Granulomatous mastitis encompasses a group of poorly characterized inflammatory breast disorders that commonly present with a breast mass or pain. Prior studies have shown that the cystic neutrophilic granulomatous mastitis (CNGM) subtype of so-called “idiopathic” granulomatous mastitis is associated with Gram-positive bacteria, particularly Corynebacterium species [1]. Histologically, CNGM is characterized by non-necrotizing, lobule-centered granulomatous inflammation accompanied by a heterogeneous inflammatory infiltrate. In certain cases, the granulomas exhibit cystic lipid spaces lined by neutrophils, a distinctive feature commonly associated with cystic neutrophilic granulomatous mastitis [2]. Studies have shown that CNGM predominantly affects women between 25 and 49 years of age, with a median age of 34 years [2]. Treatment of CNGM remains challenging and generally consists of a combination of surgery, antimicrobial agents, and corticosteroids. Disease recurrence and prolonged clinical courses frequently result in the need for repeated interventions. Nevertheless, there is currently no standardized or widely accepted therapeutic regimen for these patients [2]. We present the case of a 28-year-old pregnant woman who presented with a short history of progressively worsening left breast pain and swelling.

A 28-year-old pregnant Hispanic woman presented at 25 weeks of gestation with an 8-day history of worsening pain, swelling, and inflammation of the left breast. Physical examination revealed a firm, irregular, mildly tender mass with limited mobility. Initial treatment with dicloxacillin failed to produce clinical improvement. Given the characteristics of the mass, malignancy was also considered in the differential diagnosis. Consequently, the patient was followed weekly by the oncology service for close monitoring. The patient’s clinical history was notable for pregnancy, a relevant factor because CNGM predominantly affects women of reproductive age and has been reported in association with pregnancy and lactation. Histopathologic findings supported a diagnosis of CNGM (Figure 1) and prompted empiric treatment with azithromycin 500 mg daily for 6 weeks, cephalexin 500 mg every 8 hours for 4 weeks, prednisone 30 mg twice daily, and topical hydrocortisone 1% cream applied twice daily.

Following initiation of this treatment regimen, the patient experienced significant clinical improvement. The phlegmonous lesion decreased in size from greater than 7 cm to 3.3 cm after one month of therapy. However, in the weeks preceding delivery, the patient temporarily discontinued her prescribed medications, resulting in recurrence of her symptoms.

Management of CNGM associated with Corynebacterium kroppenstedtii typically involves prolonged antibiotic therapy directed against lipophilic Corynebacterium species, including macrolides, tetracyclines, or clindamycin, and often requires a multidisciplinary approach to care.

Cystic neutrophilic granulomatous mastitis (CNGM) is a distinct and reproducible histopathologic entity rather than an idiopathic inflammatory process. During pregnancy, physiologic breast changes, including increased glandular tissue, vascularity, and background parenchymal enhancement, can complicate clinical and radiologic assessment, resulting in overlapping with benign inflammatory conditions and pregnancy-associated breast cancer. Multiple studies have demonstrated that Corynebacterium kroppenstedtii is a true pathogen in this setting rather than a skin commensal [6].

Diagnosis can be challenging because C. kroppenstedtii is slow-growing and lipophilic, often resulting in negative routine cultures when standard microbiologic methods are used [7][8]. The use of lipid-enriched culture media has been shown to improve diagnostic yield [8]. Although microbiologic cultures were not obtained in the present case, recognition of the characteristic CNGM histopathologic pattern, together with identification of Gram-positive bacilli, supported the diagnosis. Recognition of this pattern is therefore essential, as it can prompt appropriate microbiologic evaluation and facilitate timely, targeted therapy.

CNGM is a characteristic histopathologic pattern with direct therapeutic implications. Its recognition should prompt consideration of Corynebacterium kroppenstedtii and guide appropriate antimicrobial therapy.

Dr. Twisha Oza: Case diagnosis, conceptualization, supervision, manuscript drafting, and critical revision. Dr. Upadhyay: Data collection, clinical history and findings, and preparation of the initial draft. Dr. Moultrie: Data collection. Dr. Hendrix: Clinical care and assistance with clinical findings. All authors reviewed and approved the final manuscript.

Shared with consent for the purpose of advancing education and teaching; though not applicable to de-identified single case reports.

^#Co-first authors.