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Background: Granulomatous mastitis is a relatively rare benign inflammatory disease of the breast, but it is sometimes difficult to distinguish from breast cancer by imaging. We experienced a case that was definitively diagnosed as granulomatous mastitis from the surgical specimen. The mass appeared as a large cystic lesion on imaging, which is unusual for granulomatous mastitis, and was initially suspected to be an encapsulated papillary carcinoma., Case Presentation: A 43-year-old woman presented with a painful mass in her right breast. Ultrasonography revealed a cystic mass lesion with internal solid components, with partially indistinct cyst walls and abundant blood flow. Additionally, lymphadenopathy of one axillary lymph node was observed. Magnetic resonance imaging findings showed irregularly spreading enhanced nodules within the cystic lesion, raising the suspicion of encapsulated papillary carcinoma. Although the histological findings from a needle biopsy were consistent with granulomatous mastitis, the possibility of malignancy could not be ruled out based on imaging, prompting a diagnostic probe lumpectomy. However, the surgical specimens did not reveal any tumorous lesions, and we reached a final diagnosis of granulomatous mastitis. Postoperatively, the patient was followed-up without steroid therapy and has been free from recurrence of mastitis for 22 months after surgery., Conclusions: We report a case of granulomatous mastitis that was detected as a large cystic lesion with a well-defined border on imaging and a definitive diagnosis was made from a surgical specimen., (© 2024. The Author(s).)