Cystic Neutrophilic Granulomatous Mastitis in Accessory Breast Tissue: A Case Report.

Background: Cystic neutrophilic granulomatous mastitis (CNGM) is a rare inflammatory breast disease characterized by granulomatous inflammation with cystic spaces and neutrophilic infiltration. CNGM involving accessory breast tissue is exceptionally uncommon and has only rarely been reported. Due to its nonspecific clinical manifestations, CNGM may be misdiagnosed, potentially resulting in delayed diagnosis and treatment. Case Presentation: A 37-year-old woman presented with swelling and pain in the right axillary region. Ultrasonography revealed an inflammatory lesion within accessory breast tissue, and core needle biopsy confirmed the diagnosis of CNGM. Histopathological examination demonstrated granulomatous inflammation with characteristic cystic neutrophilic features. The patient received antibiotic therapy, traditional Chinese medicine, and other conservative treatments. Her clinical symptoms and inflammatory markers improved during short-term follow-up. Conclusions: CNGM arising in accessory breast tissue is exceptionally rare and may pose a diagnostic challenge because of its nonspecific clinical and imaging features. This case expands the recognized clinical spectrum of CNGM and highlights the importance of considering CNGM in the differential diagnosis of inflammatory lesions arising in accessory breast tissue. Histopathological evaluation is essential for establishing the diagnosis and excluding malignancy or other inflammatory conditions.