mind that GM can present as a well-defined mass, as in our case.

Although steroid therapy is generally used as a treatment for GM, no treatment method has been established [5, 13]. In clinical practice, long-term steroid therapy is often ineffective, or the disease repeatedly relapses after remission, making it difficult to treat. The efficacy of surgical treatment has long been studied. Postoperative recurrence rates of 14%-25% have been reported when surgical resection is the treatment of choice [10, 14,15,16,17]. On the other hand, a meta-analysis comparing the efficacy of steroid therapy to surgical resection reported cure rates of 72%, 91%, and 95% for steroid therapy, surgical resection, and a combination of the two, respectively [18], with relapse rates after remission reported to be 21%, 7%, and 4%, respectively. Although surgical treatment may be overly invasive and should be considered with caution, it should be actively considered as a treatment option in cases of poor response to steroid therapy [10]. In this case the primary goal of surgical resection was to confirm the diagnosis, but as a result, the patient was cured without the use of steroids.

Corynebacterium kroppenstedtii has been identified as a cause of GM [19]. It likely triggers granuloma formation through a localized immune response as immune cells attempt to contain the infection [20]. Although often viewed as a contaminant due to its presence in normal skin flora, its deep breast tissue localization within cystic spaces surrounded by granulomatous inflammation indicates a pathogenic role [20]. Hyperprolactinemia, which can cause ductal ectasia and milk stasis, may also promote GM [21]. A combination of these factors might explain why symptoms appear in certain individuals, though further research is needed to fully understand these mechanisms. Detecting Corynebacterium spp is challenging due to their slow growth and the need for specific staining, often leading to underdiagnosis. However, it is generally susceptible to various antibacterial drugs; thus, it is essential to make an accurate detection for appropriate treatment.