[A case of diabetic mastopathy which was difficult to diagnose].

The patient was a 73-year-old woman with diabetes mellitus who was receiving insulin therapy. A poorly demarcated mass of 2 cm in diameter was palpated in the C region of the left breast. Mammography showed a dense locally asymmetric shadow. Ultrasonography revealed an irregular, poorly demarcated, hypoechoic mass measuring 14×21×10 mm accompanied by an attenuated posterior echo. Needle biopsy showed no evidence of malignancy, and the patient was kept under observation. An ultrasonographic examination performed 6 months later showed no change, but the possibility of cancer could not be ruled out on contrast-enhanced computed tomography and magnetic resonance imaging. Tumor resection at the patient's request was therefore performed. Histopathological examination of the breast revealed interstitial fibrosis with superimposed ground-glass opacities and lymphocyte infiltration around the ducts, leading to a diagnosis of diabetic mastopathy. Diabetic mastopathy occurs primarily in patients with a prolonged history of diabetes mellitus. It is difficult to distinguish diabetic mastopathy from breast cancer by palpation and imaging studies. Most cases are conclusively diagnosed by needle biopsy. Clinicians should be aware of diabetic mastopathy to avoid overdiagnosis and overtreatment. In our patient, diabetic mastopathy could be diagnosed on the basis of clinical characteristics and needle biopsy.