Diagnosis of an extremely rare pleomorphic adenoma of the breast with core needle biopsy: A case report

INTRODUCTION: Pleomorphic adenoma (PA) rarely originates in the mammary gland. This tumor is a benign mixed tumor and is commonly found in the salivary glands, but rarely shows findings similar to those of breast cancer. The tumor is misdiagnosed in 30–50% of the patients, including overdiagnosis of malignancy preoperatively, leading to unnecessary surgery for breast cancer. CASE PRESENTATION: The present patient was a 45-year-old woman who exhibited no subjective symptoms. She visited the Takahashi Breast and Gastroenterology Clinic for breast cancer screening. A mass measuring 1.5 cm was palpated in the E region of the left breast. Mammography (MMG) showed a tumor shadow in the left S region. There was a high-density area inside the tumor, suggesting bone or cartilage formation, and a diagnosis of category 3 on MMG was made. Ultrasonography (US) revealed a poorly demarcated tumor measuring 14.3 × 14.8 × 10.7 mm with relatively smooth margins and inhomogeneous high-echo content. Core needle biopsy (CNB) was performed, which led to the diagnosis of PA. The tumor showed proliferation of small cells forming glandular duct-like structures or anastomosed funicular structures. Moreover, the cells in the basilar portion were spindle shaped and transitioned to the stroma. Cartilaginous metaplasia, calcification, and ossification were observed in some areas. Intraductal papilloma was also observed in some areas. CONCLUSION: PA may be diagnosed based on the histological findings of CNB. Thus, unnecessary surgery for breast cancer may be avoided.